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[perl.git] / sv.c

/*    sv.c
 *
 *    Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
 *    2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 by Larry Wall
 *    and others
 *
 *    You may distribute under the terms of either the GNU General Public
 *    License or the Artistic License, as specified in the README file.
 *
 */

/*
 * 'I wonder what the Entish is for "yes" and "no",' he thought.
 *                                                      --Pippin
 *
 *     [p.480 of _The Lord of the Rings_, III/iv: "Treebeard"]
 */

/*
 *
 *
 * This file contains the code that creates, manipulates and destroys
 * scalar values (SVs). The other types (AV, HV, GV, etc.) reuse the
 * structure of an SV, so their creation and destruction is handled
 * here; higher-level functions are in av.c, hv.c, and so on. Opcode
 * level functions (eg. substr, split, join) for each of the types are
 * in the pp*.c files.
 */

#include "EXTERN.h"
#define PERL_IN_SV_C
#include "perl.h"
#include "regcomp.h"

#ifndef HAS_C99
# if __STDC_VERSION__ >= 199901L && !defined(VMS)
#  define HAS_C99 1
# endif
#endif
#if HAS_C99
# include <stdint.h>
#endif

#define FCALL *f

#ifdef __Lynx__
/* Missing proto on LynxOS */
  char *gconvert(double, int, int,  char *);
#endif

#ifdef PERL_UTF8_CACHE_ASSERT
/* if adding more checks watch out for the following tests:
 *   t/op/index.t t/op/length.t t/op/pat.t t/op/substr.t
 *   lib/utf8.t lib/Unicode/Collate/t/index.t
 * --jhi
 */
#   define ASSERT_UTF8_CACHE(cache) \
    STMT_START { if (cache) { assert((cache)[0] <= (cache)[1]); \
                              assert((cache)[2] <= (cache)[3]); \
                              assert((cache)[3] <= (cache)[1]);} \
                              } STMT_END
#else
#   define ASSERT_UTF8_CACHE(cache) NOOP
#endif

#ifdef PERL_OLD_COPY_ON_WRITE
#define SV_COW_NEXT_SV(sv)      INT2PTR(SV *,SvUVX(sv))
#define SV_COW_NEXT_SV_SET(current,next)        SvUV_set(current, PTR2UV(next))
/* This is a pessimistic view. Scalar must be purely a read-write PV to copy-
   on-write.  */
#endif

/* ============================================================================

=head1 Allocation and deallocation of SVs.

An SV (or AV, HV, etc.) is allocated in two parts: the head (struct
sv, av, hv...) contains type and reference count information, and for
many types, a pointer to the body (struct xrv, xpv, xpviv...), which
contains fields specific to each type.  Some types store all they need
in the head, so don't have a body.

In all but the most memory-paranoid configurations (ex: PURIFY), heads
and bodies are allocated out of arenas, which by default are
approximately 4K chunks of memory parcelled up into N heads or bodies.
Sv-bodies are allocated by their sv-type, guaranteeing size
consistency needed to allocate safely from arrays.

For SV-heads, the first slot in each arena is reserved, and holds a
link to the next arena, some flags, and a note of the number of slots.
Snaked through each arena chain is a linked list of free items; when
this becomes empty, an extra arena is allocated and divided up into N
items which are threaded into the free list.

SV-bodies are similar, but they use arena-sets by default, which
separate the link and info from the arena itself, and reclaim the 1st
slot in the arena.  SV-bodies are further described later.

The following global variables are associated with arenas:

    PL_sv_arenaroot     pointer to list of SV arenas
    PL_sv_root          pointer to list of free SV structures

    PL_body_arenas      head of linked-list of body arenas
    PL_body_roots[]     array of pointers to list of free bodies of svtype
                        arrays are indexed by the svtype needed

A few special SV heads are not allocated from an arena, but are
instead directly created in the interpreter structure, eg PL_sv_undef.
The size of arenas can be changed from the default by setting
PERL_ARENA_SIZE appropriately at compile time.

The SV arena serves the secondary purpose of allowing still-live SVs
to be located and destroyed during final cleanup.

At the lowest level, the macros new_SV() and del_SV() grab and free
an SV head.  (If debugging with -DD, del_SV() calls the function S_del_sv()
to return the SV to the free list with error checking.) new_SV() calls
more_sv() / sv_add_arena() to add an extra arena if the free list is empty.
SVs in the free list have their SvTYPE field set to all ones.

At the time of very final cleanup, sv_free_arenas() is called from
perl_destruct() to physically free all the arenas allocated since the
start of the interpreter.

The function visit() scans the SV arenas list, and calls a specified
function for each SV it finds which is still live - ie which has an SvTYPE
other than all 1's, and a non-zero SvREFCNT. visit() is used by the
following functions (specified as [function that calls visit()] / [function
called by visit() for each SV]):

    sv_report_used() / do_report_used()
                        dump all remaining SVs (debugging aid)

    sv_clean_objs() / do_clean_objs(),do_clean_named_objs(),
                      do_clean_named_io_objs()
                        Attempt to free all objects pointed to by RVs,
                        and try to do the same for all objects indirectly
                        referenced by typeglobs too.  Called once from
                        perl_destruct(), prior to calling sv_clean_all()
                        below.

    sv_clean_all() / do_clean_all()
                        SvREFCNT_dec(sv) each remaining SV, possibly
                        triggering an sv_free(). It also sets the
                        SVf_BREAK flag on the SV to indicate that the
                        refcnt has been artificially lowered, and thus
                        stopping sv_free() from giving spurious warnings
                        about SVs which unexpectedly have a refcnt
                        of zero.  called repeatedly from perl_destruct()
                        until there are no SVs left.

=head2 Arena allocator API Summary

Private API to rest of sv.c

    new_SV(),  del_SV(),

    new_XPVNV(), del_XPVGV(),
    etc

Public API:

    sv_report_used(), sv_clean_objs(), sv_clean_all(), sv_free_arenas()

=cut

 * ========================================================================= */

/*
 * "A time to plant, and a time to uproot what was planted..."
 */

#ifdef PERL_MEM_LOG
#  define MEM_LOG_NEW_SV(sv, file, line, func)  \
            Perl_mem_log_new_sv(sv, file, line, func)
#  define MEM_LOG_DEL_SV(sv, file, line, func)  \
            Perl_mem_log_del_sv(sv, file, line, func)
#else
#  define MEM_LOG_NEW_SV(sv, file, line, func)  NOOP
#  define MEM_LOG_DEL_SV(sv, file, line, func)  NOOP
#endif

#ifdef DEBUG_LEAKING_SCALARS
#  define FREE_SV_DEBUG_FILE(sv) STMT_START { \
        if ((sv)->sv_debug_file) PerlMemShared_free((sv)->sv_debug_file); \
    } STMT_END
#  define DEBUG_SV_SERIAL(sv)                                               \
    DEBUG_m(PerlIO_printf(Perl_debug_log, "0x%"UVxf": (%05ld) del_SV\n",    \
            PTR2UV(sv), (long)(sv)->sv_debug_serial))
#else
#  define FREE_SV_DEBUG_FILE(sv)
#  define DEBUG_SV_SERIAL(sv)   NOOP
#endif

#ifdef PERL_POISON
#  define SvARENA_CHAIN(sv)     ((sv)->sv_u.svu_rv)
#  define SvARENA_CHAIN_SET(sv,val)     (sv)->sv_u.svu_rv = MUTABLE_SV((val))
/* Whilst I'd love to do this, it seems that things like to check on
   unreferenced scalars
#  define POSION_SV_HEAD(sv)    PoisonNew(sv, 1, struct STRUCT_SV)
*/
#  define POSION_SV_HEAD(sv)    PoisonNew(&SvANY(sv), 1, void *), \
                                PoisonNew(&SvREFCNT(sv), 1, U32)
#else
#  define SvARENA_CHAIN(sv)     SvANY(sv)
#  define SvARENA_CHAIN_SET(sv,val)     SvANY(sv) = (void *)(val)
#  define POSION_SV_HEAD(sv)
#endif

/* Mark an SV head as unused, and add to free list.
 *
 * If SVf_BREAK is set, skip adding it to the free list, as this SV had
 * its refcount artificially decremented during global destruction, so
 * there may be dangling pointers to it. The last thing we want in that
 * case is for it to be reused. */

#define plant_SV(p) \
    STMT_START {                                        \
        const U32 old_flags = SvFLAGS(p);                       \
        MEM_LOG_DEL_SV(p, __FILE__, __LINE__, FUNCTION__);  \
        DEBUG_SV_SERIAL(p);                             \
        FREE_SV_DEBUG_FILE(p);                          \
        POSION_SV_HEAD(p);                              \
        SvFLAGS(p) = SVTYPEMASK;                        \
        if (!(old_flags & SVf_BREAK)) {         \
            SvARENA_CHAIN_SET(p, PL_sv_root);   \
            PL_sv_root = (p);                           \
        }                                               \
        --PL_sv_count;                                  \
    } STMT_END

#define uproot_SV(p) \
    STMT_START {                                        \
        (p) = PL_sv_root;                               \
        PL_sv_root = MUTABLE_SV(SvARENA_CHAIN(p));              \
        ++PL_sv_count;                                  \
    } STMT_END


/* make some more SVs by adding another arena */

STATIC SV*
S_more_sv(pTHX)
{
    dVAR;
    SV* sv;
    char *chunk;                /* must use New here to match call to */
    Newx(chunk,PERL_ARENA_SIZE,char);  /* Safefree() in sv_free_arenas() */
    sv_add_arena(chunk, PERL_ARENA_SIZE, 0);
    uproot_SV(sv);
    return sv;
}

/* new_SV(): return a new, empty SV head */

#ifdef DEBUG_LEAKING_SCALARS
/* provide a real function for a debugger to play with */
STATIC SV*
S_new_SV(pTHX_ const char *file, int line, const char *func)
{
    SV* sv;

    if (PL_sv_root)
        uproot_SV(sv);
    else
        sv = S_more_sv(aTHX);
    SvANY(sv) = 0;
    SvREFCNT(sv) = 1;
    SvFLAGS(sv) = 0;
    sv->sv_debug_optype = PL_op ? PL_op->op_type : 0;
    sv->sv_debug_line = (U16) (PL_parser && PL_parser->copline != NOLINE
                ? PL_parser->copline
                :  PL_curcop
                    ? CopLINE(PL_curcop)
                    : 0
            );
    sv->sv_debug_inpad = 0;
    sv->sv_debug_parent = NULL;
    sv->sv_debug_file = PL_curcop ? savesharedpv(CopFILE(PL_curcop)): NULL;

    sv->sv_debug_serial = PL_sv_serial++;

    MEM_LOG_NEW_SV(sv, file, line, func);
    DEBUG_m(PerlIO_printf(Perl_debug_log, "0x%"UVxf": (%05ld) new_SV (from %s:%d [%s])\n",
            PTR2UV(sv), (long)sv->sv_debug_serial, file, line, func));

    return sv;
}
#  define new_SV(p) (p)=S_new_SV(aTHX_ __FILE__, __LINE__, FUNCTION__)

#else
#  define new_SV(p) \
    STMT_START {                                        \
        if (PL_sv_root)                                 \
            uproot_SV(p);                               \
        else                                            \
            (p) = S_more_sv(aTHX);                      \
        SvANY(p) = 0;                                   \
        SvREFCNT(p) = 1;                                \
        SvFLAGS(p) = 0;                                 \
        MEM_LOG_NEW_SV(p, __FILE__, __LINE__, FUNCTION__);  \
    } STMT_END
#endif


/* del_SV(): return an empty SV head to the free list */

#ifdef DEBUGGING

#define del_SV(p) \
    STMT_START {                                        \
        if (DEBUG_D_TEST)                               \
            del_sv(p);                                  \
        else                                            \
            plant_SV(p);                                \
    } STMT_END

STATIC void
S_del_sv(pTHX_ SV *p)
{
    dVAR;

    PERL_ARGS_ASSERT_DEL_SV;

    if (DEBUG_D_TEST) {
        SV* sva;
        bool ok = 0;
        for (sva = PL_sv_arenaroot; sva; sva = MUTABLE_SV(SvANY(sva))) {
            const SV * const sv = sva + 1;
            const SV * const svend = &sva[SvREFCNT(sva)];
            if (p >= sv && p < svend) {
                ok = 1;
                break;
            }
        }
        if (!ok) {
            Perl_ck_warner_d(aTHX_ packWARN(WARN_INTERNAL),
                             "Attempt to free non-arena SV: 0x%"UVxf
                             pTHX__FORMAT, PTR2UV(p) pTHX__VALUE);
            return;
        }
    }
    plant_SV(p);
}

#else /* ! DEBUGGING */

#define del_SV(p)   plant_SV(p)

#endif /* DEBUGGING */


/*
=head1 SV Manipulation Functions

=for apidoc sv_add_arena

Given a chunk of memory, link it to the head of the list of arenas,
and split it into a list of free SVs.

=cut
*/

static void
S_sv_add_arena(pTHX_ char *const ptr, const U32 size, const U32 flags)
{
    dVAR;
    SV *const sva = MUTABLE_SV(ptr);
    SV* sv;
    SV* svend;

    PERL_ARGS_ASSERT_SV_ADD_ARENA;

    /* The first SV in an arena isn't an SV. */
    SvANY(sva) = (void *) PL_sv_arenaroot;              /* ptr to next arena */
    SvREFCNT(sva) = size / sizeof(SV);          /* number of SV slots */
    SvFLAGS(sva) = flags;                       /* FAKE if not to be freed */

    PL_sv_arenaroot = sva;
    PL_sv_root = sva + 1;

    svend = &sva[SvREFCNT(sva) - 1];
    sv = sva + 1;
    while (sv < svend) {
        SvARENA_CHAIN_SET(sv, (sv + 1));
#ifdef DEBUGGING
        SvREFCNT(sv) = 0;
#endif
        /* Must always set typemask because it's always checked in on cleanup
           when the arenas are walked looking for objects.  */
        SvFLAGS(sv) = SVTYPEMASK;
        sv++;
    }
    SvARENA_CHAIN_SET(sv, 0);
#ifdef DEBUGGING
    SvREFCNT(sv) = 0;
#endif
    SvFLAGS(sv) = SVTYPEMASK;
}

/* visit(): call the named function for each non-free SV in the arenas
 * whose flags field matches the flags/mask args. */

STATIC I32
S_visit(pTHX_ SVFUNC_t f, const U32 flags, const U32 mask)
{
    dVAR;
    SV* sva;
    I32 visited = 0;

    PERL_ARGS_ASSERT_VISIT;

    for (sva = PL_sv_arenaroot; sva; sva = MUTABLE_SV(SvANY(sva))) {
        const SV * const svend = &sva[SvREFCNT(sva)];
        SV* sv;
        for (sv = sva + 1; sv < svend; ++sv) {
            if (SvTYPE(sv) != (svtype)SVTYPEMASK
                    && (sv->sv_flags & mask) == flags
                    && SvREFCNT(sv))
            {
                (FCALL)(aTHX_ sv);
                ++visited;
            }
        }
    }
    return visited;
}

#ifdef DEBUGGING

/* called by sv_report_used() for each live SV */

static void
do_report_used(pTHX_ SV *const sv)
{
    if (SvTYPE(sv) != (svtype)SVTYPEMASK) {
        PerlIO_printf(Perl_debug_log, "****\n");
        sv_dump(sv);
    }
}
#endif

/*
=for apidoc sv_report_used

Dump the contents of all SVs not yet freed (debugging aid).

=cut
*/

void
Perl_sv_report_used(pTHX)
{
#ifdef DEBUGGING
    visit(do_report_used, 0, 0);
#else
    PERL_UNUSED_CONTEXT;
#endif
}

/* called by sv_clean_objs() for each live SV */

static void
do_clean_objs(pTHX_ SV *const ref)
{
    dVAR;
    assert (SvROK(ref));
    {
        SV * const target = SvRV(ref);
        if (SvOBJECT(target)) {
            DEBUG_D((PerlIO_printf(Perl_debug_log, "Cleaning object ref:\n "), sv_dump(ref)));
            if (SvWEAKREF(ref)) {
                sv_del_backref(target, ref);
                SvWEAKREF_off(ref);
                SvRV_set(ref, NULL);
            } else {
                SvROK_off(ref);
                SvRV_set(ref, NULL);
                SvREFCNT_dec(target);
            }
        }
    }

    /* XXX Might want to check arrays, etc. */
}


/* clear any slots in a GV which hold objects - except IO;
 * called by sv_clean_objs() for each live GV */

static void
do_clean_named_objs(pTHX_ SV *const sv)
{
    dVAR;
    SV *obj;
    assert(SvTYPE(sv) == SVt_PVGV);
    assert(isGV_with_GP(sv));
    if (!GvGP(sv))
        return;

    /* freeing GP entries may indirectly free the current GV;
     * hold onto it while we mess with the GP slots */
    SvREFCNT_inc(sv);

    if ( ((obj = GvSV(sv) )) && SvOBJECT(obj)) {
        DEBUG_D((PerlIO_printf(Perl_debug_log,
                "Cleaning named glob SV object:\n "), sv_dump(obj)));
        GvSV(sv) = NULL;
        SvREFCNT_dec(obj);
    }
    if ( ((obj = MUTABLE_SV(GvAV(sv)) )) && SvOBJECT(obj)) {
        DEBUG_D((PerlIO_printf(Perl_debug_log,
                "Cleaning named glob AV object:\n "), sv_dump(obj)));
        GvAV(sv) = NULL;
        SvREFCNT_dec(obj);
    }
    if ( ((obj = MUTABLE_SV(GvHV(sv)) )) && SvOBJECT(obj)) {
        DEBUG_D((PerlIO_printf(Perl_debug_log,
                "Cleaning named glob HV object:\n "), sv_dump(obj)));
        GvHV(sv) = NULL;
        SvREFCNT_dec(obj);
    }
    if ( ((obj = MUTABLE_SV(GvCV(sv)) )) && SvOBJECT(obj)) {
        DEBUG_D((PerlIO_printf(Perl_debug_log,
                "Cleaning named glob CV object:\n "), sv_dump(obj)));
        GvCV_set(sv, NULL);
        SvREFCNT_dec(obj);
    }
    SvREFCNT_dec(sv); /* undo the inc above */
}

/* clear any IO slots in a GV which hold objects (except stderr, defout);
 * called by sv_clean_objs() for each live GV */

static void
do_clean_named_io_objs(pTHX_ SV *const sv)
{
    dVAR;
    SV *obj;
    assert(SvTYPE(sv) == SVt_PVGV);
    assert(isGV_with_GP(sv));
    if (!GvGP(sv) || sv == (SV*)PL_stderrgv || sv == (SV*)PL_defoutgv)
        return;

    SvREFCNT_inc(sv);
    if ( ((obj = MUTABLE_SV(GvIO(sv)) )) && SvOBJECT(obj)) {
        DEBUG_D((PerlIO_printf(Perl_debug_log,
                "Cleaning named glob IO object:\n "), sv_dump(obj)));
        GvIOp(sv) = NULL;
        SvREFCNT_dec(obj);
    }
    SvREFCNT_dec(sv); /* undo the inc above */
}

/* Void wrapper to pass to visit() */
static void
do_curse(pTHX_ SV * const sv) {
    if ((PL_stderrgv && GvGP(PL_stderrgv) && (SV*)GvIO(PL_stderrgv) == sv)
     || (PL_defoutgv && GvGP(PL_defoutgv) && (SV*)GvIO(PL_defoutgv) == sv))
        return;
    (void)curse(sv, 0);
}

/*
=for apidoc sv_clean_objs

Attempt to destroy all objects not yet freed.

=cut
*/

void
Perl_sv_clean_objs(pTHX)
{
    dVAR;
    GV *olddef, *olderr;
    PL_in_clean_objs = TRUE;
    visit(do_clean_objs, SVf_ROK, SVf_ROK);
    /* Some barnacles may yet remain, clinging to typeglobs.
     * Run the non-IO destructors first: they may want to output
     * error messages, close files etc */
    visit(do_clean_named_objs, SVt_PVGV|SVpgv_GP, SVTYPEMASK|SVp_POK|SVpgv_GP);
    visit(do_clean_named_io_objs, SVt_PVGV|SVpgv_GP, SVTYPEMASK|SVp_POK|SVpgv_GP);
    /* And if there are some very tenacious barnacles clinging to arrays,
       closures, or what have you.... */
    visit(do_curse, SVs_OBJECT, SVs_OBJECT);
    olddef = PL_defoutgv;
    PL_defoutgv = NULL; /* disable skip of PL_defoutgv */
    if (olddef && isGV_with_GP(olddef))
        do_clean_named_io_objs(aTHX_ MUTABLE_SV(olddef));
    olderr = PL_stderrgv;
    PL_stderrgv = NULL; /* disable skip of PL_stderrgv */
    if (olderr && isGV_with_GP(olderr))
        do_clean_named_io_objs(aTHX_ MUTABLE_SV(olderr));
    SvREFCNT_dec(olddef);
    PL_in_clean_objs = FALSE;
}

/* called by sv_clean_all() for each live SV */

static void
do_clean_all(pTHX_ SV *const sv)
{
    dVAR;
    if (sv == (const SV *) PL_fdpid || sv == (const SV *)PL_strtab) {
        /* don't clean pid table and strtab */
        return;
    }
    DEBUG_D((PerlIO_printf(Perl_debug_log, "Cleaning loops: SV at 0x%"UVxf"\n", PTR2UV(sv)) ));
    SvFLAGS(sv) |= SVf_BREAK;
    SvREFCNT_dec(sv);
}

/*
=for apidoc sv_clean_all

Decrement the refcnt of each remaining SV, possibly triggering a
cleanup.  This function may have to be called multiple times to free
SVs which are in complex self-referential hierarchies.

=cut
*/

I32
Perl_sv_clean_all(pTHX)
{
    dVAR;
    I32 cleaned;
    PL_in_clean_all = TRUE;
    cleaned = visit(do_clean_all, 0,0);
    return cleaned;
}

/*
  ARENASETS: a meta-arena implementation which separates arena-info
  into struct arena_set, which contains an array of struct
  arena_descs, each holding info for a single arena.  By separating
  the meta-info from the arena, we recover the 1st slot, formerly
  borrowed for list management.  The arena_set is about the size of an
  arena, avoiding the needless malloc overhead of a naive linked-list.

  The cost is 1 arena-set malloc per ~320 arena-mallocs, + the unused
  memory in the last arena-set (1/2 on average).  In trade, we get
  back the 1st slot in each arena (ie 1.7% of a CV-arena, less for
  smaller types).  The recovery of the wasted space allows use of
  small arenas for large, rare body types, by changing array* fields
  in body_details_by_type[] below.
*/
struct arena_desc {
    char       *arena;          /* the raw storage, allocated aligned */
    size_t      size;           /* its size ~4k typ */
    svtype      utype;          /* bodytype stored in arena */
};

struct arena_set;

/* Get the maximum number of elements in set[] such that struct arena_set
   will fit within PERL_ARENA_SIZE, which is probably just under 4K, and
   therefore likely to be 1 aligned memory page.  */

#define ARENAS_PER_SET  ((PERL_ARENA_SIZE - sizeof(struct arena_set*) \
                          - 2 * sizeof(int)) / sizeof (struct arena_desc))

struct arena_set {
    struct arena_set* next;
    unsigned int   set_size;    /* ie ARENAS_PER_SET */
    unsigned int   curr;        /* index of next available arena-desc */
    struct arena_desc set[ARENAS_PER_SET];
};

/*
=for apidoc sv_free_arenas

Deallocate the memory used by all arenas.  Note that all the individual SV
heads and bodies within the arenas must already have been freed.

=cut
*/
void
Perl_sv_free_arenas(pTHX)
{
    dVAR;
    SV* sva;
    SV* svanext;
    unsigned int i;

    /* Free arenas here, but be careful about fake ones.  (We assume
       contiguity of the fake ones with the corresponding real ones.) */

    for (sva = PL_sv_arenaroot; sva; sva = svanext) {
        svanext = MUTABLE_SV(SvANY(sva));
        while (svanext && SvFAKE(svanext))
            svanext = MUTABLE_SV(SvANY(svanext));

        if (!SvFAKE(sva))
            Safefree(sva);
    }

    {
        struct arena_set *aroot = (struct arena_set*) PL_body_arenas;

        while (aroot) {
            struct arena_set *current = aroot;
            i = aroot->curr;
            while (i--) {
                assert(aroot->set[i].arena);
                Safefree(aroot->set[i].arena);
            }
            aroot = aroot->next;
            Safefree(current);
        }
    }
    PL_body_arenas = 0;

    i = PERL_ARENA_ROOTS_SIZE;
    while (i--)
        PL_body_roots[i] = 0;

    PL_sv_arenaroot = 0;
    PL_sv_root = 0;
}

/*
  Here are mid-level routines that manage the allocation of bodies out
  of the various arenas.  There are 5 kinds of arenas:

  1. SV-head arenas, which are discussed and handled above
  2. regular body arenas
  3. arenas for reduced-size bodies
  4. Hash-Entry arenas

  Arena types 2 & 3 are chained by body-type off an array of
  arena-root pointers, which is indexed by svtype.  Some of the
  larger/less used body types are malloced singly, since a large
  unused block of them is wasteful.  Also, several svtypes dont have
  bodies; the data fits into the sv-head itself.  The arena-root
  pointer thus has a few unused root-pointers (which may be hijacked
  later for arena types 4,5)

  3 differs from 2 as an optimization; some body types have several
  unused fields in the front of the structure (which are kept in-place
  for consistency).  These bodies can be allocated in smaller chunks,
  because the leading fields arent accessed.  Pointers to such bodies
  are decremented to point at the unused 'ghost' memory, knowing that
  the pointers are used with offsets to the real memory.


=head1 SV-Body Allocation

Allocation of SV-bodies is similar to SV-heads, differing as follows;
the allocation mechanism is used for many body types, so is somewhat
more complicated, it uses arena-sets, and has no need for still-live
SV detection.

At the outermost level, (new|del)_X*V macros return bodies of the
appropriate type.  These macros call either (new|del)_body_type or
(new|del)_body_allocated macro pairs, depending on specifics of the
type.  Most body types use the former pair, the latter pair is used to
allocate body types with "ghost fields".

"ghost fields" are fields that are unused in certain types, and
consequently don't need to actually exist.  They are declared because
they're part of a "base type", which allows use of functions as
methods.  The simplest examples are AVs and HVs, 2 aggregate types
which don't use the fields which support SCALAR semantics.

For these types, the arenas are carved up into appropriately sized
chunks, we thus avoid wasted memory for those unaccessed members.
When bodies are allocated, we adjust the pointer back in memory by the
size of the part not allocated, so it's as if we allocated the full
structure.  (But things will all go boom if you write to the part that
is "not there", because you'll be overwriting the last members of the
preceding structure in memory.)

We calculate the correction using the STRUCT_OFFSET macro on the first
member present. If the allocated structure is smaller (no initial NV
actually allocated) then the net effect is to subtract the size of the NV
from the pointer, to return a new pointer as if an initial NV were actually
allocated. (We were using structures named *_allocated for this, but
this turned out to be a subtle bug, because a structure without an NV
could have a lower alignment constraint, but the compiler is allowed to
optimised accesses based on the alignment constraint of the actual pointer
to the full structure, for example, using a single 64 bit load instruction
because it "knows" that two adjacent 32 bit members will be 8-byte aligned.)

This is the same trick as was used for NV and IV bodies. Ironically it
doesn't need to be used for NV bodies any more, because NV is now at
the start of the structure. IV bodies don't need it either, because
they are no longer allocated.

In turn, the new_body_* allocators call S_new_body(), which invokes
new_body_inline macro, which takes a lock, and takes a body off the
linked list at PL_body_roots[sv_type], calling Perl_more_bodies() if
necessary to refresh an empty list.  Then the lock is released, and
the body is returned.

Perl_more_bodies allocates a new arena, and carves it up into an array of N
bodies, which it strings into a linked list.  It looks up arena-size
and body-size from the body_details table described below, thus
supporting the multiple body-types.

If PURIFY is defined, or PERL_ARENA_SIZE=0, arenas are not used, and
the (new|del)_X*V macros are mapped directly to malloc/free.

For each sv-type, struct body_details bodies_by_type[] carries
parameters which control these aspects of SV handling:

Arena_size determines whether arenas are used for this body type, and if
so, how big they are.  PURIFY or PERL_ARENA_SIZE=0 set this field to
zero, forcing individual mallocs and frees.

Body_size determines how big a body is, and therefore how many fit into
each arena.  Offset carries the body-pointer adjustment needed for
"ghost fields", and is used in *_allocated macros.

But its main purpose is to parameterize info needed in
Perl_sv_upgrade().  The info here dramatically simplifies the function
vs the implementation in 5.8.8, making it table-driven.  All fields
are used for this, except for arena_size.

For the sv-types that have no bodies, arenas are not used, so those
PL_body_roots[sv_type] are unused, and can be overloaded.  In
something of a special case, SVt_NULL is borrowed for HE arenas;
PL_body_roots[HE_SVSLOT=SVt_NULL] is filled by S_more_he, but the
bodies_by_type[SVt_NULL] slot is not used, as the table is not
available in hv.c.

*/

struct body_details {
    U8 body_size;       /* Size to allocate  */
    U8 copy;            /* Size of structure to copy (may be shorter)  */
    U8 offset;
    unsigned int type : 4;          /* We have space for a sanity check.  */
    unsigned int cant_upgrade : 1;  /* Cannot upgrade this type */
    unsigned int zero_nv : 1;       /* zero the NV when upgrading from this */
    unsigned int arena : 1;         /* Allocated from an arena */
    size_t arena_size;              /* Size of arena to allocate */
};

#define HADNV FALSE
#define NONV TRUE


#ifdef PURIFY
/* With -DPURFIY we allocate everything directly, and don't use arenas.
   This seems a rather elegant way to simplify some of the code below.  */
#define HASARENA FALSE
#else
#define HASARENA TRUE
#endif
#define NOARENA FALSE

/* Size the arenas to exactly fit a given number of bodies.  A count
   of 0 fits the max number bodies into a PERL_ARENA_SIZE.block,
   simplifying the default.  If count > 0, the arena is sized to fit
   only that many bodies, allowing arenas to be used for large, rare
   bodies (XPVFM, XPVIO) without undue waste.  The arena size is
   limited by PERL_ARENA_SIZE, so we can safely oversize the
   declarations.
 */
#define FIT_ARENA0(body_size)                           \
    ((size_t)(PERL_ARENA_SIZE / body_size) * body_size)
#define FIT_ARENAn(count,body_size)                     \
    ( count * body_size <= PERL_ARENA_SIZE)             \
    ? count * body_size                                 \
    : FIT_ARENA0 (body_size)
#define FIT_ARENA(count,body_size)                      \
    count                                               \
    ? FIT_ARENAn (count, body_size)                     \
    : FIT_ARENA0 (body_size)

/* Calculate the length to copy. Specifically work out the length less any
   final padding the compiler needed to add.  See the comment in sv_upgrade
   for why copying the padding proved to be a bug.  */

#define copy_length(type, last_member) \
        STRUCT_OFFSET(type, last_member) \
        + sizeof (((type*)SvANY((const SV *)0))->last_member)

static const struct body_details bodies_by_type[] = {
    /* HEs use this offset for their arena.  */
    { 0, 0, 0, SVt_NULL, FALSE, NONV, NOARENA, 0 },

    /* The bind placeholder pretends to be an RV for now.
       Also it's marked as "can't upgrade" to stop anyone using it before it's
       implemented.  */
    { 0, 0, 0, SVt_BIND, TRUE, NONV, NOARENA, 0 },

    /* IVs are in the head, so the allocation size is 0.  */
    { 0,
      sizeof(IV), /* This is used to copy out the IV body.  */
      STRUCT_OFFSET(XPVIV, xiv_iv), SVt_IV, FALSE, NONV,
      NOARENA /* IVS don't need an arena  */, 0
    },

    { sizeof(NV), sizeof(NV),
      STRUCT_OFFSET(XPVNV, xnv_u),
      SVt_NV, FALSE, HADNV, HASARENA, FIT_ARENA(0, sizeof(NV)) },

    { sizeof(XPV) - STRUCT_OFFSET(XPV, xpv_cur),
      copy_length(XPV, xpv_len) - STRUCT_OFFSET(XPV, xpv_cur),
      + STRUCT_OFFSET(XPV, xpv_cur),
      SVt_PV, FALSE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(XPV) - STRUCT_OFFSET(XPV, xpv_cur)) },

    { sizeof(XPVIV) - STRUCT_OFFSET(XPV, xpv_cur),
      copy_length(XPVIV, xiv_u) - STRUCT_OFFSET(XPV, xpv_cur),
      + STRUCT_OFFSET(XPV, xpv_cur),
      SVt_PVIV, FALSE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(XPVIV) - STRUCT_OFFSET(XPV, xpv_cur)) },

    { sizeof(XPVNV) - STRUCT_OFFSET(XPV, xpv_cur),
      copy_length(XPVNV, xnv_u) - STRUCT_OFFSET(XPV, xpv_cur),
      + STRUCT_OFFSET(XPV, xpv_cur),
      SVt_PVNV, FALSE, HADNV, HASARENA,
      FIT_ARENA(0, sizeof(XPVNV) - STRUCT_OFFSET(XPV, xpv_cur)) },

    { sizeof(XPVMG), copy_length(XPVMG, xnv_u), 0, SVt_PVMG, FALSE, HADNV,
      HASARENA, FIT_ARENA(0, sizeof(XPVMG)) },

    { sizeof(regexp),
      sizeof(regexp),
      0,
      SVt_REGEXP, FALSE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(regexp))
    },

    { sizeof(XPVGV), sizeof(XPVGV), 0, SVt_PVGV, TRUE, HADNV,
      HASARENA, FIT_ARENA(0, sizeof(XPVGV)) },
    
    { sizeof(XPVLV), sizeof(XPVLV), 0, SVt_PVLV, TRUE, HADNV,
      HASARENA, FIT_ARENA(0, sizeof(XPVLV)) },

    { sizeof(XPVAV),
      copy_length(XPVAV, xav_alloc),
      0,
      SVt_PVAV, TRUE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(XPVAV)) },

    { sizeof(XPVHV),
      copy_length(XPVHV, xhv_max),
      0,
      SVt_PVHV, TRUE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(XPVHV)) },

    { sizeof(XPVCV),
      sizeof(XPVCV),
      0,
      SVt_PVCV, TRUE, NONV, HASARENA,
      FIT_ARENA(0, sizeof(XPVCV)) },

    { sizeof(XPVFM),
      sizeof(XPVFM),
      0,
      SVt_PVFM, TRUE, NONV, NOARENA,
      FIT_ARENA(20, sizeof(XPVFM)) },

    { sizeof(XPVIO),
      sizeof(XPVIO),
      0,
      SVt_PVIO, TRUE, NONV, HASARENA,
      FIT_ARENA(24, sizeof(XPVIO)) },
};

#define new_body_allocated(sv_type)             \
    (void *)((char *)S_new_body(aTHX_ sv_type)  \
             - bodies_by_type[sv_type].offset)

/* return a thing to the free list */

#define del_body(thing, root)                           \
    STMT_START {                                        \
        void ** const thing_copy = (void **)thing;      \
        *thing_copy = *root;                            \
        *root = (void*)thing_copy;                      \
    } STMT_END

#ifdef PURIFY

#define new_XNV()       safemalloc(sizeof(XPVNV))
#define new_XPVNV()     safemalloc(sizeof(XPVNV))
#define new_XPVMG()     safemalloc(sizeof(XPVMG))

#define del_XPVGV(p)    safefree(p)

#else /* !PURIFY */

#define new_XNV()       new_body_allocated(SVt_NV)
#define new_XPVNV()     new_body_allocated(SVt_PVNV)
#define new_XPVMG()     new_body_allocated(SVt_PVMG)

#define del_XPVGV(p)    del_body(p + bodies_by_type[SVt_PVGV].offset,   \
                                 &PL_body_roots[SVt_PVGV])

#endif /* PURIFY */

/* no arena for you! */

#define new_NOARENA(details) \
        safemalloc((details)->body_size + (details)->offset)
#define new_NOARENAZ(details) \
        safecalloc((details)->body_size + (details)->offset, 1)

void *
Perl_more_bodies (pTHX_ const svtype sv_type, const size_t body_size,
                  const size_t arena_size)
{
    dVAR;
    void ** const root = &PL_body_roots[sv_type];
    struct arena_desc *adesc;
    struct arena_set *aroot = (struct arena_set *) PL_body_arenas;
    unsigned int curr;
    char *start;
    const char *end;
    const size_t good_arena_size = Perl_malloc_good_size(arena_size);
#if defined(DEBUGGING) && !defined(PERL_GLOBAL_STRUCT_PRIVATE)
    static bool done_sanity_check;

    /* PERL_GLOBAL_STRUCT_PRIVATE cannot coexist with global
     * variables like done_sanity_check. */
    if (!done_sanity_check) {
        unsigned int i = SVt_LAST;

        done_sanity_check = TRUE;

        while (i--)
            assert (bodies_by_type[i].type == i);
    }
#endif

    assert(arena_size);

    /* may need new arena-set to hold new arena */
    if (!aroot || aroot->curr >= aroot->set_size) {
        struct arena_set *newroot;
        Newxz(newroot, 1, struct arena_set);
        newroot->set_size = ARENAS_PER_SET;
        newroot->next = aroot;
        aroot = newroot;
        PL_body_arenas = (void *) newroot;
        DEBUG_m(PerlIO_printf(Perl_debug_log, "new arenaset %p\n", (void*)aroot));
    }

    /* ok, now have arena-set with at least 1 empty/available arena-desc */
    curr = aroot->curr++;
    adesc = &(aroot->set[curr]);
    assert(!adesc->arena);
    
    Newx(adesc->arena, good_arena_size, char);
    adesc->size = good_arena_size;
    adesc->utype = sv_type;
    DEBUG_m(PerlIO_printf(Perl_debug_log, "arena %d added: %p size %"UVuf"\n", 
                          curr, (void*)adesc->arena, (UV)good_arena_size));

    start = (char *) adesc->arena;

    /* Get the address of the byte after the end of the last body we can fit.
       Remember, this is integer division:  */
    end = start + good_arena_size / body_size * body_size;

    /* computed count doesn't reflect the 1st slot reservation */
#if defined(MYMALLOC) || defined(HAS_MALLOC_GOOD_SIZE)
    DEBUG_m(PerlIO_printf(Perl_debug_log,
                          "arena %p end %p arena-size %d (from %d) type %d "
                          "size %d ct %d\n",
                          (void*)start, (void*)end, (int)good_arena_size,
                          (int)arena_size, sv_type, (int)body_size,
                          (int)good_arena_size / (int)body_size));
#else
    DEBUG_m(PerlIO_printf(Perl_debug_log,
                          "arena %p end %p arena-size %d type %d size %d ct %d\n",
                          (void*)start, (void*)end,
                          (int)arena_size, sv_type, (int)body_size,
                          (int)good_arena_size / (int)body_size));
#endif
    *root = (void *)start;

    while (1) {
        /* Where the next body would start:  */
        char * const next = start + body_size;

        if (next >= end) {
            /* This is the last body:  */
            assert(next == end);

            *(void **)start = 0;
            return *root;
        }

        *(void**) start = (void *)next;
        start = next;
    }
}

/* grab a new thing from the free list, allocating more if necessary.
   The inline version is used for speed in hot routines, and the
   function using it serves the rest (unless PURIFY).
*/
#define new_body_inline(xpv, sv_type) \
    STMT_START { \
        void ** const r3wt = &PL_body_roots[sv_type]; \
        xpv = (PTR_TBL_ENT_t*) (*((void **)(r3wt))      \
          ? *((void **)(r3wt)) : Perl_more_bodies(aTHX_ sv_type, \
                                             bodies_by_type[sv_type].body_size,\
                                             bodies_by_type[sv_type].arena_size)); \
        *(r3wt) = *(void**)(xpv); \
    } STMT_END

#ifndef PURIFY

STATIC void *
S_new_body(pTHX_ const svtype sv_type)
{
    dVAR;
    void *xpv;
    new_body_inline(xpv, sv_type);
    return xpv;
}

#endif

static const struct body_details fake_rv =
    { 0, 0, 0, SVt_IV, FALSE, NONV, NOARENA, 0 };

/*
=for apidoc sv_upgrade

Upgrade an SV to a more complex form.  Generally adds a new body type to the
SV, then copies across as much information as possible from the old body.
It croaks if the SV is already in a more complex form than requested.  You
generally want to use the C<SvUPGRADE> macro wrapper, which checks the type
before calling C<sv_upgrade>, and hence does not croak.  See also
C<svtype>.

=cut
*/

void
Perl_sv_upgrade(pTHX_ register SV *const sv, svtype new_type)
{
    dVAR;
    void*       old_body;
    void*       new_body;
    const svtype old_type = SvTYPE(sv);
    const struct body_details *new_type_details;
    const struct body_details *old_type_details
        = bodies_by_type + old_type;
    SV *referant = NULL;

    PERL_ARGS_ASSERT_SV_UPGRADE;

    if (old_type == new_type)
        return;

    /* This clause was purposefully added ahead of the early return above to
       the shared string hackery for (sort {$a <=> $b} keys %hash), with the
       inference by Nick I-S that it would fix other troublesome cases. See
       changes 7162, 7163 (f130fd4589cf5fbb24149cd4db4137c8326f49c1 and parent)

       Given that shared hash key scalars are no longer PVIV, but PV, there is
       no longer need to unshare so as to free up the IVX slot for its proper
       purpose. So it's safe to move the early return earlier.  */

    if (new_type > SVt_PVMG && SvIsCOW(sv)) {
        sv_force_normal_flags(sv, 0);
    }

    old_body = SvANY(sv);

    /* Copying structures onto other structures that have been neatly zeroed
       has a subtle gotcha. Consider XPVMG

       +------+------+------+------+------+-------+-------+
       |     NV      | CUR  | LEN  |  IV  | MAGIC | STASH |
       +------+------+------+------+------+-------+-------+
       0      4      8     12     16     20      24      28

       where NVs are aligned to 8 bytes, so that sizeof that structure is
       actually 32 bytes long, with 4 bytes of padding at the end:

       +------+------+------+------+------+-------+-------+------+
       |     NV      | CUR  | LEN  |  IV  | MAGIC | STASH | ???  |
       +------+------+------+------+------+-------+-------+------+
       0      4      8     12     16     20      24      28     32

       so what happens if you allocate memory for this structure:

       +------+------+------+------+------+-------+-------+------+------+...
       |     NV      | CUR  | LEN  |  IV  | MAGIC | STASH |  GP  | NAME |
       +------+------+------+------+------+-------+-------+------+------+...
       0      4      8     12     16     20      24      28     32     36

       zero it, then copy sizeof(XPVMG) bytes on top of it? Not quite what you
       expect, because you copy the area marked ??? onto GP. Now, ??? may have
       started out as zero once, but it's quite possible that it isn't. So now,
       rather than a nicely zeroed GP, you have it pointing somewhere random.
       Bugs ensue.

       (In fact, GP ends up pointing at a previous GP structure, because the
       principle cause of the padding in XPVMG getting garbage is a copy of
       sizeof(XPVMG) bytes from a XPVGV structure in sv_unglob. Right now
       this happens to be moot because XPVGV has been re-ordered, with GP
       no longer after STASH)

       So we are careful and work out the size of used parts of all the
       structures.  */

    switch (old_type) {
    case SVt_NULL:
        break;
    case SVt_IV:
        if (SvROK(sv)) {
            referant = SvRV(sv);
            old_type_details = &fake_rv;
            if (new_type == SVt_NV)
                new_type = SVt_PVNV;
        } else {
            if (new_type < SVt_PVIV) {
                new_type = (new_type == SVt_NV)
                    ? SVt_PVNV : SVt_PVIV;
            }
        }
        break;
    case SVt_NV:
        if (new_type < SVt_PVNV) {
            new_type = SVt_PVNV;
        }
        break;
    case SVt_PV:
        assert(new_type > SVt_PV);
        assert(SVt_IV < SVt_PV);
        assert(SVt_NV < SVt_PV);
        break;
    case SVt_PVIV:
        break;
    case SVt_PVNV:
        break;
    case SVt_PVMG:
        /* Because the XPVMG of PL_mess_sv isn't allocated from the arena,
           there's no way that it can be safely upgraded, because perl.c
           expects to Safefree(SvANY(PL_mess_sv))  */
        assert(sv != PL_mess_sv);
        /* This flag bit is used to mean other things in other scalar types.
           Given that it only has meaning inside the pad, it shouldn't be set
           on anything that can get upgraded.  */
        assert(!SvPAD_TYPED(sv));
        break;
    default:
        if (old_type_details->cant_upgrade)
            Perl_croak(aTHX_ "Can't upgrade %s (%" UVuf ") to %" UVuf,
                       sv_reftype(sv, 0), (UV) old_type, (UV) new_type);
    }

    if (old_type > new_type)
        Perl_croak(aTHX_ "sv_upgrade from type %d down to type %d",
                (int)old_type, (int)new_type);

    new_type_details = bodies_by_type + new_type;

    SvFLAGS(sv) &= ~SVTYPEMASK;
    SvFLAGS(sv) |= new_type;

    /* This can't happen, as SVt_NULL is <= all values of new_type, so one of
       the return statements above will have triggered.  */
    assert (new_type != SVt_NULL);
    switch (new_type) {
    case SVt_IV:
        assert(old_type == SVt_NULL);
        SvANY(sv) = (XPVIV*)((char*)&(sv->sv_u.svu_iv) - STRUCT_OFFSET(XPVIV, xiv_iv));
        SvIV_set(sv, 0);
        return;
    case SVt_NV:
        assert(old_type == SVt_NULL);
        SvANY(sv) = new_XNV();
        SvNV_set(sv, 0);
        return;
    case SVt_PVHV:
    case SVt_PVAV:
        assert(new_type_details->body_size);

#ifndef PURIFY  
        assert(new_type_details->arena);
        assert(new_type_details->arena_size);
        /* This points to the start of the allocated area.  */
        new_body_inline(new_body, new_type);
        Zero(new_body, new_type_details->body_size, char);
        new_body = ((char *)new_body) - new_type_details->offset;
#else
        /* We always allocated the full length item with PURIFY. To do this
           we fake things so that arena is false for all 16 types..  */
        new_body = new_NOARENAZ(new_type_details);
#endif
        SvANY(sv) = new_body;
        if (new_type == SVt_PVAV) {
            AvMAX(sv)   = -1;
            AvFILLp(sv) = -1;
            AvREAL_only(sv);
            if (old_type_details->body_size) {
                AvALLOC(sv) = 0;
            } else {
                /* It will have been zeroed when the new body was allocated.
                   Lets not write to it, in case it confuses a write-back
                   cache.  */
            }
        } else {
            assert(!SvOK(sv));
            SvOK_off(sv);
#ifndef NODEFAULT_SHAREKEYS
            HvSHAREKEYS_on(sv);         /* key-sharing on by default */
#endif
            HvMAX(sv) = 7; /* (start with 8 buckets) */
        }

        /* SVt_NULL isn't the only thing upgraded to AV or HV.
           The target created by newSVrv also is, and it can have magic.
           However, it never has SvPVX set.
        */
        if (old_type == SVt_IV) {
            assert(!SvROK(sv));
        } else if (old_type >= SVt_PV) {
            assert(SvPVX_const(sv) == 0);
        }

        if (old_type >= SVt_PVMG) {
            SvMAGIC_set(sv, ((XPVMG*)old_body)->xmg_u.xmg_magic);
            SvSTASH_set(sv, ((XPVMG*)old_body)->xmg_stash);
        } else {
            sv->sv_u.svu_array = NULL; /* or svu_hash  */
        }
        break;

    case SVt_PVIV:
        /* XXX Is this still needed?  Was it ever needed?   Surely as there is
           no route from NV to PVIV, NOK can never be true  */
        assert(!SvNOKp(sv));
        assert(!SvNOK(sv));
    case SVt_PVIO:
    case SVt_PVFM:
    case SVt_PVGV:
    case SVt_PVCV:
    case SVt_PVLV:
    case SVt_REGEXP:
    case SVt_PVMG:
    case SVt_PVNV:
    case SVt_PV:

        assert(new_type_details->body_size);
        /* We always allocated the full length item with PURIFY. To do this
           we fake things so that arena is false for all 16 types..  */
        if(new_type_details->arena) {
            /* This points to the start of the allocated area.  */
            new_body_inline(new_body, new_type);
            Zero(new_body, new_type_details->body_size, char);
            new_body = ((char *)new_body) - new_type_details->offset;
        } else {
            new_body = new_NOARENAZ(new_type_details);
        }
        SvANY(sv) = new_body;

        if (old_type_details->copy) {
            /* There is now the potential for an upgrade from something without
               an offset (PVNV or PVMG) to something with one (PVCV, PVFM)  */
            int offset = old_type_details->offset;
            int length = old_type_details->copy;

            if (new_type_details->offset > old_type_details->offset) {
                const int difference
                    = new_type_details->offset - old_type_details->offset;
                offset += difference;
                length -= difference;
            }
            assert (length >= 0);
                
            Copy((char *)old_body + offset, (char *)new_body + offset, length,
                 char);
        }

#ifndef NV_ZERO_IS_ALLBITS_ZERO
        /* If NV 0.0 is stores as all bits 0 then Zero() already creates a
         * correct 0.0 for us.  Otherwise, if the old body didn't have an
         * NV slot, but the new one does, then we need to initialise the
         * freshly created NV slot with whatever the correct bit pattern is
         * for 0.0  */
        if (old_type_details->zero_nv && !new_type_details->zero_nv
            && !isGV_with_GP(sv))
            SvNV_set(sv, 0);
#endif

        if (new_type == SVt_PVIO) {
            IO * const io = MUTABLE_IO(sv);
            GV *iogv = gv_fetchpvs("IO::File::", GV_ADD, SVt_PVHV);

            SvOBJECT_on(io);
            /* Clear the stashcache because a new IO could overrule a package
               name */
            DEBUG_o(Perl_deb(aTHX_ "sv_upgrade clearing PL_stashcache\n"));
            hv_clear(PL_stashcache);

            SvSTASH_set(io, MUTABLE_HV(SvREFCNT_inc(GvHV(iogv))));
            IoPAGE_LEN(sv) = 60;
        }
        if (new_type == SVt_REGEXP)
            sv->sv_u.svu_rx = (regexp *)new_body;
        else if (old_type < SVt_PV) {
            /* referant will be NULL unless the old type was SVt_IV emulating
               SVt_RV */
            sv->sv_u.svu_rv = referant;
        }
        break;
    default:
        Perl_croak(aTHX_ "panic: sv_upgrade to unknown type %lu",
                   (unsigned long)new_type);
    }

    if (old_type > SVt_IV) {
#ifdef PURIFY
        safefree(old_body);
#else
        /* Note that there is an assumption that all bodies of types that
           can be upgraded came from arenas. Only the more complex non-
           upgradable types are allowed to be directly malloc()ed.  */
        assert(old_type_details->arena);
        del_body((void*)((char*)old_body + old_type_details->offset),
                 &PL_body_roots[old_type]);
#endif
    }
}

/*
=for apidoc sv_backoff

Remove any string offset.  You should normally use the C<SvOOK_off> macro
wrapper instead.

=cut
*/

int
Perl_sv_backoff(pTHX_ register SV *const sv)
{
    STRLEN delta;
    const char * const s = SvPVX_const(sv);

    PERL_ARGS_ASSERT_SV_BACKOFF;
    PERL_UNUSED_CONTEXT;

    assert(SvOOK(sv));
    assert(SvTYPE(sv) != SVt_PVHV);
    assert(SvTYPE(sv) != SVt_PVAV);

    SvOOK_offset(sv, delta);
    
    SvLEN_set(sv, SvLEN(sv) + delta);
    SvPV_set(sv, SvPVX(sv) - delta);
    Move(s, SvPVX(sv), SvCUR(sv)+1, char);
    SvFLAGS(sv) &= ~SVf_OOK;
    return 0;
}

/*
=for apidoc sv_grow

Expands the character buffer in the SV.  If necessary, uses C<sv_unref> and
upgrades the SV to C<SVt_PV>.  Returns a pointer to the character buffer.
Use the C<SvGROW> wrapper instead.

=cut
*/

char *
Perl_sv_grow(pTHX_ register SV *const sv, register STRLEN newlen)
{
    char *s;

    PERL_ARGS_ASSERT_SV_GROW;

    if (PL_madskills && newlen >= 0x100000) {
        PerlIO_printf(Perl_debug_log,
                      "Allocation too large: %"UVxf"\n", (UV)newlen);
    }
#ifdef HAS_64K_LIMIT
    if (newlen >= 0x10000) {
        PerlIO_printf(Perl_debug_log,
                      "Allocation too large: %"UVxf"\n", (UV)newlen);
        my_exit(1);
    }
#endif /* HAS_64K_LIMIT */
    if (SvROK(sv))
        sv_unref(sv);
    if (SvTYPE(sv) < SVt_PV) {
        sv_upgrade(sv, SVt_PV);
        s = SvPVX_mutable(sv);
    }
    else if (SvOOK(sv)) {       /* pv is offset? */
        sv_backoff(sv);
        s = SvPVX_mutable(sv);
        if (newlen > SvLEN(sv))
            newlen += 10 * (newlen - SvCUR(sv)); /* avoid copy each time */
#ifdef HAS_64K_LIMIT
        if (newlen >= 0x10000)
            newlen = 0xFFFF;
#endif
    }
    else
        s = SvPVX_mutable(sv);

    if (newlen > SvLEN(sv)) {           /* need more room? */
        STRLEN minlen = SvCUR(sv);
        minlen += (minlen >> PERL_STRLEN_EXPAND_SHIFT) + 10;
        if (newlen < minlen)
            newlen = minlen;
#ifndef Perl_safesysmalloc_size
        newlen = PERL_STRLEN_ROUNDUP(newlen);
#endif
        if (SvLEN(sv) && s) {
            s = (char*)saferealloc(s, newlen);
        }
        else {
            s = (char*)safemalloc(newlen);
            if (SvPVX_const(sv) && SvCUR(sv)) {
                Move(SvPVX_const(sv), s, (newlen < SvCUR(sv)) ? newlen : SvCUR(sv), char);
            }
        }
        SvPV_set(sv, s);
#ifdef Perl_safesysmalloc_size
        /* Do this here, do it once, do it right, and then we will never get
           called back into sv_grow() unless there really is some growing
           needed.  */
        SvLEN_set(sv, Perl_safesysmalloc_size(s));
#else
        SvLEN_set(sv, newlen);
#endif
    }
    return s;
}

/*
=for apidoc sv_setiv

Copies an integer into the given SV, upgrading first if necessary.
Does not handle 'set' magic.  See also C<sv_setiv_mg>.

=cut
*/

void
Perl_sv_setiv(pTHX_ register SV *const sv, const IV i)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_SETIV;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    switch (SvTYPE(sv)) {
    case SVt_NULL:
    case SVt_NV:
        sv_upgrade(sv, SVt_IV);
        break;
    case SVt_PV:
        sv_upgrade(sv, SVt_PVIV);
        break;

    case SVt_PVGV:
        if (!isGV_with_GP(sv))
            break;
    case SVt_PVAV:
    case SVt_PVHV:
    case SVt_PVCV:
    case SVt_PVFM:
    case SVt_PVIO:
        /* diag_listed_as: Can't coerce %s to %s in %s */
        Perl_croak(aTHX_ "Can't coerce %s to integer in %s", sv_reftype(sv,0),
                   OP_DESC(PL_op));
    default: NOOP;
    }
    (void)SvIOK_only(sv);                       /* validate number */
    SvIV_set(sv, i);
    SvTAINT(sv);
}

/*
=for apidoc sv_setiv_mg

Like C<sv_setiv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setiv_mg(pTHX_ register SV *const sv, const IV i)
{
    PERL_ARGS_ASSERT_SV_SETIV_MG;

    sv_setiv(sv,i);
    SvSETMAGIC(sv);
}

/*
=for apidoc sv_setuv

Copies an unsigned integer into the given SV, upgrading first if necessary.
Does not handle 'set' magic.  See also C<sv_setuv_mg>.

=cut
*/

void
Perl_sv_setuv(pTHX_ register SV *const sv, const UV u)
{
    PERL_ARGS_ASSERT_SV_SETUV;

    /* With the if statement to ensure that integers are stored as IVs whenever
       possible:
       u=1.49  s=0.52  cu=72.49  cs=10.64  scripts=270  tests=20865

       without
       u=1.35  s=0.47  cu=73.45  cs=11.43  scripts=270  tests=20865

       If you wish to remove the following if statement, so that this routine
       (and its callers) always return UVs, please benchmark to see what the
       effect is. Modern CPUs may be different. Or may not :-)
    */
    if (u <= (UV)IV_MAX) {
       sv_setiv(sv, (IV)u);
       return;
    }
    sv_setiv(sv, 0);
    SvIsUV_on(sv);
    SvUV_set(sv, u);
}

/*
=for apidoc sv_setuv_mg

Like C<sv_setuv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setuv_mg(pTHX_ register SV *const sv, const UV u)
{
    PERL_ARGS_ASSERT_SV_SETUV_MG;

    sv_setuv(sv,u);
    SvSETMAGIC(sv);
}

/*
=for apidoc sv_setnv

Copies a double into the given SV, upgrading first if necessary.
Does not handle 'set' magic.  See also C<sv_setnv_mg>.

=cut
*/

void
Perl_sv_setnv(pTHX_ register SV *const sv, const NV num)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_SETNV;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    switch (SvTYPE(sv)) {
    case SVt_NULL:
    case SVt_IV:
        sv_upgrade(sv, SVt_NV);
        break;
    case SVt_PV:
    case SVt_PVIV:
        sv_upgrade(sv, SVt_PVNV);
        break;

    case SVt_PVGV:
        if (!isGV_with_GP(sv))
            break;
    case SVt_PVAV:
    case SVt_PVHV:
    case SVt_PVCV:
    case SVt_PVFM:
    case SVt_PVIO:
        /* diag_listed_as: Can't coerce %s to %s in %s */
        Perl_croak(aTHX_ "Can't coerce %s to number in %s", sv_reftype(sv,0),
                   OP_DESC(PL_op));
    default: NOOP;
    }
    SvNV_set(sv, num);
    (void)SvNOK_only(sv);                       /* validate number */
    SvTAINT(sv);
}

/*
=for apidoc sv_setnv_mg

Like C<sv_setnv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setnv_mg(pTHX_ register SV *const sv, const NV num)
{
    PERL_ARGS_ASSERT_SV_SETNV_MG;

    sv_setnv(sv,num);
    SvSETMAGIC(sv);
}

/* Print an "isn't numeric" warning, using a cleaned-up,
 * printable version of the offending string
 */

STATIC void
S_not_a_number(pTHX_ SV *const sv)
{
     dVAR;
     SV *dsv;
     char tmpbuf[64];
     const char *pv;

     PERL_ARGS_ASSERT_NOT_A_NUMBER;

     if (DO_UTF8(sv)) {
          dsv = newSVpvs_flags("", SVs_TEMP);
          pv = sv_uni_display(dsv, sv, 10, UNI_DISPLAY_ISPRINT);
     } else {
          char *d = tmpbuf;
          const char * const limit = tmpbuf + sizeof(tmpbuf) - 8;
          /* each *s can expand to 4 chars + "...\0",
             i.e. need room for 8 chars */
        
          const char *s = SvPVX_const(sv);
          const char * const end = s + SvCUR(sv);
          for ( ; s < end && d < limit; s++ ) {
               int ch = *s & 0xFF;
               if (ch & 128 && !isPRINT_LC(ch)) {
                    *d++ = 'M';
                    *d++ = '-';
                    ch &= 127;
               }
               if (ch == '\n') {
                    *d++ = '\\';
                    *d++ = 'n';
               }
               else if (ch == '\r') {
                    *d++ = '\\';
                    *d++ = 'r';
               }
               else if (ch == '\f') {
                    *d++ = '\\';
                    *d++ = 'f';
               }
               else if (ch == '\\') {
                    *d++ = '\\';
                    *d++ = '\\';
               }
               else if (ch == '\0') {
                    *d++ = '\\';
                    *d++ = '0';
               }
               else if (isPRINT_LC(ch))
                    *d++ = ch;
               else {
                    *d++ = '^';
                    *d++ = toCTRL(ch);
               }
          }
          if (s < end) {
               *d++ = '.';
               *d++ = '.';
               *d++ = '.';
          }
          *d = '\0';
          pv = tmpbuf;
    }

    if (PL_op)
        Perl_warner(aTHX_ packWARN(WARN_NUMERIC),
                    /* diag_listed_as: Argument "%s" isn't numeric%s */
                    "Argument \"%s\" isn't numeric in %s", pv,
                    OP_DESC(PL_op));
    else
        Perl_warner(aTHX_ packWARN(WARN_NUMERIC),
                    /* diag_listed_as: Argument "%s" isn't numeric%s */
                    "Argument \"%s\" isn't numeric", pv);
}

/*
=for apidoc looks_like_number

Test if the content of an SV looks like a number (or is a number).
C<Inf> and C<Infinity> are treated as numbers (so will not issue a
non-numeric warning), even if your atof() doesn't grok them.  Get-magic is
ignored.

=cut
*/

I32
Perl_looks_like_number(pTHX_ SV *const sv)
{
    const char *sbegin;
    STRLEN len;

    PERL_ARGS_ASSERT_LOOKS_LIKE_NUMBER;

    if (SvPOK(sv) || SvPOKp(sv)) {
        sbegin = SvPV_nomg_const(sv, len);
    }
    else
        return SvFLAGS(sv) & (SVf_NOK|SVp_NOK|SVf_IOK|SVp_IOK);
    return grok_number(sbegin, len, NULL);
}

STATIC bool
S_glob_2number(pTHX_ GV * const gv)
{
    PERL_ARGS_ASSERT_GLOB_2NUMBER;

    /* We know that all GVs stringify to something that is not-a-number,
        so no need to test that.  */
    if (ckWARN(WARN_NUMERIC))
    {
        SV *const buffer = sv_newmortal();
        gv_efullname3(buffer, gv, "*");
        not_a_number(buffer);
    }
    /* We just want something true to return, so that S_sv_2iuv_common
        can tail call us and return true.  */
    return TRUE;
}

/* Actually, ISO C leaves conversion of UV to IV undefined, but
   until proven guilty, assume that things are not that bad... */

/*
   NV_PRESERVES_UV:

   As 64 bit platforms often have an NV that doesn't preserve all bits of
   an IV (an assumption perl has been based on to date) it becomes necessary
   to remove the assumption that the NV always carries enough precision to
   recreate the IV whenever needed, and that the NV is the canonical form.
   Instead, IV/UV and NV need to be given equal rights. So as to not lose
   precision as a side effect of conversion (which would lead to insanity
   and the dragon(s) in t/op/numconvert.t getting very angry) the intent is
   1) to distinguish between IV/UV/NV slots that have cached a valid
      conversion where precision was lost and IV/UV/NV slots that have a
      valid conversion which has lost no precision
   2) to ensure that if a numeric conversion to one form is requested that
      would lose precision, the precise conversion (or differently
      imprecise conversion) is also performed and cached, to prevent
      requests for different numeric formats on the same SV causing
      lossy conversion chains. (lossless conversion chains are perfectly
      acceptable (still))


   flags are used:
   SvIOKp is true if the IV slot contains a valid value
   SvIOK  is true only if the IV value is accurate (UV if SvIOK_UV true)
   SvNOKp is true if the NV slot contains a valid value
   SvNOK  is true only if the NV value is accurate

   so
   while converting from PV to NV, check to see if converting that NV to an
   IV(or UV) would lose accuracy over a direct conversion from PV to
   IV(or UV). If it would, cache both conversions, return NV, but mark
   SV as IOK NOKp (ie not NOK).

   While converting from PV to IV, check to see if converting that IV to an
   NV would lose accuracy over a direct conversion from PV to NV. If it
   would, cache both conversions, flag similarly.

   Before, the SV value "3.2" could become NV=3.2 IV=3 NOK, IOK quite
   correctly because if IV & NV were set NV *always* overruled.
   Now, "3.2" will become NV=3.2 IV=3 NOK, IOKp, because the flag's meaning
   changes - now IV and NV together means that the two are interchangeable:
   SvIVX == (IV) SvNVX && SvNVX == (NV) SvIVX;

   The benefit of this is that operations such as pp_add know that if
   SvIOK is true for both left and right operands, then integer addition
   can be used instead of floating point (for cases where the result won't
   overflow). Before, floating point was always used, which could lead to
   loss of precision compared with integer addition.

   * making IV and NV equal status should make maths accurate on 64 bit
     platforms
   * may speed up maths somewhat if pp_add and friends start to use
     integers when possible instead of fp. (Hopefully the overhead in
     looking for SvIOK and checking for overflow will not outweigh the
     fp to integer speedup)
   * will slow down integer operations (callers of SvIV) on "inaccurate"
     values, as the change from SvIOK to SvIOKp will cause a call into
     sv_2iv each time rather than a macro access direct to the IV slot
   * should speed up number->string conversion on integers as IV is
     favoured when IV and NV are equally accurate

   ####################################################################
   You had better be using SvIOK_notUV if you want an IV for arithmetic:
   SvIOK is true if (IV or UV), so you might be getting (IV)SvUV.
   On the other hand, SvUOK is true iff UV.
   ####################################################################

   Your mileage will vary depending your CPU's relative fp to integer
   performance ratio.
*/

#ifndef NV_PRESERVES_UV
#  define IS_NUMBER_UNDERFLOW_IV 1
#  define IS_NUMBER_UNDERFLOW_UV 2
#  define IS_NUMBER_IV_AND_UV    2
#  define IS_NUMBER_OVERFLOW_IV  4
#  define IS_NUMBER_OVERFLOW_UV  5

/* sv_2iuv_non_preserve(): private routine for use by sv_2iv() and sv_2uv() */

/* For sv_2nv these three cases are "SvNOK and don't bother casting"  */
STATIC int
S_sv_2iuv_non_preserve(pTHX_ register SV *const sv
#  ifdef DEBUGGING
                       , I32 numtype
#  endif
                       )
{
    dVAR;

    PERL_ARGS_ASSERT_SV_2IUV_NON_PRESERVE;

    DEBUG_c(PerlIO_printf(Perl_debug_log,"sv_2iuv_non '%s', IV=0x%"UVxf" NV=%"NVgf" inttype=%"UVXf"\n", SvPVX_const(sv), SvIVX(sv), SvNVX(sv), (UV)numtype));
    if (SvNVX(sv) < (NV)IV_MIN) {
        (void)SvIOKp_on(sv);
        (void)SvNOK_on(sv);
        SvIV_set(sv, IV_MIN);
        return IS_NUMBER_UNDERFLOW_IV;
    }
    if (SvNVX(sv) > (NV)UV_MAX) {
        (void)SvIOKp_on(sv);
        (void)SvNOK_on(sv);
        SvIsUV_on(sv);
        SvUV_set(sv, UV_MAX);
        return IS_NUMBER_OVERFLOW_UV;
    }
    (void)SvIOKp_on(sv);
    (void)SvNOK_on(sv);
    /* Can't use strtol etc to convert this string.  (See truth table in
       sv_2iv  */
    if (SvNVX(sv) <= (UV)IV_MAX) {
        SvIV_set(sv, I_V(SvNVX(sv)));
        if ((NV)(SvIVX(sv)) == SvNVX(sv)) {
            SvIOK_on(sv); /* Integer is precise. NOK, IOK */
        } else {
            /* Integer is imprecise. NOK, IOKp */
        }
        return SvNVX(sv) < 0 ? IS_NUMBER_UNDERFLOW_UV : IS_NUMBER_IV_AND_UV;
    }
    SvIsUV_on(sv);
    SvUV_set(sv, U_V(SvNVX(sv)));
    if ((NV)(SvUVX(sv)) == SvNVX(sv)) {
        if (SvUVX(sv) == UV_MAX) {
            /* As we know that NVs don't preserve UVs, UV_MAX cannot
               possibly be preserved by NV. Hence, it must be overflow.
               NOK, IOKp */
            return IS_NUMBER_OVERFLOW_UV;
        }
        SvIOK_on(sv); /* Integer is precise. NOK, UOK */
    } else {
        /* Integer is imprecise. NOK, IOKp */
    }
    return IS_NUMBER_OVERFLOW_IV;
}
#endif /* !NV_PRESERVES_UV*/

STATIC bool
S_sv_2iuv_common(pTHX_ SV *const sv)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_2IUV_COMMON;

    if (SvNOKp(sv)) {
        /* erm. not sure. *should* never get NOKp (without NOK) from sv_2nv
         * without also getting a cached IV/UV from it at the same time
         * (ie PV->NV conversion should detect loss of accuracy and cache
         * IV or UV at same time to avoid this. */
        /* IV-over-UV optimisation - choose to cache IV if possible */

        if (SvTYPE(sv) == SVt_NV)
            sv_upgrade(sv, SVt_PVNV);

        (void)SvIOKp_on(sv);    /* Must do this first, to clear any SvOOK */
        /* < not <= as for NV doesn't preserve UV, ((NV)IV_MAX+1) will almost
           certainly cast into the IV range at IV_MAX, whereas the correct
           answer is the UV IV_MAX +1. Hence < ensures that dodgy boundary
           cases go to UV */
#if defined(NAN_COMPARE_BROKEN) && defined(Perl_isnan)
        if (Perl_isnan(SvNVX(sv))) {
            SvUV_set(sv, 0);
            SvIsUV_on(sv);
            return FALSE;
        }
#endif
        if (SvNVX(sv) < (NV)IV_MAX + 0.5) {
            SvIV_set(sv, I_V(SvNVX(sv)));
            if (SvNVX(sv) == (NV) SvIVX(sv)
#ifndef NV_PRESERVES_UV
                && (((UV)1 << NV_PRESERVES_UV_BITS) >
                    (UV)(SvIVX(sv) > 0 ? SvIVX(sv) : -SvIVX(sv)))
                /* Don't flag it as "accurately an integer" if the number
                   came from a (by definition imprecise) NV operation, and
                   we're outside the range of NV integer precision */
#endif
                ) {
                if (SvNOK(sv))
                    SvIOK_on(sv);  /* Can this go wrong with rounding? NWC */
                else {
                    /* scalar has trailing garbage, eg "42a" */
                }
                DEBUG_c(PerlIO_printf(Perl_debug_log,
                                      "0x%"UVxf" iv(%"NVgf" => %"IVdf") (precise)\n",
                                      PTR2UV(sv),
                                      SvNVX(sv),
                                      SvIVX(sv)));

            } else {
                /* IV not precise.  No need to convert from PV, as NV
                   conversion would already have cached IV if it detected
                   that PV->IV would be better than PV->NV->IV
                   flags already correct - don't set public IOK.  */
                DEBUG_c(PerlIO_printf(Perl_debug_log,
                                      "0x%"UVxf" iv(%"NVgf" => %"IVdf") (imprecise)\n",
                                      PTR2UV(sv),
                                      SvNVX(sv),
                                      SvIVX(sv)));
            }
            /* Can the above go wrong if SvIVX == IV_MIN and SvNVX < IV_MIN,
               but the cast (NV)IV_MIN rounds to a the value less (more
               negative) than IV_MIN which happens to be equal to SvNVX ??
               Analogous to 0xFFFFFFFFFFFFFFFF rounding up to NV (2**64) and
               NV rounding back to 0xFFFFFFFFFFFFFFFF, so UVX == UV(NVX) and
               (NV)UVX == NVX are both true, but the values differ. :-(
               Hopefully for 2s complement IV_MIN is something like
               0x8000000000000000 which will be exact. NWC */
        }
        else {
            SvUV_set(sv, U_V(SvNVX(sv)));
            if (
                (SvNVX(sv) == (NV) SvUVX(sv))
#ifndef  NV_PRESERVES_UV
                /* Make sure it's not 0xFFFFFFFFFFFFFFFF */
                /*&& (SvUVX(sv) != UV_MAX) irrelevant with code below */
                && (((UV)1 << NV_PRESERVES_UV_BITS) > SvUVX(sv))
                /* Don't flag it as "accurately an integer" if the number
                   came from a (by definition imprecise) NV operation, and
                   we're outside the range of NV integer precision */
#endif
                && SvNOK(sv)
                )
                SvIOK_on(sv);
            SvIsUV_on(sv);
            DEBUG_c(PerlIO_printf(Perl_debug_log,
                                  "0x%"UVxf" 2iv(%"UVuf" => %"IVdf") (as unsigned)\n",
                                  PTR2UV(sv),
                                  SvUVX(sv),
                                  SvUVX(sv)));
        }
    }
    else if (SvPOKp(sv)) {
        UV value;
        const int numtype = grok_number(SvPVX_const(sv), SvCUR(sv), &value);
        /* We want to avoid a possible problem when we cache an IV/ a UV which
           may be later translated to an NV, and the resulting NV is not
           the same as the direct translation of the initial string
           (eg 123.456 can shortcut to the IV 123 with atol(), but we must
           be careful to ensure that the value with the .456 is around if the
           NV value is requested in the future).
        
           This means that if we cache such an IV/a UV, we need to cache the
           NV as well.  Moreover, we trade speed for space, and do not
           cache the NV if we are sure it's not needed.
         */

        /* SVt_PVNV is one higher than SVt_PVIV, hence this order  */
        if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
             == IS_NUMBER_IN_UV) {
            /* It's definitely an integer, only upgrade to PVIV */
            if (SvTYPE(sv) < SVt_PVIV)
                sv_upgrade(sv, SVt_PVIV);
            (void)SvIOK_on(sv);
        } else if (SvTYPE(sv) < SVt_PVNV)
            sv_upgrade(sv, SVt_PVNV);

        /* If NVs preserve UVs then we only use the UV value if we know that
           we aren't going to call atof() below. If NVs don't preserve UVs
           then the value returned may have more precision than atof() will
           return, even though value isn't perfectly accurate.  */
        if ((numtype & (IS_NUMBER_IN_UV
#ifdef NV_PRESERVES_UV
                        | IS_NUMBER_NOT_INT
#endif
            )) == IS_NUMBER_IN_UV) {
            /* This won't turn off the public IOK flag if it was set above  */
            (void)SvIOKp_on(sv);

            if (!(numtype & IS_NUMBER_NEG)) {
                /* positive */;
                if (value <= (UV)IV_MAX) {
                    SvIV_set(sv, (IV)value);
                } else {
                    /* it didn't overflow, and it was positive. */
                    SvUV_set(sv, value);
                    SvIsUV_on(sv);
                }
            } else {
                /* 2s complement assumption  */
                if (value <= (UV)IV_MIN) {
                    SvIV_set(sv, -(IV)value);
                } else {
                    /* Too negative for an IV.  This is a double upgrade, but
                       I'm assuming it will be rare.  */
                    if (SvTYPE(sv) < SVt_PVNV)
                        sv_upgrade(sv, SVt_PVNV);
                    SvNOK_on(sv);
                    SvIOK_off(sv);
                    SvIOKp_on(sv);
                    SvNV_set(sv, -(NV)value);
                    SvIV_set(sv, IV_MIN);
                }
            }
        }
        /* For !NV_PRESERVES_UV and IS_NUMBER_IN_UV and IS_NUMBER_NOT_INT we
           will be in the previous block to set the IV slot, and the next
           block to set the NV slot.  So no else here.  */
        
        if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
            != IS_NUMBER_IN_UV) {
            /* It wasn't an (integer that doesn't overflow the UV). */
            SvNV_set(sv, Atof(SvPVX_const(sv)));

            if (! numtype && ckWARN(WARN_NUMERIC))
                not_a_number(sv);

#if defined(USE_LONG_DOUBLE)
            DEBUG_c(PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2iv(%" PERL_PRIgldbl ")\n",
                                  PTR2UV(sv), SvNVX(sv)));
#else
            DEBUG_c(PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2iv(%"NVgf")\n",
                                  PTR2UV(sv), SvNVX(sv)));
#endif

#ifdef NV_PRESERVES_UV
            (void)SvIOKp_on(sv);
            (void)SvNOK_on(sv);
            if (SvNVX(sv) < (NV)IV_MAX + 0.5) {
                SvIV_set(sv, I_V(SvNVX(sv)));
                if ((NV)(SvIVX(sv)) == SvNVX(sv)) {
                    SvIOK_on(sv);
                } else {
                    NOOP;  /* Integer is imprecise. NOK, IOKp */
                }
                /* UV will not work better than IV */
            } else {
                if (SvNVX(sv) > (NV)UV_MAX) {
                    SvIsUV_on(sv);
                    /* Integer is inaccurate. NOK, IOKp, is UV */
                    SvUV_set(sv, UV_MAX);
                } else {
                    SvUV_set(sv, U_V(SvNVX(sv)));
                    /* 0xFFFFFFFFFFFFFFFF not an issue in here, NVs
                       NV preservse UV so can do correct comparison.  */
                    if ((NV)(SvUVX(sv)) == SvNVX(sv)) {
                        SvIOK_on(sv);
                    } else {
                        NOOP;   /* Integer is imprecise. NOK, IOKp, is UV */
                    }
                }
                SvIsUV_on(sv);
            }
#else /* NV_PRESERVES_UV */
            if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
                == (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT)) {
                /* The IV/UV slot will have been set from value returned by
                   grok_number above.  The NV slot has just been set using
                   Atof.  */
                SvNOK_on(sv);
                assert (SvIOKp(sv));
            } else {
                if (((UV)1 << NV_PRESERVES_UV_BITS) >
                    U_V(SvNVX(sv) > 0 ? SvNVX(sv) : -SvNVX(sv))) {
                    /* Small enough to preserve all bits. */
                    (void)SvIOKp_on(sv);
                    SvNOK_on(sv);
                    SvIV_set(sv, I_V(SvNVX(sv)));
                    if ((NV)(SvIVX(sv)) == SvNVX(sv))
                        SvIOK_on(sv);
                    /* Assumption: first non-preserved integer is < IV_MAX,
                       this NV is in the preserved range, therefore: */
                    if (!(U_V(SvNVX(sv) > 0 ? SvNVX(sv) : -SvNVX(sv))
                          < (UV)IV_MAX)) {
                        Perl_croak(aTHX_ "sv_2iv assumed (U_V(fabs((double)SvNVX(sv))) < (UV)IV_MAX) but SvNVX(sv)=%"NVgf" U_V is 0x%"UVxf", IV_MAX is 0x%"UVxf"\n", SvNVX(sv), U_V(SvNVX(sv)), (UV)IV_MAX);
                    }
                } else {
                    /* IN_UV NOT_INT
                         0      0       already failed to read UV.
                         0      1       already failed to read UV.
                         1      0       you won't get here in this case. IV/UV
                                        slot set, public IOK, Atof() unneeded.
                         1      1       already read UV.
                       so there's no point in sv_2iuv_non_preserve() attempting
                       to use atol, strtol, strtoul etc.  */
#  ifdef DEBUGGING
                    sv_2iuv_non_preserve (sv, numtype);
#  else
                    sv_2iuv_non_preserve (sv);
#  endif
                }
            }
#endif /* NV_PRESERVES_UV */
        /* It might be more code efficient to go through the entire logic above
           and conditionally set with SvIOKp_on() rather than SvIOK(), but it
           gets complex and potentially buggy, so more programmer efficient
           to do it this way, by turning off the public flags:  */
        if (!numtype)
            SvFLAGS(sv) &= ~(SVf_IOK|SVf_NOK);
        }
    }
    else  {
        if (isGV_with_GP(sv))
            return glob_2number(MUTABLE_GV(sv));

        if (!SvPADTMP(sv)) {
            if (!PL_localizing && ckWARN(WARN_UNINITIALIZED))
                report_uninit(sv);
        }
        if (SvTYPE(sv) < SVt_IV)
            /* Typically the caller expects that sv_any is not NULL now.  */
            sv_upgrade(sv, SVt_IV);
        /* Return 0 from the caller.  */
        return TRUE;
    }
    return FALSE;
}

/*
=for apidoc sv_2iv_flags

Return the integer value of an SV, doing any necessary string
conversion.  If flags includes SV_GMAGIC, does an mg_get() first.
Normally used via the C<SvIV(sv)> and C<SvIVx(sv)> macros.

=cut
*/

IV
Perl_sv_2iv_flags(pTHX_ register SV *const sv, const I32 flags)
{
    dVAR;

    if (!sv)
        return 0;

    if (SvGMAGICAL(sv) && (flags & SV_GMAGIC))
        mg_get(sv);

    if (SvROK(sv)) {
        if (SvAMAGIC(sv)) {
            SV * tmpstr;
            if (flags & SV_SKIP_OVERLOAD)
                return 0;
            tmpstr = AMG_CALLunary(sv, numer_amg);
            if (tmpstr && (!SvROK(tmpstr) || (SvRV(tmpstr) != SvRV(sv)))) {
                return SvIV(tmpstr);
            }
        }
        return PTR2IV(SvRV(sv));
    }

    if (SvVALID(sv) || isREGEXP(sv)) {
        /* FBMs use the space for SvIVX and SvNVX for other purposes, and use
           the same flag bit as SVf_IVisUV, so must not let them cache IVs.
           In practice they are extremely unlikely to actually get anywhere
           accessible by user Perl code - the only way that I'm aware of is when
           a constant subroutine which is used as the second argument to index.

           Regexps have no SvIVX and SvNVX fields.
        */
        assert(isREGEXP(sv) || SvPOKp(sv));
        {
            UV value;
            const char * const ptr =
                isREGEXP(sv) ? RX_WRAPPED((REGEXP*)sv) : SvPVX_const(sv);
            const int numtype
                = grok_number(ptr, SvCUR(sv), &value);

            if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
                == IS_NUMBER_IN_UV) {
                /* It's definitely an integer */
                if (numtype & IS_NUMBER_NEG) {
                    if (value < (UV)IV_MIN)
                        return -(IV)value;
                } else {
                    if (value < (UV)IV_MAX)
                        return (IV)value;
                }
            }
            if (!numtype) {
                if (ckWARN(WARN_NUMERIC))
                    not_a_number(sv);
            }
            return I_V(Atof(ptr));
        }
    }

    if (SvTHINKFIRST(sv)) {
#ifdef PERL_OLD_COPY_ON_WRITE
        if (SvIsCOW(sv)) {
            sv_force_normal_flags(sv, 0);
        }
#endif
        if (SvREADONLY(sv) && !SvOK(sv)) {
            if (ckWARN(WARN_UNINITIALIZED))
                report_uninit(sv);
            return 0;
        }
    }

    if (!SvIOKp(sv)) {
        if (S_sv_2iuv_common(aTHX_ sv))
            return 0;
    }

    DEBUG_c(PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2iv(%"IVdf")\n",
        PTR2UV(sv),SvIVX(sv)));
    return SvIsUV(sv) ? (IV)SvUVX(sv) : SvIVX(sv);
}

/*
=for apidoc sv_2uv_flags

Return the unsigned integer value of an SV, doing any necessary string
conversion.  If flags includes SV_GMAGIC, does an mg_get() first.
Normally used via the C<SvUV(sv)> and C<SvUVx(sv)> macros.

=cut
*/

UV
Perl_sv_2uv_flags(pTHX_ register SV *const sv, const I32 flags)
{
    dVAR;

    if (!sv)
        return 0;

    if (SvGMAGICAL(sv) && (flags & SV_GMAGIC))
        mg_get(sv);

    if (SvROK(sv)) {
        if (SvAMAGIC(sv)) {
            SV *tmpstr;
            if (flags & SV_SKIP_OVERLOAD)
                return 0;
            tmpstr = AMG_CALLunary(sv, numer_amg);
            if (tmpstr && (!SvROK(tmpstr) || (SvRV(tmpstr) != SvRV(sv)))) {
                return SvUV(tmpstr);
            }
        }
        return PTR2UV(SvRV(sv));
    }

    if (SvVALID(sv) || isREGEXP(sv)) {
        /* FBMs use the space for SvIVX and SvNVX for other purposes, and use
           the same flag bit as SVf_IVisUV, so must not let them cache IVs.  
           Regexps have no SvIVX and SvNVX fields. */
        assert(isREGEXP(sv) || SvPOKp(sv));
        {
            UV value;
            const char * const ptr =
                isREGEXP(sv) ? RX_WRAPPED((REGEXP*)sv) : SvPVX_const(sv);
            const int numtype
                = grok_number(ptr, SvCUR(sv), &value);

            if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
                == IS_NUMBER_IN_UV) {
                /* It's definitely an integer */
                if (!(numtype & IS_NUMBER_NEG))
                    return value;
            }
            if (!numtype) {
                if (ckWARN(WARN_NUMERIC))
                    not_a_number(sv);
            }
            return U_V(Atof(ptr));
        }
    }

    if (SvTHINKFIRST(sv)) {
#ifdef PERL_OLD_COPY_ON_WRITE
        if (SvIsCOW(sv)) {
            sv_force_normal_flags(sv, 0);
        }
#endif
        if (SvREADONLY(sv) && !SvOK(sv)) {
            if (ckWARN(WARN_UNINITIALIZED))
                report_uninit(sv);
            return 0;
        }
    }

    if (!SvIOKp(sv)) {
        if (S_sv_2iuv_common(aTHX_ sv))
            return 0;
    }

    DEBUG_c(PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2uv(%"UVuf")\n",
                          PTR2UV(sv),SvUVX(sv)));
    return SvIsUV(sv) ? SvUVX(sv) : (UV)SvIVX(sv);
}

/*
=for apidoc sv_2nv_flags

Return the num value of an SV, doing any necessary string or integer
conversion.  If flags includes SV_GMAGIC, does an mg_get() first.
Normally used via the C<SvNV(sv)> and C<SvNVx(sv)> macros.

=cut
*/

NV
Perl_sv_2nv_flags(pTHX_ register SV *const sv, const I32 flags)
{
    dVAR;
    if (!sv)
        return 0.0;
    if (SvGMAGICAL(sv) || SvVALID(sv) || isREGEXP(sv)) {
        /* FBMs use the space for SvIVX and SvNVX for other purposes, and use
           the same flag bit as SVf_IVisUV, so must not let them cache NVs.
           Regexps have no SvIVX and SvNVX fields.  */
        const char *ptr;
        if (flags & SV_GMAGIC)
            mg_get(sv);
        if (SvNOKp(sv))
            return SvNVX(sv);
        if (SvPOKp(sv) && !SvIOKp(sv)) {
            ptr = SvPVX_const(sv);
          grokpv:
            if (!SvIOKp(sv) && ckWARN(WARN_NUMERIC) &&
                !grok_number(ptr, SvCUR(sv), NULL))
                not_a_number(sv);
            return Atof(ptr);
        }
        if (SvIOKp(sv)) {
            if (SvIsUV(sv))
                return (NV)SvUVX(sv);
            else
                return (NV)SvIVX(sv);
        }
        if (SvROK(sv)) {
            goto return_rok;
        }
        if (isREGEXP(sv)) {
            ptr = RX_WRAPPED((REGEXP *)sv);
            goto grokpv;
        }
        assert(SvTYPE(sv) >= SVt_PVMG);
        /* This falls through to the report_uninit near the end of the
           function. */
    } else if (SvTHINKFIRST(sv)) {
        if (SvROK(sv)) {
        return_rok:
            if (SvAMAGIC(sv)) {
                SV *tmpstr;
                if (flags & SV_SKIP_OVERLOAD)
                    return 0;
                tmpstr = AMG_CALLunary(sv, numer_amg);
                if (tmpstr && (!SvROK(tmpstr) || (SvRV(tmpstr) != SvRV(sv)))) {
                    return SvNV(tmpstr);
                }
            }
            return PTR2NV(SvRV(sv));
        }
#ifdef PERL_OLD_COPY_ON_WRITE
        if (SvIsCOW(sv)) {
            sv_force_normal_flags(sv, 0);
        }
#endif
        if (SvREADONLY(sv) && !SvOK(sv)) {
            if (ckWARN(WARN_UNINITIALIZED))
                report_uninit(sv);
            return 0.0;
        }
    }
    if (SvTYPE(sv) < SVt_NV) {
        /* The logic to use SVt_PVNV if necessary is in sv_upgrade.  */
        sv_upgrade(sv, SVt_NV);
#ifdef USE_LONG_DOUBLE
        DEBUG_c({
            STORE_NUMERIC_LOCAL_SET_STANDARD();
            PerlIO_printf(Perl_debug_log,
                          "0x%"UVxf" num(%" PERL_PRIgldbl ")\n",
                          PTR2UV(sv), SvNVX(sv));
            RESTORE_NUMERIC_LOCAL();
        });
#else
        DEBUG_c({
            STORE_NUMERIC_LOCAL_SET_STANDARD();
            PerlIO_printf(Perl_debug_log, "0x%"UVxf" num(%"NVgf")\n",
                          PTR2UV(sv), SvNVX(sv));
            RESTORE_NUMERIC_LOCAL();
        });
#endif
    }
    else if (SvTYPE(sv) < SVt_PVNV)
        sv_upgrade(sv, SVt_PVNV);
    if (SvNOKp(sv)) {
        return SvNVX(sv);
    }
    if (SvIOKp(sv)) {
        SvNV_set(sv, SvIsUV(sv) ? (NV)SvUVX(sv) : (NV)SvIVX(sv));
#ifdef NV_PRESERVES_UV
        if (SvIOK(sv))
            SvNOK_on(sv);
        else
            SvNOKp_on(sv);
#else
        /* Only set the public NV OK flag if this NV preserves the IV  */
        /* Check it's not 0xFFFFFFFFFFFFFFFF */
        if (SvIOK(sv) &&
            SvIsUV(sv) ? ((SvUVX(sv) != UV_MAX)&&(SvUVX(sv) == U_V(SvNVX(sv))))
                       : (SvIVX(sv) == I_V(SvNVX(sv))))
            SvNOK_on(sv);
        else
            SvNOKp_on(sv);
#endif
    }
    else if (SvPOKp(sv)) {
        UV value;
        const int numtype = grok_number(SvPVX_const(sv), SvCUR(sv), &value);
        if (!SvIOKp(sv) && !numtype && ckWARN(WARN_NUMERIC))
            not_a_number(sv);
#ifdef NV_PRESERVES_UV
        if ((numtype & (IS_NUMBER_IN_UV | IS_NUMBER_NOT_INT))
            == IS_NUMBER_IN_UV) {
            /* It's definitely an integer */
            SvNV_set(sv, (numtype & IS_NUMBER_NEG) ? -(NV)value : (NV)value);
        } else
            SvNV_set(sv, Atof(SvPVX_const(sv)));
        if (numtype)
            SvNOK_on(sv);
        else
            SvNOKp_on(sv);
#else
        SvNV_set(sv, Atof(SvPVX_const(sv)));
        /* Only set the public NV OK flag if this NV preserves the value in
           the PV at least as well as an IV/UV would.
           Not sure how to do this 100% reliably. */
        /* if that shift count is out of range then Configure's test is
           wonky. We shouldn't be in here with NV_PRESERVES_UV_BITS ==
           UV_BITS */
        if (((UV)1 << NV_PRESERVES_UV_BITS) >
            U_V(SvNVX(sv) > 0 ? SvNVX(sv) : -SvNVX(sv))) {
            SvNOK_on(sv); /* Definitely small enough to preserve all bits */
        } else if (!(numtype & IS_NUMBER_IN_UV)) {
            /* Can't use strtol etc to convert this string, so don't try.
               sv_2iv and sv_2uv will use the NV to convert, not the PV.  */
            SvNOK_on(sv);
        } else {
            /* value has been set.  It may not be precise.  */
            if ((numtype & IS_NUMBER_NEG) && (value > (UV)IV_MIN)) {
                /* 2s complement assumption for (UV)IV_MIN  */
                SvNOK_on(sv); /* Integer is too negative.  */
            } else {
                SvNOKp_on(sv);
                SvIOKp_on(sv);

                if (numtype & IS_NUMBER_NEG) {
                    SvIV_set(sv, -(IV)value);
                } else if (value <= (UV)IV_MAX) {
                    SvIV_set(sv, (IV)value);
                } else {
                    SvUV_set(sv, value);
                    SvIsUV_on(sv);
                }

                if (numtype & IS_NUMBER_NOT_INT) {
                    /* I believe that even if the original PV had decimals,
                       they are lost beyond the limit of the FP precision.
                       However, neither is canonical, so both only get p
                       flags.  NWC, 2000/11/25 */
                    /* Both already have p flags, so do nothing */
                } else {
                    const NV nv = SvNVX(sv);
                    if (SvNVX(sv) < (NV)IV_MAX + 0.5) {
                        if (SvIVX(sv) == I_V(nv)) {
                            SvNOK_on(sv);
                        } else {
                            /* It had no "." so it must be integer.  */
                        }
                        SvIOK_on(sv);
                    } else {
                        /* between IV_MAX and NV(UV_MAX).
                           Could be slightly > UV_MAX */

                        if (numtype & IS_NUMBER_NOT_INT) {
                            /* UV and NV both imprecise.  */
                        } else {
                            const UV nv_as_uv = U_V(nv);

                            if (value == nv_as_uv && SvUVX(sv) != UV_MAX) {
                                SvNOK_on(sv);
                            }
                            SvIOK_on(sv);
                        }
                    }
                }
            }
        }
        /* It might be more code efficient to go through the entire logic above
           and conditionally set with SvNOKp_on() rather than SvNOK(), but it
           gets complex and potentially buggy, so more programmer efficient
           to do it this way, by turning off the public flags:  */
        if (!numtype)
            SvFLAGS(sv) &= ~(SVf_IOK|SVf_NOK);
#endif /* NV_PRESERVES_UV */
    }
    else  {
        if (isGV_with_GP(sv)) {
            glob_2number(MUTABLE_GV(sv));
            return 0.0;
        }

        if (!PL_localizing && !SvPADTMP(sv) && ckWARN(WARN_UNINITIALIZED))
            report_uninit(sv);
        assert (SvTYPE(sv) >= SVt_NV);
        /* Typically the caller expects that sv_any is not NULL now.  */
        /* XXX Ilya implies that this is a bug in callers that assume this
           and ideally should be fixed.  */
        return 0.0;
    }
#if defined(USE_LONG_DOUBLE)
    DEBUG_c({
        STORE_NUMERIC_LOCAL_SET_STANDARD();
        PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2nv(%" PERL_PRIgldbl ")\n",
                      PTR2UV(sv), SvNVX(sv));
        RESTORE_NUMERIC_LOCAL();
    });
#else
    DEBUG_c({
        STORE_NUMERIC_LOCAL_SET_STANDARD();
        PerlIO_printf(Perl_debug_log, "0x%"UVxf" 1nv(%"NVgf")\n",
                      PTR2UV(sv), SvNVX(sv));
        RESTORE_NUMERIC_LOCAL();
    });
#endif
    return SvNVX(sv);
}

/*
=for apidoc sv_2num

Return an SV with the numeric value of the source SV, doing any necessary
reference or overload conversion.  You must use the C<SvNUM(sv)> macro to
access this function.

=cut
*/

SV *
Perl_sv_2num(pTHX_ register SV *const sv)
{
    PERL_ARGS_ASSERT_SV_2NUM;

    if (!SvROK(sv))
        return sv;
    if (SvAMAGIC(sv)) {
        SV * const tmpsv = AMG_CALLunary(sv, numer_amg);
        TAINT_IF(tmpsv && SvTAINTED(tmpsv));
        if (tmpsv && (!SvROK(tmpsv) || (SvRV(tmpsv) != SvRV(sv))))
            return sv_2num(tmpsv);
    }
    return sv_2mortal(newSVuv(PTR2UV(SvRV(sv))));
}

/* uiv_2buf(): private routine for use by sv_2pv_flags(): print an IV or
 * UV as a string towards the end of buf, and return pointers to start and
 * end of it.
 *
 * We assume that buf is at least TYPE_CHARS(UV) long.
 */

static char *
S_uiv_2buf(char *const buf, const IV iv, UV uv, const int is_uv, char **const peob)
{
    char *ptr = buf + TYPE_CHARS(UV);
    char * const ebuf = ptr;
    int sign;

    PERL_ARGS_ASSERT_UIV_2BUF;

    if (is_uv)
        sign = 0;
    else if (iv >= 0) {
        uv = iv;
        sign = 0;
    } else {
        uv = -iv;
        sign = 1;
    }
    do {
        *--ptr = '0' + (char)(uv % 10);
    } while (uv /= 10);
    if (sign)
        *--ptr = '-';
    *peob = ebuf;
    return ptr;
}

/*
=for apidoc sv_2pv_flags

Returns a pointer to the string value of an SV, and sets *lp to its length.
If flags includes SV_GMAGIC, does an mg_get() first.  Coerces sv to a
string if necessary.  Normally invoked via the C<SvPV_flags> macro.
C<sv_2pv()> and C<sv_2pv_nomg> usually end up here too.

=cut
*/

char *
Perl_sv_2pv_flags(pTHX_ register SV *const sv, STRLEN *const lp, const I32 flags)
{
    dVAR;
    char *s;

    if (!sv) {
        if (lp)
            *lp = 0;
        return (char *)"";
    }
    if (SvGMAGICAL(sv) && (flags & SV_GMAGIC))
        mg_get(sv);
    if (SvROK(sv)) {
        if (SvAMAGIC(sv)) {
            SV *tmpstr;
            if (flags & SV_SKIP_OVERLOAD)
                return NULL;
            tmpstr = AMG_CALLunary(sv, string_amg);
            TAINT_IF(tmpstr && SvTAINTED(tmpstr));
            if (tmpstr && (!SvROK(tmpstr) || (SvRV(tmpstr) != SvRV(sv)))) {
                /* Unwrap this:  */
                /* char *pv = lp ? SvPV(tmpstr, *lp) : SvPV_nolen(tmpstr);
                 */

                char *pv;
                if ((SvFLAGS(tmpstr) & (SVf_POK)) == SVf_POK) {
                    if (flags & SV_CONST_RETURN) {
                        pv = (char *) SvPVX_const(tmpstr);
                    } else {
                        pv = (flags & SV_MUTABLE_RETURN)
                            ? SvPVX_mutable(tmpstr) : SvPVX(tmpstr);
                    }
                    if (lp)
                        *lp = SvCUR(tmpstr);
                } else {
                    pv = sv_2pv_flags(tmpstr, lp, flags);
                }
                if (SvUTF8(tmpstr))
                    SvUTF8_on(sv);
                else
                    SvUTF8_off(sv);
                return pv;
            }
        }
        {
            STRLEN len;
            char *retval;
            char *buffer;
            SV *const referent = SvRV(sv);

            if (!referent) {
                len = 7;
                retval = buffer = savepvn("NULLREF", len);
            } else if (SvTYPE(referent) == SVt_REGEXP &&
                       (!(PL_curcop->cop_hints & HINT_NO_AMAGIC) ||
                        amagic_is_enabled(string_amg))) {
                REGEXP * const re = (REGEXP *)MUTABLE_PTR(referent);

                assert(re);
                        
                /* If the regex is UTF-8 we want the containing scalar to
                   have an UTF-8 flag too */
                if (RX_UTF8(re))
                    SvUTF8_on(sv);
                else
                    SvUTF8_off(sv);     

                if (lp)
                    *lp = RX_WRAPLEN(re);
 
                return RX_WRAPPED(re);
            } else {
                const char *const typestr = sv_reftype(referent, 0);
                const STRLEN typelen = strlen(typestr);
                UV addr = PTR2UV(referent);
                const char *stashname = NULL;
                STRLEN stashnamelen = 0; /* hush, gcc */
                const char *buffer_end;

                if (SvOBJECT(referent)) {
                    const HEK *const name = HvNAME_HEK(SvSTASH(referent));

                    if (name) {
                        stashname = HEK_KEY(name);
                        stashnamelen = HEK_LEN(name);

                        if (HEK_UTF8(name)) {
                            SvUTF8_on(sv);
                        } else {
                            SvUTF8_off(sv);
                        }
                    } else {
                        stashname = "__ANON__";
                        stashnamelen = 8;
                    }
                    len = stashnamelen + 1 /* = */ + typelen + 3 /* (0x */
                        + 2 * sizeof(UV) + 2 /* )\0 */;
                } else {
                    len = typelen + 3 /* (0x */
                        + 2 * sizeof(UV) + 2 /* )\0 */;
                }

                Newx(buffer, len, char);
                buffer_end = retval = buffer + len;

                /* Working backwards  */
                *--retval = '\0';
                *--retval = ')';
                do {
                    *--retval = PL_hexdigit[addr & 15];
                } while (addr >>= 4);
                *--retval = 'x';
                *--retval = '0';
                *--retval = '(';

                retval -= typelen;
                memcpy(retval, typestr, typelen);

                if (stashname) {
                    *--retval = '=';
                    retval -= stashnamelen;
                    memcpy(retval, stashname, stashnamelen);
                }
                /* retval may not necessarily have reached the start of the
                   buffer here.  */
                assert (retval >= buffer);

                len = buffer_end - retval - 1; /* -1 for that \0  */
            }
            if (lp)
                *lp = len;
            SAVEFREEPV(buffer);
            return retval;
        }
    }

    if (SvPOKp(sv)) {
        if (lp)
            *lp = SvCUR(sv);
        if (flags & SV_MUTABLE_RETURN)
            return SvPVX_mutable(sv);
        if (flags & SV_CONST_RETURN)
            return (char *)SvPVX_const(sv);
        return SvPVX(sv);
    }

    if (SvIOK(sv)) {
        /* I'm assuming that if both IV and NV are equally valid then
           converting the IV is going to be more efficient */
        const U32 isUIOK = SvIsUV(sv);
        char buf[TYPE_CHARS(UV)];
        char *ebuf, *ptr;
        STRLEN len;

        if (SvTYPE(sv) < SVt_PVIV)
            sv_upgrade(sv, SVt_PVIV);
        ptr = uiv_2buf(buf, SvIVX(sv), SvUVX(sv), isUIOK, &ebuf);
        len = ebuf - ptr;
        /* inlined from sv_setpvn */
        s = SvGROW_mutable(sv, len + 1);
        Move(ptr, s, len, char);
        s += len;
        *s = '\0';
    }
    else if (SvNOK(sv)) {
        if (SvTYPE(sv) < SVt_PVNV)
            sv_upgrade(sv, SVt_PVNV);
        if (SvNVX(sv) == 0.0) {
            s = SvGROW_mutable(sv, 2);
            *s++ = '0';
            *s = '\0';
        } else {
            dSAVE_ERRNO;
            /* The +20 is pure guesswork.  Configure test needed. --jhi */
            s = SvGROW_mutable(sv, NV_DIG + 20);
            /* some Xenix systems wipe out errno here */
            Gconvert(SvNVX(sv), NV_DIG, 0, s);
            RESTORE_ERRNO;
            while (*s) s++;
        }
#ifdef hcx
        if (s[-1] == '.')
            *--s = '\0';
#endif
    }
    else if (isGV_with_GP(sv)) {
        GV *const gv = MUTABLE_GV(sv);
        SV *const buffer = sv_newmortal();

        gv_efullname3(buffer, gv, "*");

        assert(SvPOK(buffer));
        if (SvUTF8(buffer))
            SvUTF8_on(sv);
        if (lp)
            *lp = SvCUR(buffer);
        return SvPVX(buffer);
    }
    else if (isREGEXP(sv)) {
        if (lp) *lp = RX_WRAPLEN((REGEXP *)sv);
        return RX_WRAPPED((REGEXP *)sv);
    }
    else {
        if (lp)
            *lp = 0;
        if (flags & SV_UNDEF_RETURNS_NULL)
            return NULL;
        if (!PL_localizing && !SvPADTMP(sv) && ckWARN(WARN_UNINITIALIZED))
            report_uninit(sv);
        /* Typically the caller expects that sv_any is not NULL now.  */
        if (!SvREADONLY(sv) && SvTYPE(sv) < SVt_PV)
            sv_upgrade(sv, SVt_PV);
        return (char *)"";
    }

    {
        const STRLEN len = s - SvPVX_const(sv);
        if (lp) 
            *lp = len;
        SvCUR_set(sv, len);
    }
    SvPOK_on(sv);
    DEBUG_c(PerlIO_printf(Perl_debug_log, "0x%"UVxf" 2pv(%s)\n",
                          PTR2UV(sv),SvPVX_const(sv)));
    if (flags & SV_CONST_RETURN)
        return (char *)SvPVX_const(sv);
    if (flags & SV_MUTABLE_RETURN)
        return SvPVX_mutable(sv);
    return SvPVX(sv);
}

/*
=for apidoc sv_copypv

Copies a stringified representation of the source SV into the
destination SV.  Automatically performs any necessary mg_get and
coercion of numeric values into strings.  Guaranteed to preserve
UTF8 flag even from overloaded objects.  Similar in nature to
sv_2pv[_flags] but operates directly on an SV instead of just the
string.  Mostly uses sv_2pv_flags to do its work, except when that
would lose the UTF-8'ness of the PV.

=for apidoc sv_copypv_nomg

Like sv_copypv, but doesn't invoke get magic first.

=for apidoc sv_copypv_flags

Implementation of sv_copypv and sv_copypv_nomg.  Calls get magic iff flags
include SV_GMAGIC.

=cut
*/

void
Perl_sv_copypv(pTHX_ SV *const dsv, register SV *const ssv)
{
    PERL_ARGS_ASSERT_SV_COPYPV;

    sv_copypv_flags(dsv, ssv, 0);
}

void
Perl_sv_copypv_flags(pTHX_ SV *const dsv, register SV *const ssv, const I32 flags)
{
    STRLEN len;
    const char *s;

    PERL_ARGS_ASSERT_SV_COPYPV_FLAGS;

    if ((flags & SV_GMAGIC) && SvGMAGICAL(ssv))
        mg_get(ssv);
    s = SvPV_nomg_const(ssv,len);
    sv_setpvn(dsv,s,len);
    if (SvUTF8(ssv))
        SvUTF8_on(dsv);
    else
        SvUTF8_off(dsv);
}

/*
=for apidoc sv_2pvbyte

Return a pointer to the byte-encoded representation of the SV, and set *lp
to its length.  May cause the SV to be downgraded from UTF-8 as a
side-effect.

Usually accessed via the C<SvPVbyte> macro.

=cut
*/

char *
Perl_sv_2pvbyte(pTHX_ register SV *sv, STRLEN *const lp)
{
    PERL_ARGS_ASSERT_SV_2PVBYTE;

    if (((SvREADONLY(sv) || SvFAKE(sv)) && !SvIsCOW(sv))
     || isGV_with_GP(sv) || SvROK(sv)) {
        SV *sv2 = sv_newmortal();
        sv_copypv(sv2,sv);
        sv = sv2;
    }
    else SvGETMAGIC(sv);
    sv_utf8_downgrade(sv,0);
    return lp ? SvPV_nomg(sv,*lp) : SvPV_nomg_nolen(sv);
}

/*
=for apidoc sv_2pvutf8

Return a pointer to the UTF-8-encoded representation of the SV, and set *lp
to its length.  May cause the SV to be upgraded to UTF-8 as a side-effect.

Usually accessed via the C<SvPVutf8> macro.

=cut
*/

char *
Perl_sv_2pvutf8(pTHX_ register SV *sv, STRLEN *const lp)
{
    PERL_ARGS_ASSERT_SV_2PVUTF8;

    if (((SvREADONLY(sv) || SvFAKE(sv)) && !SvIsCOW(sv))
     || isGV_with_GP(sv) || SvROK(sv))
        sv = sv_mortalcopy(sv);
    else
        SvGETMAGIC(sv);
    sv_utf8_upgrade_nomg(sv);
    return lp ? SvPV_nomg(sv,*lp) : SvPV_nomg_nolen(sv);
}


/*
=for apidoc sv_2bool

This macro is only used by sv_true() or its macro equivalent, and only if
the latter's argument is neither SvPOK, SvIOK nor SvNOK.
It calls sv_2bool_flags with the SV_GMAGIC flag.

=for apidoc sv_2bool_flags

This function is only used by sv_true() and friends,  and only if
the latter's argument is neither SvPOK, SvIOK nor SvNOK.  If the flags
contain SV_GMAGIC, then it does an mg_get() first.


=cut
*/

bool
Perl_sv_2bool_flags(pTHX_ register SV *const sv, const I32 flags)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_2BOOL_FLAGS;

    if(flags & SV_GMAGIC) SvGETMAGIC(sv);

    if (!SvOK(sv))
        return 0;
    if (SvROK(sv)) {
        if (SvAMAGIC(sv)) {
            SV * const tmpsv = AMG_CALLunary(sv, bool__amg);
            if (tmpsv && (!SvROK(tmpsv) || (SvRV(tmpsv) != SvRV(sv))))
                return cBOOL(SvTRUE(tmpsv));
        }
        return SvRV(sv) != 0;
    }
    return SvTRUE_common(sv, isGV_with_GP(sv) ? 1 : 0);
}

/*
=for apidoc sv_utf8_upgrade

Converts the PV of an SV to its UTF-8-encoded form.
Forces the SV to string form if it is not already.
Will C<mg_get> on C<sv> if appropriate.
Always sets the SvUTF8 flag to avoid future validity checks even
if the whole string is the same in UTF-8 as not.
Returns the number of bytes in the converted string

This is not a general purpose byte encoding to Unicode interface:
use the Encode extension for that.

=for apidoc sv_utf8_upgrade_nomg

Like sv_utf8_upgrade, but doesn't do magic on C<sv>.

=for apidoc sv_utf8_upgrade_flags

Converts the PV of an SV to its UTF-8-encoded form.
Forces the SV to string form if it is not already.
Always sets the SvUTF8 flag to avoid future validity checks even
if all the bytes are invariant in UTF-8.
If C<flags> has C<SV_GMAGIC> bit set,
will C<mg_get> on C<sv> if appropriate, else not.
Returns the number of bytes in the converted string
C<sv_utf8_upgrade> and
C<sv_utf8_upgrade_nomg> are implemented in terms of this function.

This is not a general purpose byte encoding to Unicode interface:
use the Encode extension for that.

=cut

The grow version is currently not externally documented.  It adds a parameter,
extra, which is the number of unused bytes the string of 'sv' is guaranteed to
have free after it upon return.  This allows the caller to reserve extra space
that it intends to fill, to avoid extra grows.

Also externally undocumented for the moment is the flag SV_FORCE_UTF8_UPGRADE,
which can be used to tell this function to not first check to see if there are
any characters that are different in UTF-8 (variant characters) which would
force it to allocate a new string to sv, but to assume there are.  Typically
this flag is used by a routine that has already parsed the string to find that
there are such characters, and passes this information on so that the work
doesn't have to be repeated.

(One might think that the calling routine could pass in the position of the
first such variant, so it wouldn't have to be found again.  But that is not the
case, because typically when the caller is likely to use this flag, it won't be
calling this routine unless it finds something that won't fit into a byte.
Otherwise it tries to not upgrade and just use bytes.  But some things that
do fit into a byte are variants in utf8, and the caller may not have been
keeping track of these.)

If the routine itself changes the string, it adds a trailing NUL.  Such a NUL
isn't guaranteed due to having other routines do the work in some input cases,
or if the input is already flagged as being in utf8.

The speed of this could perhaps be improved for many cases if someone wanted to
write a fast function that counts the number of variant characters in a string,
especially if it could return the position of the first one.

*/

STRLEN
Perl_sv_utf8_upgrade_flags_grow(pTHX_ register SV *const sv, const I32 flags, STRLEN extra)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_UTF8_UPGRADE_FLAGS_GROW;

    if (sv == &PL_sv_undef)
        return 0;
    if (!SvPOK_nog(sv)) {
        STRLEN len = 0;
        if (SvREADONLY(sv) && (SvPOKp(sv) || SvIOKp(sv) || SvNOKp(sv))) {
            (void) sv_2pv_flags(sv,&len, flags);
            if (SvUTF8(sv)) {
                if (extra) SvGROW(sv, SvCUR(sv) + extra);
                return len;
            }
        } else {
            (void) SvPV_force_flags(sv,len,flags & SV_GMAGIC);
        }
    }

    if (SvUTF8(sv)) {
        if (extra) SvGROW(sv, SvCUR(sv) + extra);
        return SvCUR(sv);
    }

    if (SvIsCOW(sv)) {
        sv_force_normal_flags(sv, 0);
    }

    if (PL_encoding && !(flags & SV_UTF8_NO_ENCODING)) {
        sv_recode_to_utf8(sv, PL_encoding);
        if (extra) SvGROW(sv, SvCUR(sv) + extra);
        return SvCUR(sv);
    }

    if (SvCUR(sv) == 0) {
        if (extra) SvGROW(sv, extra);
    } else { /* Assume Latin-1/EBCDIC */
        /* This function could be much more efficient if we
         * had a FLAG in SVs to signal if there are any variant
         * chars in the PV.  Given that there isn't such a flag
         * make the loop as fast as possible (although there are certainly ways
         * to speed this up, eg. through vectorization) */
        U8 * s = (U8 *) SvPVX_const(sv);
        U8 * e = (U8 *) SvEND(sv);
        U8 *t = s;
        STRLEN two_byte_count = 0;
        
        if (flags & SV_FORCE_UTF8_UPGRADE) goto must_be_utf8;

        /* See if really will need to convert to utf8.  We mustn't rely on our
         * incoming SV being well formed and having a trailing '\0', as certain
         * code in pp_formline can send us partially built SVs. */

        while (t < e) {
            const U8 ch = *t++;
            if (NATIVE_IS_INVARIANT(ch)) continue;

            t--;    /* t already incremented; re-point to first variant */
            two_byte_count = 1;
            goto must_be_utf8;
        }

        /* utf8 conversion not needed because all are invariants.  Mark as
         * UTF-8 even if no variant - saves scanning loop */
        SvUTF8_on(sv);
        if (extra) SvGROW(sv, SvCUR(sv) + extra);
        return SvCUR(sv);

must_be_utf8:

        /* Here, the string should be converted to utf8, either because of an
         * input flag (two_byte_count = 0), or because a character that
         * requires 2 bytes was found (two_byte_count = 1).  t points either to
         * the beginning of the string (if we didn't examine anything), or to
         * the first variant.  In either case, everything from s to t - 1 will
         * occupy only 1 byte each on output.
         *
         * There are two main ways to convert.  One is to create a new string
         * and go through the input starting from the beginning, appending each
         * converted value onto the new string as we go along.  It's probably
         * best to allocate enough space in the string for the worst possible
         * case rather than possibly running out of space and having to
         * reallocate and then copy what we've done so far.  Since everything
         * from s to t - 1 is invariant, the destination can be initialized
         * with these using a fast memory copy
         *
         * The other way is to figure out exactly how big the string should be
         * by parsing the entire input.  Then you don't have to make it big
         * enough to handle the worst possible case, and more importantly, if
         * the string you already have is large enough, you don't have to
         * allocate a new string, you can copy the last character in the input
         * string to the final position(s) that will be occupied by the
         * converted string and go backwards, stopping at t, since everything
         * before that is invariant.
         *
         * There are advantages and disadvantages to each method.
         *
         * In the first method, we can allocate a new string, do the memory
         * copy from the s to t - 1, and then proceed through the rest of the
         * string byte-by-byte.
         *
         * In the second method, we proceed through the rest of the input
         * string just calculating how big the converted string will be.  Then
         * there are two cases:
         *  1)  if the string has enough extra space to handle the converted
         *      value.  We go backwards through the string, converting until we
         *      get to the position we are at now, and then stop.  If this
         *      position is far enough along in the string, this method is
         *      faster than the other method.  If the memory copy were the same
         *      speed as the byte-by-byte loop, that position would be about
         *      half-way, as at the half-way mark, parsing to the end and back
         *      is one complete string's parse, the same amount as starting
         *      over and going all the way through.  Actually, it would be
         *      somewhat less than half-way, as it's faster to just count bytes
         *      than to also copy, and we don't have the overhead of allocating
         *      a new string, changing the scalar to use it, and freeing the
         *      existing one.  But if the memory copy is fast, the break-even
         *      point is somewhere after half way.  The counting loop could be
         *      sped up by vectorization, etc, to move the break-even point
         *      further towards the beginning.
         *  2)  if the string doesn't have enough space to handle the converted
         *      value.  A new string will have to be allocated, and one might
         *      as well, given that, start from the beginning doing the first
         *      method.  We've spent extra time parsing the string and in
         *      exchange all we've gotten is that we know precisely how big to
         *      make the new one.  Perl is more optimized for time than space,
         *      so this case is a loser.
         * So what I've decided to do is not use the 2nd method unless it is
         * guaranteed that a new string won't have to be allocated, assuming
         * the worst case.  I also decided not to put any more conditions on it
         * than this, for now.  It seems likely that, since the worst case is
         * twice as big as the unknown portion of the string (plus 1), we won't
         * be guaranteed enough space, causing us to go to the first method,
         * unless the string is short, or the first variant character is near
         * the end of it.  In either of these cases, it seems best to use the
         * 2nd method.  The only circumstance I can think of where this would
         * be really slower is if the string had once had much more data in it
         * than it does now, but there is still a substantial amount in it  */

        {
            STRLEN invariant_head = t - s;
            STRLEN size = invariant_head + (e - t) * 2 + 1 + extra;
            if (SvLEN(sv) < size) {

                /* Here, have decided to allocate a new string */

                U8 *dst;
                U8 *d;

                Newx(dst, size, U8);

                /* If no known invariants at the beginning of the input string,
                 * set so starts from there.  Otherwise, can use memory copy to
                 * get up to where we are now, and then start from here */

                if (invariant_head <= 0) {
                    d = dst;
                } else {
                    Copy(s, dst, invariant_head, char);
                    d = dst + invariant_head;
                }

                while (t < e) {
                    const UV uv = NATIVE8_TO_UNI(*t++);
                    if (UNI_IS_INVARIANT(uv))
                        *d++ = (U8)UNI_TO_NATIVE(uv);
                    else {
                        *d++ = (U8)UTF8_EIGHT_BIT_HI(uv);
                        *d++ = (U8)UTF8_EIGHT_BIT_LO(uv);
                    }
                }
                *d = '\0';
                SvPV_free(sv); /* No longer using pre-existing string */
                SvPV_set(sv, (char*)dst);
                SvCUR_set(sv, d - dst);
                SvLEN_set(sv, size);
            } else {

                /* Here, have decided to get the exact size of the string.
                 * Currently this happens only when we know that there is
                 * guaranteed enough space to fit the converted string, so
                 * don't have to worry about growing.  If two_byte_count is 0,
                 * then t points to the first byte of the string which hasn't
                 * been examined yet.  Otherwise two_byte_count is 1, and t
                 * points to the first byte in the string that will expand to
                 * two.  Depending on this, start examining at t or 1 after t.
                 * */

                U8 *d = t + two_byte_count;


                /* Count up the remaining bytes that expand to two */

                while (d < e) {
                    const U8 chr = *d++;
                    if (! NATIVE_IS_INVARIANT(chr)) two_byte_count++;
                }

                /* The string will expand by just the number of bytes that
                 * occupy two positions.  But we are one afterwards because of
                 * the increment just above.  This is the place to put the
                 * trailing NUL, and to set the length before we decrement */

                d += two_byte_count;
                SvCUR_set(sv, d - s);
                *d-- = '\0';


                /* Having decremented d, it points to the position to put the
                 * very last byte of the expanded string.  Go backwards through
                 * the string, copying and expanding as we go, stopping when we
                 * get to the part that is invariant the rest of the way down */

                e--;
                while (e >= t) {
                    const U8 ch = NATIVE8_TO_UNI(*e--);
                    if (UNI_IS_INVARIANT(ch)) {
                        *d-- = UNI_TO_NATIVE(ch);
                    } else {
                        *d-- = (U8)UTF8_EIGHT_BIT_LO(ch);
                        *d-- = (U8)UTF8_EIGHT_BIT_HI(ch);
                    }
                }
            }

            if (SvTYPE(sv) >= SVt_PVMG && SvMAGIC(sv)) {
                /* Update pos. We do it at the end rather than during
                 * the upgrade, to avoid slowing down the common case
                 * (upgrade without pos) */
                MAGIC * mg = mg_find(sv, PERL_MAGIC_regex_global);
                if (mg) {
                    I32 pos = mg->mg_len;
                    if (pos > 0 && (U32)pos > invariant_head) {
                        U8 *d = (U8*) SvPVX(sv) + invariant_head;
                        STRLEN n = (U32)pos - invariant_head;
                        while (n > 0) {
                            if (UTF8_IS_START(*d))
                                d++;
                            d++;
                            n--;
                        }
                        mg->mg_len  = d - (U8*)SvPVX(sv);
                    }
                }
                if ((mg = mg_find(sv, PERL_MAGIC_utf8)))
                    magic_setutf8(sv,mg); /* clear UTF8 cache */
            }
        }
    }

    /* Mark as UTF-8 even if no variant - saves scanning loop */
    SvUTF8_on(sv);
    return SvCUR(sv);
}

/*
=for apidoc sv_utf8_downgrade

Attempts to convert the PV of an SV from characters to bytes.
If the PV contains a character that cannot fit
in a byte, this conversion will fail;
in this case, either returns false or, if C<fail_ok> is not
true, croaks.

This is not a general purpose Unicode to byte encoding interface:
use the Encode extension for that.

=cut
*/

bool
Perl_sv_utf8_downgrade(pTHX_ register SV *const sv, const bool fail_ok)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_UTF8_DOWNGRADE;

    if (SvPOKp(sv) && SvUTF8(sv)) {
        if (SvCUR(sv)) {
            U8 *s;
            STRLEN len;
            int mg_flags = SV_GMAGIC;

            if (SvIsCOW(sv)) {
                sv_force_normal_flags(sv, 0);
            }
            if (SvTYPE(sv) >= SVt_PVMG && SvMAGIC(sv)) {
                /* update pos */
                MAGIC * mg = mg_find(sv, PERL_MAGIC_regex_global);
                if (mg) {
                    I32 pos = mg->mg_len;
                    if (pos > 0) {
                        sv_pos_b2u(sv, &pos);
                        mg_flags = 0; /* sv_pos_b2u does get magic */
                        mg->mg_len  = pos;
                    }
                }
                if ((mg = mg_find(sv, PERL_MAGIC_utf8)))
                    magic_setutf8(sv,mg); /* clear UTF8 cache */

            }
            s = (U8 *) SvPV_flags(sv, len, mg_flags);

            if (!utf8_to_bytes(s, &len)) {
                if (fail_ok)
                    return FALSE;
                else {
                    if (PL_op)
                        Perl_croak(aTHX_ "Wide character in %s",
                                   OP_DESC(PL_op));
                    else
                        Perl_croak(aTHX_ "Wide character");
                }
            }
            SvCUR_set(sv, len);
        }
    }
    SvUTF8_off(sv);
    return TRUE;
}

/*
=for apidoc sv_utf8_encode

Converts the PV of an SV to UTF-8, but then turns the C<SvUTF8>
flag off so that it looks like octets again.

=cut
*/

void
Perl_sv_utf8_encode(pTHX_ register SV *const sv)
{
    PERL_ARGS_ASSERT_SV_UTF8_ENCODE;

    if (SvREADONLY(sv)) {
        sv_force_normal_flags(sv, 0);
    }
    (void) sv_utf8_upgrade(sv);
    SvUTF8_off(sv);
}

/*
=for apidoc sv_utf8_decode

If the PV of the SV is an octet sequence in UTF-8
and contains a multiple-byte character, the C<SvUTF8> flag is turned on
so that it looks like a character.  If the PV contains only single-byte
characters, the C<SvUTF8> flag stays off.
Scans PV for validity and returns false if the PV is invalid UTF-8.

=cut
*/

bool
Perl_sv_utf8_decode(pTHX_ register SV *const sv)
{
    PERL_ARGS_ASSERT_SV_UTF8_DECODE;

    if (SvPOKp(sv)) {
        const U8 *start, *c;
        const U8 *e;

        /* The octets may have got themselves encoded - get them back as
         * bytes
         */
        if (!sv_utf8_downgrade(sv, TRUE))
            return FALSE;

        /* it is actually just a matter of turning the utf8 flag on, but
         * we want to make sure everything inside is valid utf8 first.
         */
        c = start = (const U8 *) SvPVX_const(sv);
        if (!is_utf8_string(c, SvCUR(sv)))
            return FALSE;
        e = (const U8 *) SvEND(sv);
        while (c < e) {
            const U8 ch = *c++;
            if (!UTF8_IS_INVARIANT(ch)) {
                SvUTF8_on(sv);
                break;
            }
        }
        if (SvTYPE(sv) >= SVt_PVMG && SvMAGIC(sv)) {
            /* adjust pos to the start of a UTF8 char sequence */
            MAGIC * mg = mg_find(sv, PERL_MAGIC_regex_global);
            if (mg) {
                I32 pos = mg->mg_len;
                if (pos > 0) {
                    for (c = start + pos; c > start; c--) {
                        if (UTF8_IS_START(*c))
                            break;
                    }
                    mg->mg_len  = c - start;
                }
            }
            if ((mg = mg_find(sv, PERL_MAGIC_utf8)))
                magic_setutf8(sv,mg); /* clear UTF8 cache */
        }
    }
    return TRUE;
}

/*
=for apidoc sv_setsv

Copies the contents of the source SV C<ssv> into the destination SV
C<dsv>.  The source SV may be destroyed if it is mortal, so don't use this
function if the source SV needs to be reused.  Does not handle 'set' magic.
Loosely speaking, it performs a copy-by-value, obliterating any previous
content of the destination.

You probably want to use one of the assortment of wrappers, such as
C<SvSetSV>, C<SvSetSV_nosteal>, C<SvSetMagicSV> and
C<SvSetMagicSV_nosteal>.

=for apidoc sv_setsv_flags

Copies the contents of the source SV C<ssv> into the destination SV
C<dsv>.  The source SV may be destroyed if it is mortal, so don't use this
function if the source SV needs to be reused.  Does not handle 'set' magic.
Loosely speaking, it performs a copy-by-value, obliterating any previous
content of the destination.
If the C<flags> parameter has the C<SV_GMAGIC> bit set, will C<mg_get> on
C<ssv> if appropriate, else not.  If the C<flags>
parameter has the C<NOSTEAL> bit set then the
buffers of temps will not be stolen.  <sv_setsv>
and C<sv_setsv_nomg> are implemented in terms of this function.

You probably want to use one of the assortment of wrappers, such as
C<SvSetSV>, C<SvSetSV_nosteal>, C<SvSetMagicSV> and
C<SvSetMagicSV_nosteal>.

This is the primary function for copying scalars, and most other
copy-ish functions and macros use this underneath.

=cut
*/

static void
S_glob_assign_glob(pTHX_ SV *const dstr, SV *const sstr, const int dtype)
{
    I32 mro_changes = 0; /* 1 = method, 2 = isa, 3 = recursive isa */
    HV *old_stash = NULL;

    PERL_ARGS_ASSERT_GLOB_ASSIGN_GLOB;

    if (dtype != SVt_PVGV && !isGV_with_GP(dstr)) {
        const char * const name = GvNAME(sstr);
        const STRLEN len = GvNAMELEN(sstr);
        {
            if (dtype >= SVt_PV) {
                SvPV_free(dstr);
                SvPV_set(dstr, 0);
                SvLEN_set(dstr, 0);
                SvCUR_set(dstr, 0);
            }
            SvUPGRADE(dstr, SVt_PVGV);
            (void)SvOK_off(dstr);
            /* We have to turn this on here, even though we turn it off
               below, as GvSTASH will fail an assertion otherwise. */
            isGV_with_GP_on(dstr);
        }
        GvSTASH(dstr) = GvSTASH(sstr);
        if (GvSTASH(dstr))
            Perl_sv_add_backref(aTHX_ MUTABLE_SV(GvSTASH(dstr)), dstr);
        gv_name_set(MUTABLE_GV(dstr), name, len,
                        GV_ADD | (GvNAMEUTF8(sstr) ? SVf_UTF8 : 0 ));
        SvFAKE_on(dstr);        /* can coerce to non-glob */
    }

    if(GvGP(MUTABLE_GV(sstr))) {
        /* If source has method cache entry, clear it */
        if(GvCVGEN(sstr)) {
            SvREFCNT_dec(GvCV(sstr));
            GvCV_set(sstr, NULL);
            GvCVGEN(sstr) = 0;
        }
        /* If source has a real method, then a method is
           going to change */
        else if(
         GvCV((const GV *)sstr) && GvSTASH(dstr) && HvENAME(GvSTASH(dstr))
        ) {
            mro_changes = 1;
        }
    }

    /* If dest already had a real method, that's a change as well */
    if(
        !mro_changes && GvGP(MUTABLE_GV(dstr)) && GvCVu((const GV *)dstr)
     && GvSTASH(dstr) && HvENAME(GvSTASH(dstr))
    ) {
        mro_changes = 1;
    }

    /* We don't need to check the name of the destination if it was not a
       glob to begin with. */
    if(dtype == SVt_PVGV) {
        const char * const name = GvNAME((const GV *)dstr);
        if(
            strEQ(name,"ISA")
         /* The stash may have been detached from the symbol table, so
            check its name. */
         && GvSTASH(dstr) && HvENAME(GvSTASH(dstr))
        )
            mro_changes = 2;
        else {
            const STRLEN len = GvNAMELEN(dstr);
            if ((len > 1 && name[len-2] == ':' && name[len-1] == ':')
             || (len == 1 && name[0] == ':')) {
                mro_changes = 3;

                /* Set aside the old stash, so we can reset isa caches on
                   its subclasses. */
                if((old_stash = GvHV(dstr)))
                    /* Make sure we do not lose it early. */
                    SvREFCNT_inc_simple_void_NN(
                     sv_2mortal((SV *)old_stash)
                    );
            }
        }
    }

    gp_free(MUTABLE_GV(dstr));
    isGV_with_GP_off(dstr); /* SvOK_off does not like globs. */
    (void)SvOK_off(dstr);
    isGV_with_GP_on(dstr);
    GvINTRO_off(dstr);          /* one-shot flag */
    GvGP_set(dstr, gp_ref(GvGP(sstr)));
    if (SvTAINTED(sstr))
        SvTAINT(dstr);
    if (GvIMPORTED(dstr) != GVf_IMPORTED
        && CopSTASH_ne(PL_curcop, GvSTASH(dstr)))
        {
            GvIMPORTED_on(dstr);
        }
    GvMULTI_on(dstr);
    if(mro_changes == 2) {
      if (GvAV((const GV *)sstr)) {
        MAGIC *mg;
        SV * const sref = (SV *)GvAV((const GV *)dstr);
        if (SvSMAGICAL(sref) && (mg = mg_find(sref, PERL_MAGIC_isa))) {
            if (SvTYPE(mg->mg_obj) != SVt_PVAV) {
                AV * const ary = newAV();
                av_push(ary, mg->mg_obj); /* takes the refcount */
                mg->mg_obj = (SV *)ary;
            }
            av_push((AV *)mg->mg_obj, SvREFCNT_inc_simple_NN(dstr));
        }
        else sv_magic(sref, dstr, PERL_MAGIC_isa, NULL, 0);
      }
      mro_isa_changed_in(GvSTASH(dstr));
    }
    else if(mro_changes == 3) {
        HV * const stash = GvHV(dstr);
        if(old_stash ? (HV *)HvENAME_get(old_stash) : stash)
            mro_package_moved(
                stash, old_stash,
                (GV *)dstr, 0
            );
    }
    else if(mro_changes) mro_method_changed_in(GvSTASH(dstr));
    return;
}

static void
S_glob_assign_ref(pTHX_ SV *const dstr, SV *const sstr)
{
    SV * const sref = SvREFCNT_inc(SvRV(sstr));
    SV *dref = NULL;
    const int intro = GvINTRO(dstr);
    SV **location;
    U8 import_flag = 0;
    const U32 stype = SvTYPE(sref);

    PERL_ARGS_ASSERT_GLOB_ASSIGN_REF;

    if (intro) {
        GvINTRO_off(dstr);      /* one-shot flag */
        GvLINE(dstr) = CopLINE(PL_curcop);
        GvEGV(dstr) = MUTABLE_GV(dstr);
    }
    GvMULTI_on(dstr);
    switch (stype) {
    case SVt_PVCV:
        location = (SV **) &(GvGP(dstr)->gp_cv); /* XXX bypassing GvCV_set */
        import_flag = GVf_IMPORTED_CV;
        goto common;
    case SVt_PVHV:
        location = (SV **) &GvHV(dstr);
        import_flag = GVf_IMPORTED_HV;
        goto common;
    case SVt_PVAV:
        location = (SV **) &GvAV(dstr);
        import_flag = GVf_IMPORTED_AV;
        goto common;
    case SVt_PVIO:
        location = (SV **) &GvIOp(dstr);
        goto common;
    case SVt_PVFM:
        location = (SV **) &GvFORM(dstr);
        goto common;
    default:
        location = &GvSV(dstr);
        import_flag = GVf_IMPORTED_SV;
    common:
        if (intro) {
            if (stype == SVt_PVCV) {
                /*if (GvCVGEN(dstr) && (GvCV(dstr) != (const CV *)sref || GvCVGEN(dstr))) {*/
                if (GvCVGEN(dstr)) {
                    SvREFCNT_dec(GvCV(dstr));
                    GvCV_set(dstr, NULL);
                    GvCVGEN(dstr) = 0; /* Switch off cacheness. */
                }
            }
            SAVEGENERICSV(*location);
        }
        else
            dref = *location;
        if (stype == SVt_PVCV && (*location != sref || GvCVGEN(dstr))) {
            CV* const cv = MUTABLE_CV(*location);
            if (cv) {
                if (!GvCVGEN((const GV *)dstr) &&
                    (CvROOT(cv) || CvXSUB(cv)) &&
                    /* redundant check that avoids creating the extra SV
                       most of the time: */
                    (CvCONST(cv) || ckWARN(WARN_REDEFINE)))
                    {
                        SV * const new_const_sv =
                            CvCONST((const CV *)sref)
                                 ? cv_const_sv((const CV *)sref)
                                 : NULL;
                        report_redefined_cv(
                           sv_2mortal(Perl_newSVpvf(aTHX_
                                "%"HEKf"::%"HEKf,
                                HEKfARG(
                                 HvNAME_HEK(GvSTASH((const GV *)dstr))
                                ),
                                HEKfARG(GvENAME_HEK(MUTABLE_GV(dstr)))
                           )),
                           cv,
                           CvCONST((const CV *)sref) ? &new_const_sv : NULL
                        );
                    }
                if (!intro)
                    cv_ckproto_len_flags(cv, (const GV *)dstr,
                                   SvPOK(sref) ? CvPROTO(sref) : NULL,
                                   SvPOK(sref) ? CvPROTOLEN(sref) : 0,
                                   SvPOK(sref) ? SvUTF8(sref) : 0);
            }
            GvCVGEN(dstr) = 0; /* Switch off cacheness. */
            GvASSUMECV_on(dstr);
            if(GvSTASH(dstr)) mro_method_changed_in(GvSTASH(dstr)); /* sub foo { 1 } sub bar { 2 } *bar = \&foo */
        }
        *location = sref;
        if (import_flag && !(GvFLAGS(dstr) & import_flag)
            && CopSTASH_ne(PL_curcop, GvSTASH(dstr))) {
            GvFLAGS(dstr) |= import_flag;
        }
        if (stype == SVt_PVHV) {
            const char * const name = GvNAME((GV*)dstr);
            const STRLEN len = GvNAMELEN(dstr);
            if (
                (
                   (len > 1 && name[len-2] == ':' && name[len-1] == ':')
                || (len == 1 && name[0] == ':')
                )
             && (!dref || HvENAME_get(dref))
            ) {
                mro_package_moved(
                    (HV *)sref, (HV *)dref,
                    (GV *)dstr, 0
                );
            }
        }
        else if (
            stype == SVt_PVAV && sref != dref
         && strEQ(GvNAME((GV*)dstr), "ISA")
         /* The stash may have been detached from the symbol table, so
            check its name before doing anything. */
         && GvSTASH(dstr) && HvENAME(GvSTASH(dstr))
        ) {
            MAGIC *mg;
            MAGIC * const omg = dref && SvSMAGICAL(dref)
                                 ? mg_find(dref, PERL_MAGIC_isa)
                                 : NULL;
            if (SvSMAGICAL(sref) && (mg = mg_find(sref, PERL_MAGIC_isa))) {
                if (SvTYPE(mg->mg_obj) != SVt_PVAV) {
                    AV * const ary = newAV();
                    av_push(ary, mg->mg_obj); /* takes the refcount */
                    mg->mg_obj = (SV *)ary;
                }
                if (omg) {
                    if (SvTYPE(omg->mg_obj) == SVt_PVAV) {
                        SV **svp = AvARRAY((AV *)omg->mg_obj);
                        I32 items = AvFILLp((AV *)omg->mg_obj) + 1;
                        while (items--)
                            av_push(
                             (AV *)mg->mg_obj,
                             SvREFCNT_inc_simple_NN(*svp++)
                            );
                    }
                    else
                        av_push(
                         (AV *)mg->mg_obj,
                         SvREFCNT_inc_simple_NN(omg->mg_obj)
                        );
                }
                else
                    av_push((AV *)mg->mg_obj,SvREFCNT_inc_simple_NN(dstr));
            }
            else
            {
                sv_magic(
                 sref, omg ? omg->mg_obj : dstr, PERL_MAGIC_isa, NULL, 0
                );
                mg = mg_find(sref, PERL_MAGIC_isa);
            }
            /* Since the *ISA assignment could have affected more than
               one stash, don't call mro_isa_changed_in directly, but let
               magic_clearisa do it for us, as it already has the logic for
               dealing with globs vs arrays of globs. */
            assert(mg);
            Perl_magic_clearisa(aTHX_ NULL, mg);
        }
        else if (stype == SVt_PVIO) {
            DEBUG_o(Perl_deb(aTHX_ "glob_assign_ref clearing PL_stashcache\n"));
            /* It's a cache. It will rebuild itself quite happily.
               It's a lot of effort to work out exactly which key (or keys)
               might be invalidated by the creation of the this file handle.
            */
            hv_clear(PL_stashcache);
        }
        break;
    }
    SvREFCNT_dec(dref);
    if (SvTAINTED(sstr))
        SvTAINT(dstr);
    return;
}

void
Perl_sv_setsv_flags(pTHX_ SV *dstr, register SV* sstr, const I32 flags)
{
    dVAR;
    U32 sflags;
    int dtype;
    svtype stype;

    PERL_ARGS_ASSERT_SV_SETSV_FLAGS;

    if (sstr == dstr)
        return;

    if (SvIS_FREED(dstr)) {
        Perl_croak(aTHX_ "panic: attempt to copy value %" SVf
                   " to a freed scalar %p", SVfARG(sstr), (void *)dstr);
    }
    SV_CHECK_THINKFIRST_COW_DROP(dstr);
    if (!sstr)
        sstr = &PL_sv_undef;
    if (SvIS_FREED(sstr)) {
        Perl_croak(aTHX_ "panic: attempt to copy freed scalar %p to %p",
                   (void*)sstr, (void*)dstr);
    }
    stype = SvTYPE(sstr);
    dtype = SvTYPE(dstr);

    /* There's a lot of redundancy below but we're going for speed here */

    switch (stype) {
    case SVt_NULL:
      undef_sstr:
        if (dtype != SVt_PVGV && dtype != SVt_PVLV) {
            (void)SvOK_off(dstr);
            return;
        }
        break;
    case SVt_IV:
        if (SvIOK(sstr)) {
            switch (dtype) {
            case SVt_NULL:
                sv_upgrade(dstr, SVt_IV);
                break;
            case SVt_NV:
            case SVt_PV:
                sv_upgrade(dstr, SVt_PVIV);
                break;
            case SVt_PVGV:
            case SVt_PVLV:
                goto end_of_first_switch;
            }
            (void)SvIOK_only(dstr);
            SvIV_set(dstr,  SvIVX(sstr));
            if (SvIsUV(sstr))
                SvIsUV_on(dstr);
            /* SvTAINTED can only be true if the SV has taint magic, which in
               turn means that the SV type is PVMG (or greater). This is the
               case statement for SVt_IV, so this cannot be true (whatever gcov
               may say).  */
            assert(!SvTAINTED(sstr));
            return;
        }
        if (!SvROK(sstr))
            goto undef_sstr;
        if (dtype < SVt_PV && dtype != SVt_IV)
            sv_upgrade(dstr, SVt_IV);
        break;

    case SVt_NV:
        if (SvNOK(sstr)) {
            switch (dtype) {
            case SVt_NULL:
            case SVt_IV:
                sv_upgrade(dstr, SVt_NV);
                break;
            case SVt_PV:
            case SVt_PVIV:
                sv_upgrade(dstr, SVt_PVNV);
                break;
            case SVt_PVGV:
            case SVt_PVLV:
                goto end_of_first_switch;
            }
            SvNV_set(dstr, SvNVX(sstr));
            (void)SvNOK_only(dstr);
            /* SvTAINTED can only be true if the SV has taint magic, which in
               turn means that the SV type is PVMG (or greater). This is the
               case statement for SVt_NV, so this cannot be true (whatever gcov
               may say).  */
            assert(!SvTAINTED(sstr));
            return;
        }
        goto undef_sstr;

    case SVt_PV:
        if (dtype < SVt_PV)
            sv_upgrade(dstr, SVt_PV);
        break;
    case SVt_PVIV:
        if (dtype < SVt_PVIV)
            sv_upgrade(dstr, SVt_PVIV);
        break;
    case SVt_PVNV:
        if (dtype < SVt_PVNV)
            sv_upgrade(dstr, SVt_PVNV);
        break;
    default:
        {
        const char * const type = sv_reftype(sstr,0);
        if (PL_op)
            /* diag_listed_as: Bizarre copy of %s */
            Perl_croak(aTHX_ "Bizarre copy of %s in %s", type, OP_DESC(PL_op));
        else
            Perl_croak(aTHX_ "Bizarre copy of %s", type);
        }
        break;

    case SVt_REGEXP:
      upgregexp:
        if (dtype < SVt_REGEXP)
        {
            if (dtype >= SVt_PV) {
                SvPV_free(dstr);
                SvPV_set(dstr, 0);
                SvLEN_set(dstr, 0);
                SvCUR_set(dstr, 0);
            }
            sv_upgrade(dstr, SVt_REGEXP);
        }
        break;

        /* case SVt_BIND: */
    case SVt_PVLV:
    case SVt_PVGV:
    case SVt_PVMG:
        if (SvGMAGICAL(sstr) && (flags & SV_GMAGIC)) {
            mg_get(sstr);
            if (SvTYPE(sstr) != stype)
                stype = SvTYPE(sstr);
        }
        if (isGV_with_GP(sstr) && dtype <= SVt_PVLV) {
                    glob_assign_glob(dstr, sstr, dtype);
                    return;
        }
        if (stype == SVt_PVLV)
        {
            if (isREGEXP(sstr)) goto upgregexp;
            SvUPGRADE(dstr, SVt_PVNV);
        }
        else
            SvUPGRADE(dstr, (svtype)stype);
    }
 end_of_first_switch:

    /* dstr may have been upgraded.  */
    dtype = SvTYPE(dstr);
    sflags = SvFLAGS(sstr);

    if (dtype == SVt_PVCV) {
        /* Assigning to a subroutine sets the prototype.  */
        if (SvOK(sstr)) {
            STRLEN len;
            const char *const ptr = SvPV_const(sstr, len);

            SvGROW(dstr, len + 1);
            Copy(ptr, SvPVX(dstr), len + 1, char);
            SvCUR_set(dstr, len);
            SvPOK_only(dstr);
            SvFLAGS(dstr) |= sflags & SVf_UTF8;
            CvAUTOLOAD_off(dstr);
        } else {
            SvOK_off(dstr);
        }
    }
    else if (dtype == SVt_PVAV || dtype == SVt_PVHV || dtype == SVt_PVFM) {
        const char * const type = sv_reftype(dstr,0);
        if (PL_op)
            /* diag_listed_as: Cannot copy to %s */
            Perl_croak(aTHX_ "Cannot copy to %s in %s", type, OP_DESC(PL_op));
        else
            Perl_croak(aTHX_ "Cannot copy to %s", type);
    } else if (sflags & SVf_ROK) {
        if (isGV_with_GP(dstr)
            && SvTYPE(SvRV(sstr)) == SVt_PVGV && isGV_with_GP(SvRV(sstr))) {
            sstr = SvRV(sstr);
            if (sstr == dstr) {
                if (GvIMPORTED(dstr) != GVf_IMPORTED
                    && CopSTASH_ne(PL_curcop, GvSTASH(dstr)))
                {
                    GvIMPORTED_on(dstr);
                }
                GvMULTI_on(dstr);
                return;
            }
            glob_assign_glob(dstr, sstr, dtype);
            return;
        }

        if (dtype >= SVt_PV) {
            if (isGV_with_GP(dstr)) {
                glob_assign_ref(dstr, sstr);
                return;
            }
            if (SvPVX_const(dstr)) {
                SvPV_free(dstr);
                SvLEN_set(dstr, 0);
                SvCUR_set(dstr, 0);
            }
        }
        (void)SvOK_off(dstr);
        SvRV_set(dstr, SvREFCNT_inc(SvRV(sstr)));
        SvFLAGS(dstr) |= sflags & SVf_ROK;
        assert(!(sflags & SVp_NOK));
        assert(!(sflags & SVp_IOK));
        assert(!(sflags & SVf_NOK));
        assert(!(sflags & SVf_IOK));
    }
    else if (isGV_with_GP(dstr)) {
        if (!(sflags & SVf_OK)) {
            Perl_ck_warner(aTHX_ packWARN(WARN_MISC),
                           "Undefined value assigned to typeglob");
        }
        else {
            GV *gv = gv_fetchsv_nomg(sstr, GV_ADD, SVt_PVGV);
            if (dstr != (const SV *)gv) {
                const char * const name = GvNAME((const GV *)dstr);
                const STRLEN len = GvNAMELEN(dstr);
                HV *old_stash = NULL;
                bool reset_isa = FALSE;
                if ((len > 1 && name[len-2] == ':' && name[len-1] == ':')
                 || (len == 1 && name[0] == ':')) {
                    /* Set aside the old stash, so we can reset isa caches
                       on its subclasses. */
                    if((old_stash = GvHV(dstr))) {
                        /* Make sure we do not lose it early. */
                        SvREFCNT_inc_simple_void_NN(
                         sv_2mortal((SV *)old_stash)
                        );
                    }
                    reset_isa = TRUE;
                }

                if (GvGP(dstr))
                    gp_free(MUTABLE_GV(dstr));
                GvGP_set(dstr, gp_ref(GvGP(gv)));

                if (reset_isa) {
                    HV * const stash = GvHV(dstr);
                    if(
                        old_stash ? (HV *)HvENAME_get(old_stash) : stash
                    )
                        mro_package_moved(
                         stash, old_stash,
                         (GV *)dstr, 0
                        );
                }
            }
        }
    }
    else if ((dtype == SVt_REGEXP || dtype == SVt_PVLV)
          && (stype == SVt_REGEXP || isREGEXP(sstr))) {
        reg_temp_copy((REGEXP*)dstr, (REGEXP*)sstr);
    }
    else if (sflags & SVp_POK) {
        bool isSwipe = 0;

        /*
         * Check to see if we can just swipe the string.  If so, it's a
         * possible small lose on short strings, but a big win on long ones.
         * It might even be a win on short strings if SvPVX_const(dstr)
         * has to be allocated and SvPVX_const(sstr) has to be freed.
         * Likewise if we can set up COW rather than doing an actual copy, we
         * drop to the else clause, as the swipe code and the COW setup code
         * have much in common.
         */

        /* Whichever path we take through the next code, we want this true,
           and doing it now facilitates the COW check.  */
        (void)SvPOK_only(dstr);

        if (
            /* If we're already COW then this clause is not true, and if COW
               is allowed then we drop down to the else and make dest COW 
               with us.  If caller hasn't said that we're allowed to COW
               shared hash keys then we don't do the COW setup, even if the
               source scalar is a shared hash key scalar.  */
            (((flags & SV_COW_SHARED_HASH_KEYS)
               ? !(sflags & SVf_IsCOW)
               : 1 /* If making a COW copy is forbidden then the behaviour we
                       desire is as if the source SV isn't actually already
                       COW, even if it is.  So we act as if the source flags
                       are not COW, rather than actually testing them.  */
              )
#ifndef PERL_OLD_COPY_ON_WRITE
             /* The change that added SV_COW_SHARED_HASH_KEYS makes the logic
                when PERL_OLD_COPY_ON_WRITE is defined a little wrong.
                Conceptually PERL_OLD_COPY_ON_WRITE being defined should
                override SV_COW_SHARED_HASH_KEYS, because it means "always COW"
                but in turn, it's somewhat dead code, never expected to go
                live, but more kept as a placeholder on how to do it better
                in a newer implementation.  */
             /* If we are COW and dstr is a suitable target then we drop down
                into the else and make dest a COW of us.  */
             || (SvFLAGS(dstr) & SVf_BREAK)
#endif
             )
            &&
            !(isSwipe =
                 (sflags & SVs_TEMP) &&   /* slated for free anyway? */
                 !(sflags & SVf_OOK) &&   /* and not involved in OOK hack? */
                 (!(flags & SV_NOSTEAL)) &&
                                        /* and we're allowed to steal temps */
                 SvREFCNT(sstr) == 1 &&   /* and no other references to it? */
                 SvLEN(sstr))             /* and really is a string */
#ifdef PERL_OLD_COPY_ON_WRITE
            && ((flags & SV_COW_SHARED_HASH_KEYS)
                ? (!((sflags & CAN_COW_MASK) == CAN_COW_FLAGS
                     && (SvFLAGS(dstr) & CAN_COW_MASK) == CAN_COW_FLAGS
                     && SvTYPE(sstr) >= SVt_PVIV))
                : 1)
#endif
            ) {
            /* Failed the swipe test, and it's not a shared hash key either.
               Have to copy the string.  */
            STRLEN len = SvCUR(sstr);
            SvGROW(dstr, len + 1);      /* inlined from sv_setpvn */
            Move(SvPVX_const(sstr),SvPVX(dstr),len,char);
            SvCUR_set(dstr, len);
            *SvEND(dstr) = '\0';
        } else {
            /* If PERL_OLD_COPY_ON_WRITE is not defined, then isSwipe will always
               be true in here.  */
            /* Either it's a shared hash key, or it's suitable for
               copy-on-write or we can swipe the string.  */
            if (DEBUG_C_TEST) {
                PerlIO_printf(Perl_debug_log, "Copy on write: sstr --> dstr\n");
                sv_dump(sstr);
                sv_dump(dstr);
            }
#ifdef PERL_OLD_COPY_ON_WRITE
            if (!isSwipe) {
                if (!(sflags & SVf_IsCOW)) {
                    SvIsCOW_on(sstr);
                    /* Make the source SV into a loop of 1.
                       (about to become 2) */
                    SV_COW_NEXT_SV_SET(sstr, sstr);
                }
            }
#endif
            /* Initial code is common.  */
            if (SvPVX_const(dstr)) {    /* we know that dtype >= SVt_PV */
                SvPV_free(dstr);
            }

            if (!isSwipe) {
                /* making another shared SV.  */
                STRLEN cur = SvCUR(sstr);
                STRLEN len = SvLEN(sstr);
#ifdef PERL_OLD_COPY_ON_WRITE
                if (len) {
                    assert (SvTYPE(dstr) >= SVt_PVIV);
                    /* SvIsCOW_normal */
                    /* splice us in between source and next-after-source.  */
                    SV_COW_NEXT_SV_SET(dstr, SV_COW_NEXT_SV(sstr));
                    SV_COW_NEXT_SV_SET(sstr, dstr);
                    SvPV_set(dstr, SvPVX_mutable(sstr));
                } else
#endif
                {
                    /* SvIsCOW_shared_hash */
                    DEBUG_C(PerlIO_printf(Perl_debug_log,
                                          "Copy on write: Sharing hash\n"));

                    assert (SvTYPE(dstr) >= SVt_PV);
                    SvPV_set(dstr,
                             HEK_KEY(share_hek_hek(SvSHARED_HEK_FROM_PV(SvPVX_const(sstr)))));
                }
                SvLEN_set(dstr, len);
                SvCUR_set(dstr, cur);
                SvIsCOW_on(dstr);
            }
            else
                {       /* Passes the swipe test.  */
                SvPV_set(dstr, SvPVX_mutable(sstr));
                SvLEN_set(dstr, SvLEN(sstr));
                SvCUR_set(dstr, SvCUR(sstr));

                SvTEMP_off(dstr);
                (void)SvOK_off(sstr);   /* NOTE: nukes most SvFLAGS on sstr */
                SvPV_set(sstr, NULL);
                SvLEN_set(sstr, 0);
                SvCUR_set(sstr, 0);
                SvTEMP_off(sstr);
            }
        }
        if (sflags & SVp_NOK) {
            SvNV_set(dstr, SvNVX(sstr));
        }
        if (sflags & SVp_IOK) {
            SvIV_set(dstr, SvIVX(sstr));
            /* Must do this otherwise some other overloaded use of 0x80000000
               gets confused. I guess SVpbm_VALID */
            if (sflags & SVf_IVisUV)
                SvIsUV_on(dstr);
        }
        SvFLAGS(dstr) |= sflags & (SVf_IOK|SVp_IOK|SVf_NOK|SVp_NOK|SVf_UTF8);
        {
            const MAGIC * const smg = SvVSTRING_mg(sstr);
            if (smg) {
                sv_magic(dstr, NULL, PERL_MAGIC_vstring,
                         smg->mg_ptr, smg->mg_len);
                SvRMAGICAL_on(dstr);
            }
        }
    }
    else if (sflags & (SVp_IOK|SVp_NOK)) {
        (void)SvOK_off(dstr);
        SvFLAGS(dstr) |= sflags & (SVf_IOK|SVp_IOK|SVf_IVisUV|SVf_NOK|SVp_NOK);
        if (sflags & SVp_IOK) {
            /* XXXX Do we want to set IsUV for IV(ROK)?  Be extra safe... */
            SvIV_set(dstr, SvIVX(sstr));
        }
        if (sflags & SVp_NOK) {
            SvNV_set(dstr, SvNVX(sstr));
        }
    }
    else {
        if (isGV_with_GP(sstr)) {
            gv_efullname3(dstr, MUTABLE_GV(sstr), "*");
        }
        else
            (void)SvOK_off(dstr);
    }
    if (SvTAINTED(sstr))
        SvTAINT(dstr);
}

/*
=for apidoc sv_setsv_mg

Like C<sv_setsv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setsv_mg(pTHX_ SV *const dstr, register SV *const sstr)
{
    PERL_ARGS_ASSERT_SV_SETSV_MG;

    sv_setsv(dstr,sstr);
    SvSETMAGIC(dstr);
}

#ifdef PERL_OLD_COPY_ON_WRITE
SV *
Perl_sv_setsv_cow(pTHX_ SV *dstr, SV *sstr)
{
    STRLEN cur = SvCUR(sstr);
    STRLEN len = SvLEN(sstr);
    char *new_pv;

    PERL_ARGS_ASSERT_SV_SETSV_COW;

    if (DEBUG_C_TEST) {
        PerlIO_printf(Perl_debug_log, "Fast copy on write: %p -> %p\n",
                      (void*)sstr, (void*)dstr);
        sv_dump(sstr);
        if (dstr)
                    sv_dump(dstr);
    }

    if (dstr) {
        if (SvTHINKFIRST(dstr))
            sv_force_normal_flags(dstr, SV_COW_DROP_PV);
        else if (SvPVX_const(dstr))
            Safefree(SvPVX_mutable(dstr));
    }
    else
        new_SV(dstr);
    SvUPGRADE(dstr, SVt_PVIV);

    assert (SvPOK(sstr));
    assert (SvPOKp(sstr));
    assert (!SvIOK(sstr));
    assert (!SvIOKp(sstr));
    assert (!SvNOK(sstr));
    assert (!SvNOKp(sstr));

    if (SvIsCOW(sstr)) {

        if (SvLEN(sstr) == 0) {
            /* source is a COW shared hash key.  */
            DEBUG_C(PerlIO_printf(Perl_debug_log,
                                  "Fast copy on write: Sharing hash\n"));
            new_pv = HEK_KEY(share_hek_hek(SvSHARED_HEK_FROM_PV(SvPVX_const(sstr))));
            goto common_exit;
        }
        SV_COW_NEXT_SV_SET(dstr, SV_COW_NEXT_SV(sstr));
    } else {
        assert ((SvFLAGS(sstr) & CAN_COW_MASK) == CAN_COW_FLAGS);
        SvUPGRADE(sstr, SVt_PVIV);
        SvIsCOW_on(sstr);
        DEBUG_C(PerlIO_printf(Perl_debug_log,
                              "Fast copy on write: Converting sstr to COW\n"));
        SV_COW_NEXT_SV_SET(dstr, sstr);
    }
    SV_COW_NEXT_SV_SET(sstr, dstr);
    new_pv = SvPVX_mutable(sstr);

  common_exit:
    SvPV_set(dstr, new_pv);
    SvFLAGS(dstr) = (SVt_PVIV|SVf_POK|SVp_POK|SVf_IsCOW);
    if (SvUTF8(sstr))
        SvUTF8_on(dstr);
    SvLEN_set(dstr, len);
    SvCUR_set(dstr, cur);
    if (DEBUG_C_TEST) {
        sv_dump(dstr);
    }
    return dstr;
}
#endif

/*
=for apidoc sv_setpvn

Copies a string into an SV.  The C<len> parameter indicates the number of
bytes to be copied.  If the C<ptr> argument is NULL the SV will become
undefined.  Does not handle 'set' magic.  See C<sv_setpvn_mg>.

=cut
*/

void
Perl_sv_setpvn(pTHX_ register SV *const sv, register const char *const ptr, register const STRLEN len)
{
    dVAR;
    char *dptr;

    PERL_ARGS_ASSERT_SV_SETPVN;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    if (!ptr) {
        (void)SvOK_off(sv);
        return;
    }
    else {
        /* len is STRLEN which is unsigned, need to copy to signed */
        const IV iv = len;
        if (iv < 0)
            Perl_croak(aTHX_ "panic: sv_setpvn called with negative strlen %"
                       IVdf, iv);
    }
    SvUPGRADE(sv, SVt_PV);

    dptr = SvGROW(sv, len + 1);
    Move(ptr,dptr,len,char);
    dptr[len] = '\0';
    SvCUR_set(sv, len);
    (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    SvTAINT(sv);
    if (SvTYPE(sv) == SVt_PVCV) CvAUTOLOAD_off(sv);
}

/*
=for apidoc sv_setpvn_mg

Like C<sv_setpvn>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setpvn_mg(pTHX_ register SV *const sv, register const char *const ptr, register const STRLEN len)
{
    PERL_ARGS_ASSERT_SV_SETPVN_MG;

    sv_setpvn(sv,ptr,len);
    SvSETMAGIC(sv);
}

/*
=for apidoc sv_setpv

Copies a string into an SV.  The string must be null-terminated.  Does not
handle 'set' magic.  See C<sv_setpv_mg>.

=cut
*/

void
Perl_sv_setpv(pTHX_ register SV *const sv, register const char *const ptr)
{
    dVAR;
    STRLEN len;

    PERL_ARGS_ASSERT_SV_SETPV;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    if (!ptr) {
        (void)SvOK_off(sv);
        return;
    }
    len = strlen(ptr);
    SvUPGRADE(sv, SVt_PV);

    SvGROW(sv, len + 1);
    Move(ptr,SvPVX(sv),len+1,char);
    SvCUR_set(sv, len);
    (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    SvTAINT(sv);
    if (SvTYPE(sv) == SVt_PVCV) CvAUTOLOAD_off(sv);
}

/*
=for apidoc sv_setpv_mg

Like C<sv_setpv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_setpv_mg(pTHX_ register SV *const sv, register const char *const ptr)
{
    PERL_ARGS_ASSERT_SV_SETPV_MG;

    sv_setpv(sv,ptr);
    SvSETMAGIC(sv);
}

void
Perl_sv_sethek(pTHX_ register SV *const sv, const HEK *const hek)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_SETHEK;

    if (!hek) {
        return;
    }

    if (HEK_LEN(hek) == HEf_SVKEY) {
        sv_setsv(sv, *(SV**)HEK_KEY(hek));
        return;
    } else {
        const int flags = HEK_FLAGS(hek);
        if (flags & HVhek_WASUTF8) {
            STRLEN utf8_len = HEK_LEN(hek);
            char *as_utf8 = (char *)bytes_to_utf8((U8*)HEK_KEY(hek), &utf8_len);
            sv_usepvn_flags(sv, as_utf8, utf8_len, SV_HAS_TRAILING_NUL);
            SvUTF8_on(sv);
            return;
        } else if (flags & HVhek_UNSHARED) {
            sv_setpvn(sv, HEK_KEY(hek), HEK_LEN(hek));
            if (HEK_UTF8(hek))
                SvUTF8_on(sv);
            else SvUTF8_off(sv);
            return;
        }
        {
            SV_CHECK_THINKFIRST_COW_DROP(sv);
            SvUPGRADE(sv, SVt_PV);
            Safefree(SvPVX(sv));
            SvPV_set(sv,(char *)HEK_KEY(share_hek_hek(hek)));
            SvCUR_set(sv, HEK_LEN(hek));
            SvLEN_set(sv, 0);
            SvIsCOW_on(sv);
            SvPOK_on(sv);
            if (HEK_UTF8(hek))
                SvUTF8_on(sv);
            else SvUTF8_off(sv);
            return;
        }
    }
}


/*
=for apidoc sv_usepvn_flags

Tells an SV to use C<ptr> to find its string value.  Normally the
string is stored inside the SV but sv_usepvn allows the SV to use an
outside string.  The C<ptr> should point to memory that was allocated
by C<malloc>.  It must be the start of a mallocked block
of memory, and not a pointer to the middle of it.  The
string length, C<len>, must be supplied.  By default
this function will realloc (i.e. move) the memory pointed to by C<ptr>,
so that pointer should not be freed or used by the programmer after
giving it to sv_usepvn, and neither should any pointers from "behind"
that pointer (e.g. ptr + 1) be used.

If C<flags> & SV_SMAGIC is true, will call SvSETMAGIC.  If C<flags> &
SV_HAS_TRAILING_NUL is true, then C<ptr[len]> must be NUL, and the realloc
will be skipped (i.e. the buffer is actually at least 1 byte longer than
C<len>, and already meets the requirements for storing in C<SvPVX>).

=cut
*/

void
Perl_sv_usepvn_flags(pTHX_ SV *const sv, char *ptr, const STRLEN len, const U32 flags)
{
    dVAR;
    STRLEN allocate;

    PERL_ARGS_ASSERT_SV_USEPVN_FLAGS;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    SvUPGRADE(sv, SVt_PV);
    if (!ptr) {
        (void)SvOK_off(sv);
        if (flags & SV_SMAGIC)
            SvSETMAGIC(sv);
        return;
    }
    if (SvPVX_const(sv))
        SvPV_free(sv);

#ifdef DEBUGGING
    if (flags & SV_HAS_TRAILING_NUL)
        assert(ptr[len] == '\0');
#endif

    allocate = (flags & SV_HAS_TRAILING_NUL)
        ? len + 1 :
#ifdef Perl_safesysmalloc_size
        len + 1;
#else 
        PERL_STRLEN_ROUNDUP(len + 1);
#endif
    if (flags & SV_HAS_TRAILING_NUL) {
        /* It's long enough - do nothing.
           Specifically Perl_newCONSTSUB is relying on this.  */
    } else {
#ifdef DEBUGGING
        /* Force a move to shake out bugs in callers.  */
        char *new_ptr = (char*)safemalloc(allocate);
        Copy(ptr, new_ptr, len, char);
        PoisonFree(ptr,len,char);
        Safefree(ptr);
        ptr = new_ptr;
#else
        ptr = (char*) saferealloc (ptr, allocate);
#endif
    }
#ifdef Perl_safesysmalloc_size
    SvLEN_set(sv, Perl_safesysmalloc_size(ptr));
#else
    SvLEN_set(sv, allocate);
#endif
    SvCUR_set(sv, len);
    SvPV_set(sv, ptr);
    if (!(flags & SV_HAS_TRAILING_NUL)) {
        ptr[len] = '\0';
    }
    (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    SvTAINT(sv);
    if (flags & SV_SMAGIC)
        SvSETMAGIC(sv);
}

#ifdef PERL_OLD_COPY_ON_WRITE
/* Need to do this *after* making the SV normal, as we need the buffer
   pointer to remain valid until after we've copied it.  If we let go too early,
   another thread could invalidate it by unsharing last of the same hash key
   (which it can do by means other than releasing copy-on-write Svs)
   or by changing the other copy-on-write SVs in the loop.  */
STATIC void
S_sv_release_COW(pTHX_ register SV *sv, const char *pvx, SV *after)
{
    PERL_ARGS_ASSERT_SV_RELEASE_COW;

    { /* this SV was SvIsCOW_normal(sv) */
         /* we need to find the SV pointing to us.  */
        SV *current = SV_COW_NEXT_SV(after);

        if (current == sv) {
            /* The SV we point to points back to us (there were only two of us
               in the loop.)
               Hence other SV is no longer copy on write either.  */
            SvIsCOW_off(after);
        } else {
            /* We need to follow the pointers around the loop.  */
            SV *next;
            while ((next = SV_COW_NEXT_SV(current)) != sv) {
                assert (next);
                current = next;
                 /* don't loop forever if the structure is bust, and we have
                    a pointer into a closed loop.  */
                assert (current != after);
                assert (SvPVX_const(current) == pvx);
            }
            /* Make the SV before us point to the SV after us.  */
            SV_COW_NEXT_SV_SET(current, after);
        }
    }
}
#endif
/*
=for apidoc sv_force_normal_flags

Undo various types of fakery on an SV, where fakery means
"more than" a string: if the PV is a shared string, make
a private copy; if we're a ref, stop refing; if we're a glob, downgrade to
an xpvmg; if we're a copy-on-write scalar, this is the on-write time when
we do the copy, and is also used locally; if this is a
vstring, drop the vstring magic.  If C<SV_COW_DROP_PV> is set
then a copy-on-write scalar drops its PV buffer (if any) and becomes
SvPOK_off rather than making a copy.  (Used where this
scalar is about to be set to some other value.)  In addition,
the C<flags> parameter gets passed to C<sv_unref_flags()>
when unreffing.  C<sv_force_normal> calls this function
with flags set to 0.

=cut
*/

void
Perl_sv_force_normal_flags(pTHX_ register SV *const sv, const U32 flags)
{
    dVAR;

    PERL_ARGS_ASSERT_SV_FORCE_NORMAL_FLAGS;

#ifdef PERL_OLD_COPY_ON_WRITE
    if (SvREADONLY(sv)) {
        if (IN_PERL_RUNTIME)
            Perl_croak_no_modify(aTHX);
    }
    else
        if (SvIsCOW(sv)) {
            const char * const pvx = SvPVX_const(sv);
            const STRLEN len = SvLEN(sv);
            const STRLEN cur = SvCUR(sv);
            /* next COW sv in the loop.  If len is 0 then this is a shared-hash
               key scalar, so we mustn't attempt to call SV_COW_NEXT_SV(), as
               we'll fail an assertion.  */
            SV * const next = len ? SV_COW_NEXT_SV(sv) : 0;

            if (DEBUG_C_TEST) {
                PerlIO_printf(Perl_debug_log,
                              "Copy on write: Force normal %ld\n",
                              (long) flags);
                sv_dump(sv);
            }
            SvIsCOW_off(sv);
            /* This SV doesn't own the buffer, so need to Newx() a new one:  */
            SvPV_set(sv, NULL);
            SvLEN_set(sv, 0);
            if (flags & SV_COW_DROP_PV) {
                /* OK, so we don't need to copy our buffer.  */
                SvPOK_off(sv);
            } else {
                SvGROW(sv, cur + 1);
                Move(pvx,SvPVX(sv),cur,char);
                SvCUR_set(sv, cur);
                *SvEND(sv) = '\0';
            }
            if (len) {
                sv_release_COW(sv, pvx, next);
            } else {
                unshare_hek(SvSHARED_HEK_FROM_PV(pvx));
            }
            if (DEBUG_C_TEST) {
                sv_dump(sv);
            }
        }
#else
    if (SvREADONLY(sv)) {
        if (IN_PERL_RUNTIME)
            Perl_croak_no_modify();
    }
    else
        if (SvIsCOW(sv)) {
            const char * const pvx = SvPVX_const(sv);
            const STRLEN len = SvCUR(sv);
            SvIsCOW_off(sv);
            SvPV_set(sv, NULL);
            SvLEN_set(sv, 0);
            if (flags & SV_COW_DROP_PV) {
                /* OK, so we don't need to copy our buffer.  */
                SvPOK_off(sv);
            } else {
                SvGROW(sv, len + 1);
                Move(pvx,SvPVX(sv),len,char);
                *SvEND(sv) = '\0';
            }
            unshare_hek(SvSHARED_HEK_FROM_PV(pvx));
        }
#endif
    if (SvROK(sv))
        sv_unref_flags(sv, flags);
    else if (SvFAKE(sv) && isGV_with_GP(sv))
        sv_unglob(sv, flags);
    else if (SvFAKE(sv) && isREGEXP(sv)) {
        /* Need to downgrade the REGEXP to a simple(r) scalar. This is analogous
           to sv_unglob. We only need it here, so inline it.  */
        const bool islv = SvTYPE(sv) == SVt_PVLV;
        const svtype new_type =
          islv ? SVt_NULL : SvMAGIC(sv) || SvSTASH(sv) ? SVt_PVMG : SVt_PV;
        SV *const temp = newSV_type(new_type);
        regexp *const temp_p = ReANY((REGEXP *)sv);

        if (new_type == SVt_PVMG) {
            SvMAGIC_set(temp, SvMAGIC(sv));
            SvMAGIC_set(sv, NULL);
            SvSTASH_set(temp, SvSTASH(sv));
            SvSTASH_set(sv, NULL);
        }
        if (!islv) SvCUR_set(temp, SvCUR(sv));
        /* Remember that SvPVX is in the head, not the body.  But
           RX_WRAPPED is in the body. */
        assert(ReANY((REGEXP *)sv)->mother_re);
        /* Their buffer is already owned by someone else. */
        if (flags & SV_COW_DROP_PV) {
            /* SvLEN is already 0.  For SVt_REGEXP, we have a brand new
               zeroed body.  For SVt_PVLV, it should have been set to 0
               before turning into a regexp. */
            assert(!SvLEN(islv ? sv : temp));
            sv->sv_u.svu_pv = 0;
        }
        else {
            sv->sv_u.svu_pv = savepvn(RX_WRAPPED((REGEXP *)sv), SvCUR(sv));
            SvLEN_set(islv ? sv : temp, SvCUR(sv)+1);
            SvPOK_on(sv);
        }

        /* Now swap the rest of the bodies. */

        SvFAKE_off(sv);
        if (!islv) {
            SvFLAGS(sv) &= ~SVTYPEMASK;
            SvFLAGS(sv) |= new_type;
            SvANY(sv) = SvANY(temp);
        }

        SvFLAGS(temp) &= ~(SVTYPEMASK);
        SvFLAGS(temp) |= SVt_REGEXP|SVf_FAKE;
        SvANY(temp) = temp_p;
        temp->sv_u.svu_rx = (regexp *)temp_p;

        SvREFCNT_dec(temp);
    }
    else if (SvVOK(sv)) sv_unmagic(sv, PERL_MAGIC_vstring);
}

/*
=for apidoc sv_chop

Efficient removal of characters from the beginning of the string buffer.
SvPOK(sv), or at least SvPOKp(sv), must be true and the C<ptr> must be a
pointer to somewhere inside the string buffer.  The C<ptr> becomes the first
character of the adjusted string.  Uses the "OOK hack".  On return, only
SvPOK(sv) and SvPOKp(sv) among the OK flags will be true.

Beware: after this function returns, C<ptr> and SvPVX_const(sv) may no longer
refer to the same chunk of data.

The unfortunate similarity of this function's name to that of Perl's C<chop>
operator is strictly coincidental.  This function works from the left;
C<chop> works from the right.

=cut
*/

void
Perl_sv_chop(pTHX_ register SV *const sv, register const char *const ptr)
{
    STRLEN delta;
    STRLEN old_delta;
    U8 *p;
#ifdef DEBUGGING
    const U8 *evacp;
    STRLEN evacn;
#endif
    STRLEN max_delta;

    PERL_ARGS_ASSERT_SV_CHOP;

    if (!ptr || !SvPOKp(sv))
        return;
    delta = ptr - SvPVX_const(sv);
    if (!delta) {
        /* Nothing to do.  */
        return;
    }
    max_delta = SvLEN(sv) ? SvLEN(sv) : SvCUR(sv);
    if (delta > max_delta)
        Perl_croak(aTHX_ "panic: sv_chop ptr=%p, start=%p, end=%p",
                   ptr, SvPVX_const(sv), SvPVX_const(sv) + max_delta);
    /* SvPVX(sv) may move in SV_CHECK_THINKFIRST(sv), so don't use ptr any more */
    SV_CHECK_THINKFIRST(sv);
    SvPOK_only_UTF8(sv);

    if (!SvOOK(sv)) {
        if (!SvLEN(sv)) { /* make copy of shared string */
            const char *pvx = SvPVX_const(sv);
            const STRLEN len = SvCUR(sv);
            SvGROW(sv, len + 1);
            Move(pvx,SvPVX(sv),len,char);
            *SvEND(sv) = '\0';
        }
        SvOOK_on(sv);
        old_delta = 0;
    } else {
        SvOOK_offset(sv, old_delta);
    }
    SvLEN_set(sv, SvLEN(sv) - delta);
    SvCUR_set(sv, SvCUR(sv) - delta);
    SvPV_set(sv, SvPVX(sv) + delta);

    p = (U8 *)SvPVX_const(sv);

#ifdef DEBUGGING
    /* how many bytes were evacuated?  we will fill them with sentinel
       bytes, except for the part holding the new offset of course. */
    evacn = delta;
    if (old_delta)
        evacn += (old_delta < 0x100 ? 1 : 1 + sizeof(STRLEN));
    assert(evacn);
    assert(evacn <= delta + old_delta);
    evacp = p - evacn;
#endif

    delta += old_delta;
    assert(delta);
    if (delta < 0x100) {
        *--p = (U8) delta;
    } else {
        *--p = 0;
        p -= sizeof(STRLEN);
        Copy((U8*)&delta, p, sizeof(STRLEN), U8);
    }

#ifdef DEBUGGING
    /* Fill the preceding buffer with sentinals to verify that no-one is
       using it.  */
    while (p > evacp) {
        --p;
        *p = (U8)PTR2UV(p);
    }
#endif
}

/*
=for apidoc sv_catpvn

Concatenates the string onto the end of the string which is in the SV.  The
C<len> indicates number of bytes to copy.  If the SV has the UTF-8
status set, then the bytes appended should be valid UTF-8.
Handles 'get' magic, but not 'set' magic.  See C<sv_catpvn_mg>.

=for apidoc sv_catpvn_flags

Concatenates the string onto the end of the string which is in the SV.  The
C<len> indicates number of bytes to copy.  If the SV has the UTF-8
status set, then the bytes appended should be valid UTF-8.
If C<flags> has the C<SV_SMAGIC> bit set, will
C<mg_set> on C<dsv> afterwards if appropriate.
C<sv_catpvn> and C<sv_catpvn_nomg> are implemented
in terms of this function.

=cut
*/

void
Perl_sv_catpvn_flags(pTHX_ register SV *const dsv, register const char *sstr, register const STRLEN slen, const I32 flags)
{
    dVAR;
    STRLEN dlen;
    const char * const dstr = SvPV_force_flags(dsv, dlen, flags);

    PERL_ARGS_ASSERT_SV_CATPVN_FLAGS;
    assert((flags & (SV_CATBYTES|SV_CATUTF8)) != (SV_CATBYTES|SV_CATUTF8));

    if (!(flags & SV_CATBYTES) || !SvUTF8(dsv)) {
      if (flags & SV_CATUTF8 && !SvUTF8(dsv)) {
         sv_utf8_upgrade_flags_grow(dsv, 0, slen + 1);
         dlen = SvCUR(dsv);
      }
      else SvGROW(dsv, dlen + slen + 1);
      if (sstr == dstr)
        sstr = SvPVX_const(dsv);
      Move(sstr, SvPVX(dsv) + dlen, slen, char);
      SvCUR_set(dsv, SvCUR(dsv) + slen);
    }
    else {
        /* We inline bytes_to_utf8, to avoid an extra malloc. */
        const char * const send = sstr + slen;
        U8 *d;

        /* Something this code does not account for, which I think is
           impossible; it would require the same pv to be treated as
           bytes *and* utf8, which would indicate a bug elsewhere. */
        assert(sstr != dstr);

        SvGROW(dsv, dlen + slen * 2 + 1);
        d = (U8 *)SvPVX(dsv) + dlen;

        while (sstr < send) {
            const UV uv = NATIVE_TO_ASCII((U8)*sstr++);
            if (UNI_IS_INVARIANT(uv))
                *d++ = (U8)UTF_TO_NATIVE(uv);
            else {
                *d++ = (U8)UTF8_EIGHT_BIT_HI(uv);
                *d++ = (U8)UTF8_EIGHT_BIT_LO(uv);
            }
        }
        SvCUR_set(dsv, d-(const U8 *)SvPVX(dsv));
    }
    *SvEND(dsv) = '\0';
    (void)SvPOK_only_UTF8(dsv);         /* validate pointer */
    SvTAINT(dsv);
    if (flags & SV_SMAGIC)
        SvSETMAGIC(dsv);
}

/*
=for apidoc sv_catsv

Concatenates the string from SV C<ssv> onto the end of the string in SV
C<dsv>.  If C<ssv> is null, does nothing; otherwise modifies only C<dsv>.
Handles 'get' magic on both SVs, but no 'set' magic.  See C<sv_catsv_mg> and
C<sv_catsv_nomg>.

=for apidoc sv_catsv_flags

Concatenates the string from SV C<ssv> onto the end of the string in SV
C<dsv>.  If C<ssv> is null, does nothing; otherwise modifies only C<dsv>.
If C<flags> include C<SV_GMAGIC> bit set, will call C<mg_get> on both SVs if
appropriate.  If C<flags> include C<SV_SMAGIC>, C<mg_set> will be called on
the modified SV afterward, if appropriate.  C<sv_catsv>, C<sv_catsv_nomg>,
and C<sv_catsv_mg> are implemented in terms of this function.

=cut */

void
Perl_sv_catsv_flags(pTHX_ SV *const dsv, register SV *const ssv, const I32 flags)
{
    dVAR;
 
    PERL_ARGS_ASSERT_SV_CATSV_FLAGS;

    if (ssv) {
        STRLEN slen;
        const char *spv = SvPV_flags_const(ssv, slen, flags);
        if (spv) {
            if (flags & SV_GMAGIC)
                SvGETMAGIC(dsv);
            sv_catpvn_flags(dsv, spv, slen,
                            DO_UTF8(ssv) ? SV_CATUTF8 : SV_CATBYTES);
            if (flags & SV_SMAGIC)
                SvSETMAGIC(dsv);
        }
    }
}

/*
=for apidoc sv_catpv

Concatenates the string onto the end of the string which is in the SV.
If the SV has the UTF-8 status set, then the bytes appended should be
valid UTF-8.  Handles 'get' magic, but not 'set' magic.  See C<sv_catpv_mg>.

=cut */

void
Perl_sv_catpv(pTHX_ register SV *const sv, register const char *ptr)
{
    dVAR;
    STRLEN len;
    STRLEN tlen;
    char *junk;

    PERL_ARGS_ASSERT_SV_CATPV;

    if (!ptr)
        return;
    junk = SvPV_force(sv, tlen);
    len = strlen(ptr);
    SvGROW(sv, tlen + len + 1);
    if (ptr == junk)
        ptr = SvPVX_const(sv);
    Move(ptr,SvPVX(sv)+tlen,len+1,char);
    SvCUR_set(sv, SvCUR(sv) + len);
    (void)SvPOK_only_UTF8(sv);          /* validate pointer */
    SvTAINT(sv);
}

/*
=for apidoc sv_catpv_flags

Concatenates the string onto the end of the string which is in the SV.
If the SV has the UTF-8 status set, then the bytes appended should
be valid UTF-8.  If C<flags> has the C<SV_SMAGIC> bit set, will C<mg_set>
on the modified SV if appropriate.

=cut
*/

void
Perl_sv_catpv_flags(pTHX_ SV *dstr, const char *sstr, const I32 flags)
{
    PERL_ARGS_ASSERT_SV_CATPV_FLAGS;
    sv_catpvn_flags(dstr, sstr, strlen(sstr), flags);
}

/*
=for apidoc sv_catpv_mg

Like C<sv_catpv>, but also handles 'set' magic.

=cut
*/

void
Perl_sv_catpv_mg(pTHX_ register SV *const sv, register const char *const ptr)
{
    PERL_ARGS_ASSERT_SV_CATPV_MG;

    sv_catpv(sv,ptr);
    SvSETMAGIC(sv);
}

/*
=for apidoc newSV

Creates a new SV.  A non-zero C<len> parameter indicates the number of
bytes of preallocated string space the SV should have.  An extra byte for a
trailing NUL is also reserved.  (SvPOK is not set for the SV even if string
space is allocated.)  The reference count for the new SV is set to 1.

In 5.9.3, newSV() replaces the older NEWSV() API, and drops the first
parameter, I<x>, a debug aid which allowed callers to identify themselves.
This aid has been superseded by a new build option, PERL_MEM_LOG (see
L<perlhacktips/PERL_MEM_LOG>).  The older API is still there for use in XS
modules supporting older perls.

=cut
*/

SV *
Perl_newSV(pTHX_ const STRLEN len)
{
    dVAR;
    SV *sv;

    new_SV(sv);
    if (len) {
        sv_upgrade(sv, SVt_PV);
        SvGROW(sv, len + 1);
    }
    return sv;
}
/*
=for apidoc sv_magicext

Adds magic to an SV, upgrading it if necessary.  Applies the
supplied vtable and returns a pointer to the magic added.

Note that C<sv_magicext> will allow things that C<sv_magic> will not.
In particular, you can add magic to SvREADONLY SVs, and add more than
one instance of the same 'how'.

If C<namlen> is greater than zero then a C<savepvn> I<copy> of C<name> is
stored, if C<namlen> is zero then C<name> is stored as-is and - as another
special case - if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
to contain an C<SV*> and is stored as-is with its REFCNT incremented.

(This is now used as a subroutine by C<sv_magic>.)

=cut
*/
MAGIC * 
Perl_sv_magicext(pTHX_ SV *const sv, SV *const obj, const int how, 
                const MGVTBL *const vtable, const char *const name, const I32 namlen)
{
    dVAR;
    MAGIC* mg;

    PERL_ARGS_ASSERT_SV_MAGICEXT;

    SvUPGRADE(sv, SVt_PVMG);
    Newxz(mg, 1, MAGIC);
    mg->mg_moremagic = SvMAGIC(sv);
    SvMAGIC_set(sv, mg);

    /* Sometimes a magic contains a reference loop, where the sv and
       object refer to each other.  To prevent a reference loop that
       would prevent such objects being freed, we look for such loops
       and if we find one we avoid incrementing the object refcount.

       Note we cannot do this to avoid self-tie loops as intervening RV must
       have its REFCNT incremented to keep it in existence.

    */
    if (!obj || obj == sv ||
        how == PERL_MAGIC_arylen ||
        how == PERL_MAGIC_symtab ||
        (SvTYPE(obj) == SVt_PVGV &&
            (GvSV(obj) == sv || GvHV(obj) == (const HV *)sv
             || GvAV(obj) == (const AV *)sv || GvCV(obj) == (const CV *)sv
             || GvIOp(obj) == (const IO *)sv || GvFORM(obj) == (const CV *)sv)))
    {
        mg->mg_obj = obj;
    }
    else {
        mg->mg_obj = SvREFCNT_inc_simple(obj);
        mg->mg_flags |= MGf_REFCOUNTED;
    }

    /* Normal self-ties simply pass a null object, and instead of
       using mg_obj directly, use the SvTIED_obj macro to produce a
       new RV as needed.  For glob "self-ties", we are tieing the PVIO
       with an RV obj pointing to the glob containing the PVIO.  In
       this case, to avoid a reference loop, we need to weaken the
       reference.
    */

    if (how == PERL_MAGIC_tiedscalar && SvTYPE(sv) == SVt_PVIO &&
        obj && SvROK(obj) && GvIO(SvRV(obj)) == (const IO *)sv)
    {
      sv_rvweaken(obj);
    }

    mg->mg_type = how;
    mg->mg_len = namlen;
    if (name) {
        if (namlen > 0)
            mg->mg_ptr = savepvn(name, namlen);
        else if (namlen == HEf_SVKEY) {
            /* Yes, this is casting away const. This is only for the case of
               HEf_SVKEY. I think we need to document this aberation of the
               constness of the API, rather than making name non-const, as
               that change propagating outwards a long way.  */
            mg->mg_ptr = (char*)SvREFCNT_inc_simple_NN((SV *)name);
        } else
            mg->mg_ptr = (char *) name;
    }
    mg->mg_virtual = (MGVTBL *) vtable;

    mg_magical(sv);
    return mg;
}

/*
=for apidoc sv_magic

Adds magic to an SV.  First upgrades C<sv> to type C<SVt_PVMG> if
necessary, then adds a new magic item of type C<how> to the head of the
magic list.

See C<sv_magicext> (which C<sv_magic> now calls) for a description of the
handling of the C<name> and C<namlen> arguments.

You need to use C<sv_magicext> to add magic to SvREADONLY SVs and also
to add more than one instance of the same 'how'.

=cut
*/

void
Perl_sv_magic(pTHX_ register SV *const sv, SV *const obj, const int how, 
             const char *const name, const I32 namlen)
{
    dVAR;
    const MGVTBL *vtable;
    MAGIC* mg;
    unsigned int flags;
    unsigned int vtable_index;

    PERL_ARGS_ASSERT_SV_MAGIC;

    if (how < 0 || (unsigned)how > C_ARRAY_LENGTH(PL_magic_data)
        || ((flags = PL_magic_data[how]),
            (vtable_index = flags & PERL_MAGIC_VTABLE_MASK)
            > magic_vtable_max))
        Perl_croak(aTHX_ "Don't know how to handle magic of type \\%o", how);

    /* PERL_MAGIC_ext is reserved for use by extensions not perl internals.
       Useful for attaching extension internal data to perl vars.
       Note that multiple extensions may clash if magical scalars
       etc holding private data from one are passed to another. */

    vtable = (vtable_index == magic_vtable_max)
        ? NULL : PL_magic_vtables + vtable_index;

#ifdef PERL_OLD_COPY_ON_WRITE
    if (SvIsCOW(sv))
        sv_force_normal_flags(sv, 0);
#endif
    if (SvREADONLY(sv)) {
        if (
            /* its okay to attach magic to shared strings */
            !SvIsCOW(sv)

            && IN_PERL_RUNTIME
            && !PERL_MAGIC_TYPE_READONLY_ACCEPTABLE(how)
           )
        {
            Perl_croak_no_modify();
        }
    }
    if (SvMAGICAL(sv) || (how == PERL_MAGIC_taint && SvTYPE(sv) >= SVt_PVMG)) {
        if (SvMAGIC(sv) && (mg = mg_find(sv, how))) {
            /* sv_magic() refuses to add a magic of the same 'how' as an
               existing one
             */
            if (how == PERL_MAGIC_taint)
                mg->mg_len |= 1;
            return;
        }
    }

    /* Rest of work is done else where */
    mg = sv_magicext(sv,obj,how,vtable,name,namlen);

    switch (how) {
    case PERL_MAGIC_taint:
        mg->mg_len = 1;
        break;
    case PERL_MAGIC_ext:
    case PERL_MAGIC_dbfile:
        SvRMAGICAL_on(sv);
        break;
    }
}

static int
S_sv_unmagicext_flags(pTHX_ SV *const sv, const int type, MGVTBL *vtbl, const U32 flags)
{
    MAGIC* mg;
    MAGIC** mgp;

    assert(flags <= 1);

    if (SvTYPE(sv) < SVt_PVMG || !SvMAGIC(sv))
        return 0;
    mgp = &(((XPVMG*) SvANY(sv))->xmg_u.xmg_magic);
    for (mg = *mgp; mg; mg = *mgp) {
        const MGVTBL* const virt = mg->mg_virtual;
        if (mg->mg_type == type && (!flags || virt == vtbl)) {
            *mgp = mg->mg_moremagic;
            if (virt && virt->svt_free)
                virt->svt_free(aTHX_ sv, mg);
            if (mg->mg_ptr && mg->mg_type != PERL_MAGIC_regex_global) {
                if (mg->mg_len > 0)
                    Safefree(mg->mg_ptr);
                else if (mg->mg_len == HEf_SVKEY)
                    SvREFCNT_dec(MUTABLE_SV(mg->mg_ptr));
                else if (mg->mg_type == PERL_MAGIC_utf8)
                    Safefree(mg->mg_ptr);
            }
            if (mg->mg_flags & MGf_REFCOUNTED)
                SvREFCNT_dec(mg->mg_obj);
            Safefree(mg);
        }
        else
            mgp = &mg->mg_moremagic;
    }
    if (SvMAGIC(sv)) {
        if (SvMAGICAL(sv))      /* if we're under save_magic, wait for restore_magic; */
            mg_magical(sv);     /*    else fix the flags now */
    }
    else {
        SvMAGICAL_off(sv);
        SvFLAGS(sv) |= (SvFLAGS(sv) & (SVp_IOK|SVp_NOK|SVp_POK)) >> PRIVSHIFT;
    }
    return 0;
}

/*
=for apidoc sv_unmagic

Removes all magic of type C<type> from an SV.

=cut
*/

int
Perl_sv_unmagic(pTHX_ SV *const sv, const int type)
{
    PERL_ARGS_ASSERT_SV_UNMAGIC;
    return S_sv_unmagicext_flags(aTHX_ sv, type, NULL, 0);
}

/*
=for apidoc sv_unmagicext

Removes all magic of type C<type> with the specified C<vtbl> from an SV.

=cut
*/

int
Perl_sv_unmagicext(pTHX_ SV *const sv, const int type, MGVTBL *vtbl)
{
    PERL_ARGS_ASSERT_SV_UNMAGICEXT;
    return S_sv_unmagicext_flags(aTHX_ sv, type, vtbl, 1);
}

/*
=for apidoc sv_rvweaken

Weaken a reference: set the C<SvWEAKREF> flag on this RV; give the
referred-to SV C<PERL_MAGIC_backref> magic if it hasn't already; and
push a back-reference to this RV onto the array of backreferences
associated with that magic.  If the RV is magical, set magic will be
called after the RV is cleared.

=cut
*/

SV *
Perl_sv_rvweaken(pTHX_ SV *const sv)
{
    SV *tsv;

    PERL_ARGS_ASSERT_SV_RVWEAKEN;

    if (!SvOK(sv))  /* let undefs pass */
        return sv;
    if (!SvROK(sv))
        Perl_croak(aTHX_ "Can't weaken a nonreference");
    else if (SvWEAKREF(sv)) {
        Perl_ck_warner(aTHX_ packWARN(WARN_MISC), "Reference is already weak");
        return sv;
    }
    else if (SvREADONLY(sv)) croak_no_modify();
    tsv = SvRV(sv);
    Perl_sv_add_backref(aTHX_ tsv, sv);
    SvWEAKREF_on(sv);
    SvREFCNT_dec(tsv);
    return sv;
}

/* Give tsv backref magic if it hasn't already got it, then push a
 * back-reference to sv onto the array associated with the backref magic.
 *
 * As an optimisation, if there's only one backref and it's not an AV,
 * store it directly in the HvAUX or mg_obj slot, avoiding the need to
 * allocate an AV. (Whether the slot holds an AV tells us whether this is
 * active.)
 */

/* A discussion about the backreferences array and its refcount:
 *
 * The AV holding the backreferences is pointed to either as the mg_obj of
 * PERL_MAGIC_backref, or in the specific case of a HV, from the
 * xhv_backreferences field. The array is created with a refcount
 * of 2. This means that if during global destruction the array gets
 * picked on before its parent to have its refcount decremented by the
 * random zapper, it won't actually be freed, meaning it's still there for
 * when its parent gets freed.
 *
 * When the parent SV is freed, the extra ref is killed by
 * Perl_sv_kill_backrefs.  The other ref is killed, in the case of magic,
 * by mg_free() / MGf_REFCOUNTED, or for a hash, by Perl_hv_kill_backrefs.
 *
 * When a single backref SV is stored directly, it is not reference
 * counted.
 */

void
Perl_sv_add_backref(pTHX_ SV *const tsv, SV *const sv)
{
    dVAR;
    SV **svp;
    AV *av = NULL;
    MAGIC *mg = NULL;

    PERL_ARGS_ASSERT_SV_ADD_BACKREF;

    /* find slot to store array or singleton backref */

    if (SvTYPE(tsv) == SVt_PVHV) {
        svp = (SV**)Perl_hv_backreferences_p(aTHX_ MUTABLE_HV(tsv));
    } else {
        if (! ((mg =
            (SvMAGICAL(tsv) ? mg_find(tsv, PERL_MAGIC_backref) : NULL))))
        {
            sv_magic(tsv, NULL, PERL_MAGIC_backref, NULL, 0);
            mg = mg_find(tsv, PERL_MAGIC_backref);
        }
        svp = &(mg->mg_obj);
    }

    /* create or retrieve the array */

    if (   (!*svp && SvTYPE(sv) == SVt_PVAV)
        || (*svp && SvTYPE(*svp) != SVt_PVAV)
    ) {
        /* create array */
        av = newAV();
        AvREAL_off(av);
        SvREFCNT_inc_simple_void(av);
        /* av now has a refcnt of 2; see discussion above */
        if (*svp) {
            /* move single existing backref to the array */
            av_extend(av, 1);
            AvARRAY(av)[++AvFILLp(av)] = *svp; /* av_push() */
        }
        *svp = (SV*)av;
        if (mg)
            mg->mg_flags |= MGf_REFCOUNTED;
    }
    else
        av = MUTABLE_AV(*svp);

    if (!av) {
        /* optimisation: store single backref directly in HvAUX or mg_obj */
        *svp = sv;
        return;
    }
    /* push new backref */
    assert(SvTYPE(av) == SVt_PVAV);
    if (AvFILLp(av) >= AvMAX(av)) {
        av_extend(av, AvFILLp(av)+1);
    }
    AvARRAY(av)[++AvFILLp(av)] = sv; /* av_push() */
}

/* delete a back-reference to ourselves from the backref magic associated
 * with the SV we point to.
 */

void
Perl_sv_del_backref(pTHX_ SV *const tsv, SV *const sv)
{
    dVAR;
    SV **svp = NULL;

    PERL_ARGS_ASSERT_SV_DEL_BACKREF;

    if (SvTYPE(tsv) == SVt_PVHV) {
        if (SvOOK(tsv))
            svp = (SV**)Perl_hv_backreferences_p(aTHX_ MUTABLE_HV(tsv));
    }
    else if (SvIS_FREED(tsv) && PL_phase == PERL_PHASE_DESTRUCT) {
        /* It's possible for the the last (strong) reference to tsv to have
           become freed *before* the last thing holding a weak reference.
           If both survive longer than the backreferences array, then when
           the referent's reference count drops to 0 and it is freed, it's
           not able to chase the backreferences, so they aren't NULLed.

           For example, a CV holds a weak reference to its stash. If both the
           CV and the stash survive longer than the backreferences array,
           and the CV gets picked for the SvBREAK() treatment first,
           *and* it turns out that the stash is only being kept alive because
           of an our variable in the pad of the CV, then midway during CV
           destruction the stash gets freed, but CvSTASH() isn't set to NULL.
           It ends up pointing to the freed HV. Hence it's chased in here, and
           if this block wasn't here, it would hit the !svp panic just below.

           I don't believe that "better" destruction ordering is going to help
           here - during global destruction there's always going to be the
           chance that something goes out of order. We've tried to make it
           foolproof before, and it only resulted in evolutionary pressure on
           fools. Which made us look foolish for our hubris. :-(
        */
        return;
    }
    else {
        MAGIC *const mg
            = SvMAGICAL(tsv) ? mg_find(tsv, PERL_MAGIC_backref) : NULL;
        svp =  mg ? &(mg->mg_obj) : NULL;
    }

    if (!svp)
        Perl_croak(aTHX_ "panic: del_backref, svp=0");
    if (!*svp) {
        /* It's possible that sv is being freed recursively part way through the
           freeing of tsv. If this happens, the backreferences array of tsv has
           already been freed, and so svp will be NULL. If this is the case,
           we should not panic. Instead, nothing needs doing, so return.  */
        if (PL_phase == PERL_PHASE_DESTRUCT && SvREFCNT(tsv) == 0)
            return;
        Perl_croak(aTHX_ "panic: del_backref, *svp=%p phase=%s refcnt=%" UVuf,
                   *svp, PL_phase_names[PL_phase], (UV)SvREFCNT(tsv));
    }

    if (SvTYPE(*svp) == SVt_PVAV) {
#ifdef DEBUGGING
        int count = 1;
#endif
        AV * const av = (AV*)*svp;
        SSize_t fill;
        assert(!SvIS_FREED(av));
        fill = AvFILLp(av);
        assert(fill > -1);
        svp = AvARRAY(av);
        /* for an SV with N weak references to it, if all those
         * weak refs are deleted, then sv_del_backref will be called
         * N times and O(N^2) compares will be done within the backref
         * array. To ameliorate this potential slowness, we:
         * 1) make sure this code is as tight as possible;
         * 2) when looking for SV, look for it at both the head and tail of the
         *    array first before searching the rest, since some create/destroy
         *    patterns will cause the backrefs to be freed in order.
         */
        if (*svp == sv) {
            AvARRAY(av)++;
            AvMAX(av)--;
        }
        else {
            SV **p = &svp[fill];
            SV *const topsv = *p;
            if (topsv != sv) {
#ifdef DEBUGGING
                count = 0;
#endif
                while (--p > svp) {
                    if (*p == sv) {
                        /* We weren't the last entry.
                           An unordered list has this property that you
                           can take the last element off the end to fill
                           the hole, and it's still an unordered list :-)
                        */
                        *p = topsv;
#ifdef DEBUGGING
                        count++;
#else
                        break; /* should only be one */
#endif
                    }
                }
            }
        }
        assert(count ==1);
        AvFILLp(av) = fill-1;
    }
    else if (SvIS_FREED(*svp) && PL_phase == PERL_PHASE_DESTRUCT) {
        /* freed AV; skip */
    }
    else {
        /* optimisation: only a single backref, stored directly */
        if (*svp != sv)
            Perl_croak(aTHX_ "panic: del_backref, *svp=%p, sv=%p", *svp, sv);
        *svp = NULL;
    }

}

void
Perl_sv_kill_backrefs(pTHX_ SV *const sv, AV *const av)
{
    SV **svp;
    SV **last;
    bool is_array;

    PERL_ARGS_ASSERT_SV_KILL_BACKREFS;

    if (!av)
        return;

    /* after multiple passes through Perl_sv_clean_all() for a thingy
     * that has badly leaked, the backref array may have gotten freed,
     * since we only protect it against 1 round of cleanup */
    if (SvIS_FREED(av)) {
        if (PL_in_clean_all) /* All is fair */
            return;
        Perl_croak(aTHX_
                   "panic: magic_killbackrefs (freed backref AV/SV)");
    }


    is_array = (SvTYPE(av) == SVt_PVAV);
    if (is_array) {
        assert(!SvIS_FREED(av));
        svp = AvARRAY(av);
        if (svp)
            last = svp + AvFILLp(av);
    }
    else {
        /* optimisation: only a single backref, stored directly */
        svp = (SV**)&av;
        last = svp;
    }

    if (svp) {
        while (svp <= last) {
            if (*svp) {
                SV *const referrer = *svp;
                if (SvWEAKREF(referrer)) {
                    /* XXX Should we check that it hasn't changed? */
                    assert(SvROK(referrer));
                    SvRV_set(referrer, 0);
                    SvOK_off(referrer);
                    SvWEAKREF_off(referrer);
                    SvSETMAGIC(referrer);
                } else if (SvTYPE(referrer) == SVt_PVGV ||
                           SvTYPE(referrer) == SVt_PVLV) {
                    assert(SvTYPE(sv) == SVt_PVHV); /* stash backref */
                    /* You lookin' at me?  */
                    assert(GvSTASH(referrer));
                    assert(GvSTASH(referrer) == (const HV *)sv);
                    GvSTASH(referrer) = 0;
                } else if (SvTYPE(referrer) == SVt_PVCV ||
                           SvTYPE(referrer) == SVt_PVFM) {
                    if (SvTYPE(sv) == SVt_PVHV) { /* stash backref */
                        /* You lookin' at me?  */
                        assert(CvSTASH(referrer));
                        assert(CvSTASH(referrer) == (const HV *)sv);
                        SvANY(MUTABLE_CV(referrer))->xcv_stash = 0;
                    }
                    else {
                        assert(SvTYPE(sv) == SVt_PVGV);
                        /* You lookin' at me?  */
                        assert(CvGV(referrer));
                        assert(CvGV(referrer) == (const GV *)sv);
                        anonymise_cv_maybe(MUTABLE_GV(sv),
                                                MUTABLE_CV(referrer));
                    }

                } else {
                    Perl_croak(aTHX_
                               "panic: magic_killbackrefs (flags=%"UVxf")",
                               (UV)SvFLAGS(referrer));
                }

                if (is_array)
                    *svp = NULL;
            }
            svp++;
        }
    }
    if (is_array) {
        AvFILLp(av) = -1;
        SvREFCNT_dec(av); /* remove extra count added by sv_add_backref() */
    }
    return;
}

/*
=for apidoc sv_insert

Inserts a string at the specified offset/length within the SV.  Similar to
the Perl substr() function.  Handles get magic.

=for apidoc sv_insert_flags

Same as C<sv_insert>, but the extra C<flags> are passed to the
C<SvPV_force_flags> that applies to C<bigstr>.

=cut
*/

void
Perl_sv_insert_flags(pTHX_ SV *const bigstr, const STRLEN offset, const STRLEN len, const char *const little, const STRLEN littlelen, const U32 flags)
{
    dVAR;
    char *big;
    char *mid;
    char *midend;
    char *bigend;
    SSize_t i;          /* better be sizeof(STRLEN) or bad things happen */
    STRLEN curlen;

    PERL_ARGS_ASSERT_SV_INSERT_FLAGS;

    if (!bigstr)
        Perl_croak(aTHX_ "Can't modify nonexistent substring");
    SvPV_force_flags(bigstr, curlen, flags);
    (void)SvPOK_only_UTF8(bigstr);
    if (offset + len > curlen) {
        SvGROW(bigstr, offset+len+1);
        Zero(SvPVX(bigstr)+curlen, offset+len-curlen, char);
        SvCUR_set(bigstr, offset+len);
    }

    SvTAINT(bigstr);
    i = littlelen - len;
    if (i > 0) {                        /* string might grow */
        big = SvGROW(bigstr, SvCUR(bigstr) + i + 1);
        mid = big + offset + len;
        midend = bigend = big + SvCUR(bigstr);
        bigend += i;
        *bigend = '\0';
        while (midend > mid)            /* shove everything down */
            *--bigend = *--midend;
        Move(little,big+offset,littlelen,char);
        SvCUR_set(bigstr, SvCUR(bigstr) + i);
        SvSETMAGIC(bigstr);
        return;
    }
    else if (i == 0) {
        Move(little,SvPVX(bigstr)+offset,len,char);
        SvSETMAGIC(bigstr);
        return;
    }

    big = SvPVX(bigstr);
    mid = big + offset;
    midend = mid + len;
    bigend = big + SvCUR(bigstr);

    if (midend > bigend)
        Perl_croak(aTHX_ "panic: sv_insert, midend=%p, bigend=%p",
                   midend, bigend);

    if (mid - big > bigend - midend) {  /* faster to shorten from end */
        if (littlelen) {
            Move(little, mid, littlelen,char);
            mid += littlelen;
        }
        i = bigend - midend;
        if (i > 0) {
            Move(midend, mid, i,char);
            mid += i;
        }
        *mid = '\0';
        SvCUR_set(bigstr, mid - big);
    }
    else if ((i = mid - big)) { /* faster from front */
        midend -= littlelen;
        mid = midend;
        Move(big, midend - i, i, char);
        sv_chop(bigstr,midend-i);
        if (littlelen)
            Move(little, mid, littlelen,char);
    }
    else if (littlelen) {
        midend -= littlelen;
        sv_chop(bigstr,midend);
        Move(little,midend,littlelen,char);
    }
    else {
        sv_chop(bigstr,midend);
    }
    SvSETMAGIC(bigstr);
}

/*
=for apidoc sv_replace

Make the first argument a copy of the second, then delete the original.
The target SV physically takes over ownership of the body of the source SV
and inherits its flags; however, the target keeps any magic it owns,
and any magic in the source is discarded.
Note that this is a rather specialist SV copying operation; most of the
time you'll want to use C<sv_setsv> or one of its many macro front-ends.

=cut
*/

void
Perl_sv_replace(pTHX_ register SV *const sv, register SV *const nsv)
{
    dVAR;
    const U32 refcnt = SvREFCNT(sv);

    PERL_ARGS_ASSERT_SV_REPLACE;

    SV_CHECK_THINKFIRST_COW_DROP(sv);
    if (SvREFCNT(nsv) != 1) {
        Perl_croak(aTHX_ "panic: reference miscount on nsv in sv_replace()"
                   " (%" UVuf " != 1)", (UV) SvREFCNT(nsv));
    }
    if (SvMAGICAL(sv)) {
        if (SvMAGICAL(nsv))
            mg_free(nsv);
        else
            sv_upgrade(nsv, SVt_PVMG);
        SvMAGIC_set(nsv, SvMAGIC(sv));
        SvFLAGS(nsv) |= SvMAGICAL(sv);
        SvMAGICAL_off(sv);
        SvMAGIC_set(sv, NULL);
    }
    SvREFCNT(sv) = 0;
    sv_clear(sv);
    assert(!SvREFCNT(sv));
#ifdef DEBUG_LEAKING_SCALARS
    sv->sv_flags  = nsv->sv_flags;
    sv->sv_any    = nsv->sv_any;
    sv->sv_refcnt = nsv->sv_refcnt;
    sv->sv_u      = nsv->sv_u;
#else
    StructCopy(nsv,sv,SV);
#endif
    if(SvTYPE(sv) == SVt_IV) {
        SvANY(sv)
            = (XPVIV*)((char*)&(sv->sv_u.svu_iv) - STRUCT_OFFSET(XPVIV, xiv_iv));
    }
        

#ifdef PERL_OLD_COPY_ON_WRITE
    if (SvIsCOW_normal(nsv)) {
        /* We need to follow the pointers around the loop to make the
           previous SV point to sv, rather than nsv.  */
        SV *next;
        SV *current = nsv;
        while ((next = SV_COW_NEXT_SV(current)) != nsv) {
            assert(next);
            current = next;
            assert(SvPVX_const(current) == SvPVX_const(nsv));
        }
        /* Make the SV before us point to the SV after us.  */
        if (DEBUG_C_TEST) {
            PerlIO_printf(Perl_debug_log, "previous is\n");
            sv_dump(current);
            PerlIO_printf(Perl_debug_log,
                          "move it from 0x%"UVxf" to 0x%"UVxf"\n",
                          (UV) SV_COW_NEXT_SV(current), (UV) sv);
        }
        SV_COW_NEXT_SV_SET(current, sv);
    }
#endif
    SvREFCNT(sv) = refcnt;
    SvFLAGS(nsv) |= SVTYPEMASK;         /* Mark as freed */
    SvREFCNT(nsv) = 0;
    del_SV(nsv);
}

/* We're about to free a GV which has a CV that refers back to us.
 * If that CV will outlive us, make it anonymous (i.e. fix up its CvGV
 * field) */

STATIC void
S_anonymise_cv_maybe(pTHX_ GV *gv, CV* cv)
{
    SV *gvname;
    GV *anongv;

    PERL_ARGS_ASSERT_ANONYMISE_CV_MAYBE;

    /* be assertive! */
    assert(SvREFCNT(gv) == 0);
    assert(isGV(gv) && isGV_with_GP(gv));
    assert(GvGP(gv));
    assert(!CvANON(cv));
    assert(CvGV(cv) == gv);
    assert(!CvNAMED(cv));

    /* will the CV shortly be freed by gp_free() ? */
    if (GvCV(gv) == cv && GvGP(gv)->gp_refcnt < 2 && SvREFCNT(cv) < 2) {
        SvANY(cv)->xcv_gv_u.xcv_gv = NULL;
        return;
    }

    /* if not, anonymise: */
    gvname = (GvSTASH(gv) && HvNAME(GvSTASH(gv)) && HvENAME(GvSTASH(gv)))
                    ? newSVhek(HvENAME_HEK(GvSTASH(gv)))
                    : newSVpvn_flags( "__ANON__", 8, 0 );
    sv_catpvs(gvname, "::__ANON__");
    anongv = gv_fetchsv(gvname, GV_ADDMULTI, SVt_PVCV);
    SvREFCNT_dec(gvname);

    CvANON_on(cv);
    CvCVGV_RC_on(cv);
    SvANY(cv)->xcv_gv_u.xcv_gv = MUTABLE_GV(SvREFCNT_inc(anongv));
}


/*
=for apidoc sv_clear

Clear an SV: call any destructors, free up any memory used by the body,
and free the body itself.  The SV's head is I<not> freed, although
its type is set to all 1's so that it won't inadvertently be assumed
to be live during global destruction etc.
This function should only be called when REFCNT is zero.  Most of the time
you'll want to call C<sv_free()> (or its macro wrapper C<SvREFCNT_dec>)
instead.

=cut
*/

void
Perl_sv_clear(pTHX_ SV *const orig_sv)
{
    dVAR;
    HV *stash;
    U32 type;
    const struct body_details *sv_type_details;
    SV* iter_sv = NULL;
    SV* next_sv = NULL;
    SV *sv = orig_sv;
    STRLEN hash_index;

    PERL_ARGS_ASSERT_SV_CLEAR;

    /* within this loop, sv is the SV currently being freed, and
     * iter_sv is the most recent AV or whatever that's being iterated
     * over to provide more SVs */

    while (sv) {

        type = SvTYPE(sv);

        assert(SvREFCNT(sv) == 0);
        assert(SvTYPE(sv) != (svtype)SVTYPEMASK);

        if (type <= SVt_IV) {
            /* See the comment in sv.h about the collusion between this
             * early return and the overloading of the NULL slots in the
             * size table.  */
            if (SvROK(sv))
                goto free_rv;
            SvFLAGS(sv) &= SVf_BREAK;
            SvFLAGS(sv) |= SVTYPEMASK;
            goto free_head;
        }

        assert(!SvOBJECT(sv) || type >= SVt_PVMG); /* objs are always >= MG */

        if (type >= SVt_PVMG) {
            if (SvOBJECT(sv)) {
                if (!curse(sv, 1)) goto get_next_sv;
                type = SvTYPE(sv); /* destructor may have changed it */
            }
            /* Free back-references before magic, in case the magic calls
             * Perl code that has weak references to sv. */
            if (type == SVt_PVHV) {
                Perl_hv_kill_backrefs(aTHX_ MUTABLE_HV(sv));
                if (SvMAGIC(sv))
                    mg_free(sv);
            }
            else if (type == SVt_PVMG && SvPAD_OUR(sv)) {
                SvREFCNT_dec(SvOURSTASH(sv));
            } else if (SvMAGIC(sv)) {
                /* Free back-references before other types of magic. */
                sv_unmagic(sv, PERL_MAGIC_backref);
                mg_free(sv);
            }
            SvMAGICAL_off(sv);
            if (type == SVt_PVMG && SvPAD_TYPED(sv))
                SvREFCNT_dec(SvSTASH(sv));
        }
        switch (type) {
            /* case SVt_BIND: */
        case SVt_PVIO:
            if (IoIFP(sv) &&
                IoIFP(sv) != PerlIO_stdin() &&
                IoIFP(sv) != PerlIO_stdout() &&
                IoIFP(sv) != PerlIO_stderr() &&
                !(IoFLAGS(sv) & IOf_FAKE_DIRP))
            {
                io_close(MUTABLE_IO(sv), FALSE);
            }
            if (IoDIRP(sv) && !(IoFLAGS(sv) & IOf_FAKE_DIRP))
                PerlDir_close(IoDIRP(sv));
            IoDIRP(sv) = (DIR*)NULL;
            Safefree(IoTOP_NAME(sv));
            Safefree(IoFMT_NAME(sv));
            Safefree(IoBOTTOM_NAME(sv));
            if ((const GV *)sv == PL_statgv)
                PL_statgv = NULL;
            goto freescalar;
        case SVt_REGEXP:
            /* FIXME for plugins */
          freeregexp:
            pregfree2((REGEXP*) sv);
            goto freescalar;
        case SVt_PVCV:
        case SVt_PVFM:
            cv_undef(MUTABLE_CV(sv));
            /* If we're in a stash, we don't own a reference to it.
             * However it does have a back reference to us, which needs to
             * be cleared.  */
            if ((stash = CvSTASH(sv)))
                sv_del_backref(MUTABLE_SV(stash), sv);
            goto freescalar;
        case SVt_PVHV:
            if (PL_last_swash_hv == (const HV *)sv) {
                PL_last_swash_hv = NULL;
            }
            if (HvTOTALKEYS((HV*)sv) > 0) {
                const char *name;
                /* this statement should match the one at the beginning of
                 * hv_undef_flags() */
                if (   PL_phase != PERL_PHASE_DESTRUCT
                    && (name = HvNAME((HV*)sv)))
                {
                    if (PL_stashcache) {
                    DEBUG_o(Perl_deb(aTHX_ "sv_clear clearing PL_stashcache for '%"SVf"'\n",
                                     sv));
                        (void)hv_delete(PL_stashcache, name,
                            HvNAMEUTF8((HV*)sv) ? -HvNAMELEN_get((HV*)sv) : HvNAMELEN_get((HV*)sv), G_DISCARD);
                    }
                    hv_name_set((HV*)sv, NULL, 0, 0);
                }

                /* save old iter_sv in unused SvSTASH field */
                assert(!SvOBJECT(sv));
                SvSTASH(sv) = (HV*)iter_sv;
                iter_sv = sv;

                /* save old hash_index in unused SvMAGIC field */
                assert(!SvMAGICAL(sv));
                assert(!SvMAGIC(sv));
                ((XPVMG*) SvANY(sv))->xmg_u.xmg_hash_index = hash_index;
                hash_index = 0;

                next_sv = Perl_hfree_next_entry(aTHX_ (HV*)sv, &hash_index);
                goto get_next_sv; /* process this new sv */
            }
            /* free empty hash */
            Perl_hv_undef_flags(aTHX_ MUTABLE_HV(sv), HV_NAME_SETALL);
            assert(!HvARRAY((HV*)sv));
            break;
        case SVt_PVAV:
            {
                AV* av = MUTABLE_AV(sv);
                if (PL_comppad == av) {
                    PL_comppad = NULL;
                    PL_curpad = NULL;
                }
                if (AvREAL(av) && AvFILLp(av) > -1) {
                    next_sv = AvARRAY(av)[AvFILLp(av)--];
                    /* save old iter_sv in top-most slot of AV,
                     * and pray that it doesn't get wiped in the meantime */
                    AvARRAY(av)[AvMAX(av)] = iter_sv;
                    iter_sv = sv;
                    goto get_next_sv; /* process this new sv */
                }
                Safefree(AvALLOC(av));
            }

            break;
        case SVt_PVLV:
            if (LvTYPE(sv) == 'T') { /* for tie: return HE to pool */
                SvREFCNT_dec(HeKEY_sv((HE*)LvTARG(sv)));
                HeNEXT((HE*)LvTARG(sv)) = PL_hv_fetch_ent_mh;
                PL_hv_fetch_ent_mh = (HE*)LvTARG(sv);
            }
            else if (LvTYPE(sv) != 't') /* unless tie: unrefcnted fake SV**  */
                SvREFCNT_dec(LvTARG(sv));
            if (isREGEXP(sv)) goto freeregexp;
        case SVt_PVGV:
            if (isGV_with_GP(sv)) {
                if(GvCVu((const GV *)sv) && (stash = GvSTASH(MUTABLE_GV(sv)))
                   && HvENAME_get(stash))
                    mro_method_changed_in(stash);
                gp_free(MUTABLE_GV(sv));
                if (GvNAME_HEK(sv))
                    unshare_hek(GvNAME_HEK(sv));
                /* If we're in a stash, we don't own a reference to it.
                 * However it does have a back reference to us, which
                 * needs to be cleared.  */
                if (!SvVALID(sv) && (stash = GvSTASH(sv)))
                        sv_del_backref(MUTABLE_SV(stash), sv);
            }
            /* FIXME. There are probably more unreferenced pointers to SVs
             * in the interpreter struct that we should check and tidy in
             * a similar fashion to this:  */
            /* See also S_sv_unglob, which does the same thing. */
            if ((const GV *)sv == PL_last_in_gv)
                PL_last_in_gv = NULL;
            else if ((const GV *)sv == PL_statgv)
                PL_statgv = NULL;
        case SVt_PVMG:
        case SVt_PVNV:
        case SVt_PVIV:
        case SVt_PV:
          freescalar:
            /* Don't bother with SvOOK_off(sv); as we're only going to
             * free it.  */
            if (SvOOK(sv)) {
                STRLEN offset;
                SvOOK_offset(sv, offset);
                SvPV_set(sv, SvPVX_mutable(sv) - offset);
                /* Don't even bother with turning off the OOK flag.  */
            }
            if (SvROK(sv)) {
            free_rv:
                {
                    SV * const target = SvRV(sv);
                    if (SvWEAKREF(sv))
                        sv_del_backref(target, sv);
                    else
                        next_sv = target;
                }
            }
#ifdef PERL_OLD_COPY_ON_WRITE
            else if (SvPVX_const(sv)
                     && !(SvTYPE(sv) == SVt_PVIO
                     && !(IoFLAGS(sv) & IOf_FAKE_DIRP)))
            {
                if (SvIsCOW(sv)) {
                    if (DEBUG_C_TEST) {
                        PerlIO_printf(Perl_debug_log, "Copy on write: clear\n");
                        sv_dump(sv);
                    }
                    if (SvLEN(sv)) {
                        sv_release_COW(sv, SvPVX_const(sv), SV_COW_NEXT_SV(sv));
                    } else {
                        unshare_hek(SvSHARED_HEK_FROM_PV(SvPVX_const(sv)));
                    }

                } else if (SvLEN(sv)) {
                    Safefree(SvPVX_mutable(sv));
                }
            }
#else
            else if (SvPVX_const(sv) && SvLEN(sv)
                     && !(SvTYPE(sv) == SVt_PVIO
                     && !(IoFLAGS(sv) & IOf_FAKE_DIRP)))
                Safefree(SvPVX_mutable(sv));
            else if (SvPVX_const(sv) && SvIsCOW(sv)) {
                unshare_hek(SvSHARED_HEK_FROM_PV(SvPVX_const(sv)));
            }
#endif
            break;
        case SVt_NV:
            break;
        }

      free_body:

        SvFLAGS(sv) &= SVf_BREAK;
        SvFLAGS(sv) |= SVTYPEMASK;

        sv_type_details = bodies_by_type + type;
        if (sv_type_details->arena) {
            del_body(((char *)SvANY(sv) + sv_type_details->offset),
                     &PL_body_roots[type]);
        }
        else if (sv_type_details->body_size) {
            safefree(SvANY(sv));
        }

      free_head:
        /* caller is responsible for freeing the head of the original sv */
        if (sv != orig_sv && !SvREFCNT(sv))
            del_SV(sv);

        /* grab and free next sv, if any */
      get_next_sv:
        while (1) {
            sv = NULL;
            if (next_sv) {
                sv = next_sv;
                next_sv = NULL;
            }
            else if (!iter_sv) {
                break;
            } else if (SvTYPE(iter_sv) == SVt_PVAV) {
                AV *const av = (AV*)iter_sv;
                if (AvFILLp(av) > -1) {
                    sv = AvARRAY(av)[AvFILLp(av)--];
                }
                else { /* no more elements of current AV to free */
                    sv = iter_sv;
                    type = SvTYPE(sv);
                    /* restore previous value, squirrelled away */
                    iter_sv = AvARRAY(av)[AvMAX(av)];
                    Safefree(AvALLOC(av));
                    goto free_body;
                }
            } else if (SvTYPE(iter_sv) == SVt_PVHV) {
                sv = Perl_hfree_next_entry(aTHX_ (HV*)iter_sv, &hash_index);
                if (!sv && !HvTOTALKEYS((HV *)iter_sv)) {
                    /* no more elements of current HV to free */
                    sv = iter_sv;
                    type = SvTYPE(sv);
                    /* Restore previous values of iter_sv and hash_index,
                     * squirrelled away */
                    assert(!SvOBJECT(sv));
                    iter_sv = (SV*)SvSTASH(sv);
                    assert(!SvMAGICAL(sv));
                    hash_index = ((XPVMG*) SvANY(sv))->xmg_u.xmg_hash_index;
#ifdef DEBUGGING
                    /* perl -DA does not like rubbish in SvMAGIC. */
                    SvMAGIC_set(sv, 0);
#endif

                    /* free any remaining detritus from the hash struct */
                    Perl_hv_undef_flags(aTHX_ MUTABLE_HV(sv), HV_NAME_SETALL);
                    assert(!HvARRAY((HV*)sv));
                    goto free_body;
                }
            }

            /* unrolled SvREFCNT_dec and sv_free2 follows: */

            if (!sv)
                continue;
            if (!SvREFCNT(sv)) {
                sv_free(sv);
                continue;
            }
            if (--(SvREFCNT(sv)))
                continue;
#ifdef DEBUGGING
            if (SvTEMP(sv)) {
                Perl_ck_warner_d(aTHX_ packWARN(WARN_DEBUGGING),
                         "Attempt to free temp prematurely: SV 0x%"UVxf
                         pTHX__FORMAT, PTR2UV(sv) pTHX__VALUE);
                continue;
            }
#endif
            if (SvREADONLY(sv) && SvIMMORTAL(sv)) {
                /* make sure SvREFCNT(sv)==0 happens very seldom */
                SvREFCNT(sv) = (~(U32)0)/2;
                continue;
            }
            break;
        } /* while 1 */

    } /* while sv */
}

/* This routine curses the sv itself, not the object referenced by sv. So
   sv does not have to be ROK. */

static bool
S_curse(pTHX_ SV * const sv, const bool check_refcnt) {
    dVAR;

    PERL_ARGS_ASSERT_CURSE;
    assert(SvOBJECT(sv));

    if (PL_defstash &&  /* Still have a symbol table? */
        SvDESTROYABLE(sv))
    {
        dSP;
        HV* stash;
        do {
          if ((stash = SvSTASH(sv)) && HvNAME(stash)) {
            CV* destructor = NULL;
            if (!SvOBJECT(stash)) destructor = (CV *)SvSTASH(stash);
            if (!destructor) {
                GV * const gv =
                    gv_fetchmeth_autoload(stash, "DESTROY", 7, 0);
                if (gv && (destructor = GvCV(gv))) {
                    if (!SvOBJECT(stash))
                        SvSTASH(stash) = (HV *)destructor;
                }
            }
            if (destructor
                /* A constant subroutine can have no side effects, so
                   don't bother calling it.  */
                && !CvCONST(destructor)
                /* Don't bother calling an empty destructor or one that
                   returns immediately. */
                && (CvISXSUB(destructor)
                || (CvSTART(destructor)
                    && (CvSTART(destructor)->op_next->op_type
                                        != OP_LEAVESUB)
                    && (CvSTART(destructor)->op_next->op_type
                                        != OP_PUSHMARK
                        || CvSTART(destructor)->op_next->op_next->op_type
                                        != OP_RETURN
                       )
                   ))
               )
            {
                SV* const tmpref = newRV(sv);
                SvREADONLY_on(tmpref); /* DESTROY() could be naughty */
                ENTER;
                PUSHSTACKi(PERLSI_DESTROY);
                EXTEND(SP, 2);
                PUSHMARK(SP);
                PUSHs(tmpref);
                PUTBACK;
                call_sv(MUTABLE_SV(destructor),
                            G_DISCARD|G_EVAL|G_KEEPERR|G_VOID);
                POPSTACK;
                SPAGAIN;
                LEAVE;
                if(SvREFCNT(tmpref) < 2) {
                    /* tmpref is not kept alive! */
                    SvREFCNT(sv)--;
                    SvRV_set(tmpref, NULL);
                    SvROK_off(tmpref);
                }
                SvREFCNT_dec(tmpref);
            }
          }
        } while (SvOBJECT(sv) && SvSTASH(sv) != stash);


        if (check_refcnt && SvREFCNT(sv)) {
            if (PL_in_clean_objs)
                Perl_croak(aTHX_
                  "DESTROY created new reference to dead object '%"HEKf"'",
                   HEKfARG(HvNAME_HEK(stash)));
            /* DESTROY gave object new lease on life */
            return FALSE;
        }
    }

    if (SvOBJECT(sv)) {
        SvREFCNT_dec(SvSTASH(sv)); /* possibly of changed persuasion */
        SvOBJECT_off(sv);       /* Curse the object. */
        if (SvTYPE(sv) != SVt_PVIO)
            --PL_sv_objcount;/* XXX Might want something more general */
    }
    return TRUE;
}

/*
=for apidoc sv_newref

Increment an SV's reference count.  Use the C<SvREFCNT_inc()> wrapper
instead.

=cut
*/

SV *
Perl_sv_newref(pTHX_ SV *const sv)
{
    PERL_UNUSED_CONTEXT;
    if (sv)
        (SvREFCNT(sv))++;
    return sv;
}

/*
=for apidoc sv_free

Decrement an SV's reference count, and if it drops to zero, call
C<sv_clear> to invoke destructors and free up any memory used by
the body; finally, deallocate the SV's head itself.
Normally called via a wrapper macro C<SvREFCNT_dec>.

=cut
*/

void
Perl_sv_free(pTHX_ SV *const sv)
{
    dVAR;
    if (!sv)
        return;
    if (SvREFCNT(sv) == 0) {
        if (SvFLAGS(sv) & SVf_BREAK)
            /* this SV's refcnt has been artificially decremented to
             * trigger cleanup */
            return;
        if (PL_in_clean_all) /* All is fair */
            return;
        if (SvREADONLY(sv) && SvIMMORTAL(sv)) {
            /* make sure SvREFCNT(sv)==0 happens very seldom */
            SvREFCNT(sv) = (~(U32)0)/2;
            return;
        }
        if (ckWARN_d(WARN_INTERNAL)) {
#ifdef DEBUG_LEAKING_SCALARS_FORK_DUMP
            Perl_dump_sv_child(aTHX_ sv);
#else
  #ifdef DEBUG_LEAKING_SCALARS
            sv_dump(sv);
  #endif
#ifdef DEBUG_LEAKING_SCALARS_ABORT
            if (PL_warnhook == PERL_WARNHOOK_FATAL
                || ckDEAD(packWARN(WARN_INTERNAL))) {
                /* Don't let Perl_warner cause us to escape our fate:  */
                abort();
            }
#endif
            /* This may not return:  */
            Perl_warner(aTHX_ packWARN(WARN_INTERNAL),
                        "Attempt to free unreferenced scalar: SV 0x%"UVxf
                        pTHX__FORMAT, PTR2UV(sv) pTHX__VALUE);
#endif