harmonise S_pushav() and pp_padav()

[perl5.git] / pp_hot.c

/*    pp_hot.c
 *
 *    Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
 *    2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 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.
 *
 */

/*
 * Then he heard Merry change the note, and up went the Horn-cry of Buckland,
 * shaking the air.
 *
 *                  Awake!  Awake!  Fear, Fire, Foes!  Awake!
 *                               Fire, Foes!  Awake!
 *
 *     [p.1007 of _The Lord of the Rings_, VI/viii: "The Scouring of the Shire"]
 */

/* This file contains 'hot' pp ("push/pop") functions that
 * execute the opcodes that make up a perl program. A typical pp function
 * expects to find its arguments on the stack, and usually pushes its
 * results onto the stack, hence the 'pp' terminology. Each OP structure
 * contains a pointer to the relevant pp_foo() function.
 *
 * By 'hot', we mean common ops whose execution speed is critical.
 * By gathering them together into a single file, we encourage
 * CPU cache hits on hot code. Also it could be taken as a warning not to
 * change any code in this file unless you're sure it won't affect
 * performance.
 */

#include "EXTERN.h"
#define PERL_IN_PP_HOT_C
#include "perl.h"

/* Hot code. */

PP(pp_const)
{
    dSP;
    XPUSHs(cSVOP_sv);
    RETURN;
}

PP(pp_nextstate)
{
    PL_curcop = (COP*)PL_op;
    TAINT_NOT;		/* Each statement is presumed innocent */
    PL_stack_sp = PL_stack_base + CX_CUR()->blk_oldsp;
    FREETMPS;
    PERL_ASYNC_CHECK();
    return NORMAL;
}

PP(pp_gvsv)
{
    dSP;
    EXTEND(SP,1);
    if (UNLIKELY(PL_op->op_private & OPpLVAL_INTRO))
	PUSHs(save_scalar(cGVOP_gv));
    else
	PUSHs(GvSVn(cGVOP_gv));
    RETURN;
}


/* also used for: pp_lineseq() pp_regcmaybe() pp_scalar() pp_scope() */

PP(pp_null)
{
    return NORMAL;
}

/* This is sometimes called directly by pp_coreargs, pp_grepstart and
   amagic_call. */
PP(pp_pushmark)
{
    PUSHMARK(PL_stack_sp);
    return NORMAL;
}

PP(pp_stringify)
{
    dSP; dTARGET;
    SV * const sv = TOPs;
    SETs(TARG);
    sv_copypv(TARG, sv);
    SvSETMAGIC(TARG);
    /* no PUTBACK, SETs doesn't inc/dec SP */
    return NORMAL;
}

PP(pp_gv)
{
    dSP;
    XPUSHs(MUTABLE_SV(cGVOP_gv));
    RETURN;
}


/* also used for: pp_andassign() */

PP(pp_and)
{
    PERL_ASYNC_CHECK();
    {
	/* SP is not used to remove a variable that is saved across the
	  sv_2bool_flags call in SvTRUE_NN, if a RISC/CISC or low/high machine
	  register or load/store vs direct mem ops macro is introduced, this
	  should be a define block between direct PL_stack_sp and dSP operations,
	  presently, using PL_stack_sp is bias towards CISC cpus */
	SV * const sv = *PL_stack_sp;
	if (!SvTRUE_NN(sv))
	    return NORMAL;
	else {
	    if (PL_op->op_type == OP_AND)
		--PL_stack_sp;
	    return cLOGOP->op_other;
	}
    }
}

PP(pp_sassign)
{
    dSP;
    /* sassign keeps its args in the optree traditionally backwards.
       So we pop them differently.
    */
    SV *left = POPs; SV *right = TOPs;

    if (PL_op->op_private & OPpASSIGN_BACKWARDS) { /* {or,and,dor}assign */
	SV * const temp = left;
	left = right; right = temp;
    }
    assert(TAINTING_get || !TAINT_get);
    if (UNLIKELY(TAINT_get) && !SvTAINTED(right))
	TAINT_NOT;
    if (UNLIKELY(PL_op->op_private & OPpASSIGN_CV_TO_GV)) {
        /* *foo =\&bar */
	SV * const cv = SvRV(right);
	const U32 cv_type = SvTYPE(cv);
	const bool is_gv = isGV_with_GP(left);
	const bool got_coderef = cv_type == SVt_PVCV || cv_type == SVt_PVFM;

	if (!got_coderef) {
	    assert(SvROK(cv));
	}

	/* Can do the optimisation if left (LVALUE) is not a typeglob,
	   right (RVALUE) is a reference to something, and we're in void
	   context. */
	if (!got_coderef && !is_gv && GIMME_V == G_VOID) {
	    /* Is the target symbol table currently empty?  */
	    GV * const gv = gv_fetchsv_nomg(left, GV_NOINIT, SVt_PVGV);
	    if (SvTYPE(gv) != SVt_PVGV && !SvOK(gv)) {
		/* Good. Create a new proxy constant subroutine in the target.
		   The gv becomes a(nother) reference to the constant.  */
		SV *const value = SvRV(cv);

		SvUPGRADE(MUTABLE_SV(gv), SVt_IV);
		SvPCS_IMPORTED_on(gv);
		SvRV_set(gv, value);
		SvREFCNT_inc_simple_void(value);
		SETs(left);
		RETURN;
	    }
	}

	/* Need to fix things up.  */
	if (!is_gv) {
	    /* Need to fix GV.  */
	    left = MUTABLE_SV(gv_fetchsv_nomg(left,GV_ADD, SVt_PVGV));
	}

	if (!got_coderef) {
	    /* We've been returned a constant rather than a full subroutine,
	       but they expect a subroutine reference to apply.  */
	    if (SvROK(cv)) {
		ENTER_with_name("sassign_coderef");
		SvREFCNT_inc_void(SvRV(cv));
		/* newCONSTSUB takes a reference count on the passed in SV
		   from us.  We set the name to NULL, otherwise we get into
		   all sorts of fun as the reference to our new sub is
		   donated to the GV that we're about to assign to.
		*/
		SvRV_set(right, MUTABLE_SV(newCONSTSUB(GvSTASH(left), NULL,
						      SvRV(cv))));
		SvREFCNT_dec_NN(cv);
		LEAVE_with_name("sassign_coderef");
	    } else {
		/* What can happen for the corner case *{"BONK"} = \&{"BONK"};
		   is that
		   First:   ops for \&{"BONK"}; return us the constant in the
			    symbol table
		   Second:  ops for *{"BONK"} cause that symbol table entry
			    (and our reference to it) to be upgraded from RV
			    to typeblob)
		   Thirdly: We get here. cv is actually PVGV now, and its
			    GvCV() is actually the subroutine we're looking for

		   So change the reference so that it points to the subroutine
		   of that typeglob, as that's what they were after all along.
		*/
		GV *const upgraded = MUTABLE_GV(cv);
		CV *const source = GvCV(upgraded);

		assert(source);
		assert(CvFLAGS(source) & CVf_CONST);

		SvREFCNT_inc_simple_void_NN(source);
		SvREFCNT_dec_NN(upgraded);
		SvRV_set(right, MUTABLE_SV(source));
	    }
	}

    }
    if (
      UNLIKELY(SvTEMP(left)) && !SvSMAGICAL(left) && SvREFCNT(left) == 1 &&
      (!isGV_with_GP(left) || SvFAKE(left)) && ckWARN(WARN_MISC)
    )
	Perl_warner(aTHX_
	    packWARN(WARN_MISC), "Useless assignment to a temporary"
	);
    SvSetMagicSV(left, right);
    SETs(left);
    RETURN;
}

PP(pp_cond_expr)
{
    dSP;
    SV *sv;

    PERL_ASYNC_CHECK();
    sv = POPs;
    RETURNOP(SvTRUE_NN(sv) ? cLOGOP->op_other : cLOGOP->op_next);
}

PP(pp_unstack)
{
    PERL_CONTEXT *cx;
    PERL_ASYNC_CHECK();
    TAINT_NOT;		/* Each statement is presumed innocent */
    cx  = CX_CUR();
    PL_stack_sp = PL_stack_base + cx->blk_oldsp;
    FREETMPS;
    if (!(PL_op->op_flags & OPf_SPECIAL)) {
        assert(CxTYPE(cx) == CXt_BLOCK || CxTYPE_is_LOOP(cx));
	CX_LEAVE_SCOPE(cx);
    }
    return NORMAL;
}

PP(pp_concat)
{
  dSP; dATARGET; tryAMAGICbin_MG(concat_amg, AMGf_assign);
  {
    dPOPTOPssrl;
    bool lbyte;
    STRLEN rlen;
    const char *rpv = NULL;
    bool rbyte = FALSE;
    bool rcopied = FALSE;

    if (TARG == right && right != left) { /* $r = $l.$r */
	rpv = SvPV_nomg_const(right, rlen);
	rbyte = !DO_UTF8(right);
	right = newSVpvn_flags(rpv, rlen, SVs_TEMP);
	rpv = SvPV_const(right, rlen);	/* no point setting UTF-8 here */
	rcopied = TRUE;
    }

    if (TARG != left) { /* not $l .= $r */
        STRLEN llen;
        const char* const lpv = SvPV_nomg_const(left, llen);
	lbyte = !DO_UTF8(left);
	sv_setpvn(TARG, lpv, llen);
	if (!lbyte)
	    SvUTF8_on(TARG);
	else
	    SvUTF8_off(TARG);
    }
    else { /* $l .= $r   and   left == TARG */
	if (!SvOK(left)) {
            if ((left == right                          /* $l .= $l */
                 || (PL_op->op_private & OPpTARGET_MY)) /* $l = $l . $r */
                && ckWARN(WARN_UNINITIALIZED)
                )
                report_uninit(left);
            SvPVCLEAR(left);
	}
        else {
            SvPV_force_nomg_nolen(left);
        }
	lbyte = !DO_UTF8(left);
	if (IN_BYTES)
	    SvUTF8_off(left);
    }

    if (!rcopied) {
	rpv = SvPV_nomg_const(right, rlen);
	rbyte = !DO_UTF8(right);
    }
    if (lbyte != rbyte) {
	if (lbyte)
	    sv_utf8_upgrade_nomg(TARG);
	else {
	    if (!rcopied)
		right = newSVpvn_flags(rpv, rlen, SVs_TEMP);
	    sv_utf8_upgrade_nomg(right);
	    rpv = SvPV_nomg_const(right, rlen);
	}
    }
    sv_catpvn_nomg(TARG, rpv, rlen);

    SETTARG;
    RETURN;
  }
}

/* push the elements of av onto the stack */

STATIC void
S_pushav(pTHX_ AV* const av)
{
    dSP;
    const SSize_t maxarg = AvFILL(av) + 1;
    EXTEND(SP, maxarg);
    if (UNLIKELY(SvRMAGICAL(av))) {
        PADOFFSET i;
        for (i=0; i < (PADOFFSET)maxarg; i++) {
            SV ** const svp = av_fetch(av, i, FALSE);
            SP[i+1] = svp ? *svp : &PL_sv_undef;
        }
    }
    else {
        PADOFFSET i;
        for (i=0; i < (PADOFFSET)maxarg; i++) {
            SV * const sv = AvARRAY(av)[i];
            SP[i+1] = LIKELY(sv) ? sv : &PL_sv_undef;
        }
    }
    SP += maxarg;
    PUTBACK;
}


/* ($lex1,@lex2,...)   or my ($lex1,@lex2,...)  */

PP(pp_padrange)
{
    dSP;
    PADOFFSET base = PL_op->op_targ;
    int count = (int)(PL_op->op_private) & OPpPADRANGE_COUNTMASK;
    if (PL_op->op_flags & OPf_SPECIAL) {
        /* fake the RHS of my ($x,$y,..) = @_ */
        PUSHMARK(SP);
        S_pushav(aTHX_ GvAVn(PL_defgv));
        SPAGAIN;
    }

    /* note, this is only skipped for compile-time-known void cxt */
    if ((PL_op->op_flags & OPf_WANT) != OPf_WANT_VOID) {
        int i;

        EXTEND(SP, count);
        PUSHMARK(SP);
        for (i = 0; i <count; i++)
            *++SP = PAD_SV(base+i);
    }
    if (PL_op->op_private & OPpLVAL_INTRO) {
        SV **svp = &(PAD_SVl(base));
        const UV payload = (UV)(
                      (base << (OPpPADRANGE_COUNTSHIFT + SAVE_TIGHT_SHIFT))
                    | (count << SAVE_TIGHT_SHIFT)
                    | SAVEt_CLEARPADRANGE);
        int i;

        STATIC_ASSERT_STMT(OPpPADRANGE_COUNTMASK + 1 == (1 << OPpPADRANGE_COUNTSHIFT));
        assert((payload >> (OPpPADRANGE_COUNTSHIFT+SAVE_TIGHT_SHIFT))
                == (Size_t)base);
        {
            dSS_ADD;
            SS_ADD_UV(payload);
            SS_ADD_END(1);
        }

        for (i = 0; i <count; i++)
            SvPADSTALE_off(*svp++); /* mark lexical as active */
    }
    RETURN;
}


PP(pp_padsv)
{
    dSP;
    EXTEND(SP, 1);
    {
	OP * const op = PL_op;
	/* access PL_curpad once */
	SV ** const padentry = &(PAD_SVl(op->op_targ));
	{
	    dTARG;
	    TARG = *padentry;
	    PUSHs(TARG);
	    PUTBACK; /* no pop/push after this, TOPs ok */
	}
	if (op->op_flags & OPf_MOD) {
	    if (op->op_private & OPpLVAL_INTRO)
		if (!(op->op_private & OPpPAD_STATE))
		    save_clearsv(padentry);
	    if (op->op_private & OPpDEREF) {
		/* TOPs is equivalent to TARG here.  Using TOPs (SP) rather
		   than TARG reduces the scope of TARG, so it does not
		   span the call to save_clearsv, resulting in smaller
		   machine code. */
		TOPs = vivify_ref(TOPs, op->op_private & OPpDEREF);
	    }
	}
	return op->op_next;
    }
}

PP(pp_readline)
{
    dSP;
    /* pp_coreargs pushes a NULL to indicate no args passed to
     * CORE::readline() */
    if (TOPs) {
	SvGETMAGIC(TOPs);
	tryAMAGICunTARGETlist(iter_amg, 0);
	PL_last_in_gv = MUTABLE_GV(*PL_stack_sp--);
    }
    else PL_last_in_gv = PL_argvgv, PL_stack_sp--;
    if (!isGV_with_GP(PL_last_in_gv)) {
	if (SvROK(PL_last_in_gv) && isGV_with_GP(SvRV(PL_last_in_gv)))
	    PL_last_in_gv = MUTABLE_GV(SvRV(PL_last_in_gv));
	else {
	    dSP;
	    XPUSHs(MUTABLE_SV(PL_last_in_gv));
	    PUTBACK;
	    Perl_pp_rv2gv(aTHX);
	    PL_last_in_gv = MUTABLE_GV(*PL_stack_sp--);
	    if (PL_last_in_gv == (GV *)&PL_sv_undef)
		PL_last_in_gv = NULL;
	    else
		assert(isGV_with_GP(PL_last_in_gv));
	}
    }
    return do_readline();
}

PP(pp_eq)
{
    dSP;
    SV *left, *right;

    tryAMAGICbin_MG(eq_amg, AMGf_set|AMGf_numeric);
    right = POPs;
    left  = TOPs;
    SETs(boolSV(
	(SvIOK_notUV(left) && SvIOK_notUV(right))
	? (SvIVX(left) == SvIVX(right))
	: ( do_ncmp(left, right) == 0)
    ));
    RETURN;
}


/* also used for: pp_i_preinc() */

PP(pp_preinc)
{
    SV *sv = *PL_stack_sp;

    if (LIKELY(((sv->sv_flags &
                        (SVf_THINKFIRST|SVs_GMG|SVf_IVisUV|
                         SVf_IOK|SVf_NOK|SVf_POK|SVp_NOK|SVp_POK|SVf_ROK))
                == SVf_IOK))
        && SvIVX(sv) != IV_MAX)
    {
	SvIV_set(sv, SvIVX(sv) + 1);
    }
    else /* Do all the PERL_PRESERVE_IVUV and hard cases in sv_inc */
	sv_inc(sv);
    SvSETMAGIC(sv);
    return NORMAL;
}


/* also used for: pp_i_predec() */

PP(pp_predec)
{
    SV *sv = *PL_stack_sp;

    if (LIKELY(((sv->sv_flags &
                        (SVf_THINKFIRST|SVs_GMG|SVf_IVisUV|
                         SVf_IOK|SVf_NOK|SVf_POK|SVp_NOK|SVp_POK|SVf_ROK))
                == SVf_IOK))
        && SvIVX(sv) != IV_MIN)
    {
	SvIV_set(sv, SvIVX(sv) - 1);
    }
    else /* Do all the PERL_PRESERVE_IVUV and hard cases  in sv_dec */
	sv_dec(sv);
    SvSETMAGIC(sv);
    return NORMAL;
}


/* also used for: pp_orassign() */

PP(pp_or)
{
    dSP;
    SV *sv;
    PERL_ASYNC_CHECK();
    sv = TOPs;
    if (SvTRUE_NN(sv))
	RETURN;
    else {
	if (PL_op->op_type == OP_OR)
            --SP;
	RETURNOP(cLOGOP->op_other);
    }
}


/* also used for: pp_dor() pp_dorassign() */

PP(pp_defined)
{
    dSP;
    SV* sv;
    bool defined;
    const int op_type = PL_op->op_type;
    const bool is_dor = (op_type == OP_DOR || op_type == OP_DORASSIGN);

    if (is_dor) {
	PERL_ASYNC_CHECK();
        sv = TOPs;
        if (UNLIKELY(!sv || !SvANY(sv))) {
	    if (op_type == OP_DOR)
		--SP;
            RETURNOP(cLOGOP->op_other);
        }
    }
    else {
	/* OP_DEFINED */
        sv = POPs;
        if (UNLIKELY(!sv || !SvANY(sv)))
            RETPUSHNO;
    }

    defined = FALSE;
    switch (SvTYPE(sv)) {
    case SVt_PVAV:
	if (AvMAX(sv) >= 0 || SvGMAGICAL(sv) || (SvRMAGICAL(sv) && mg_find(sv, PERL_MAGIC_tied)))
	    defined = TRUE;
	break;
    case SVt_PVHV:
	if (HvARRAY(sv) || SvGMAGICAL(sv) || (SvRMAGICAL(sv) && mg_find(sv, PERL_MAGIC_tied)))
	    defined = TRUE;
	break;
    case SVt_PVCV:
	if (CvROOT(sv) || CvXSUB(sv))
	    defined = TRUE;
	break;
    default:
	SvGETMAGIC(sv);
	if (SvOK(sv))
	    defined = TRUE;
	break;
    }

    if (is_dor) {
        if(defined) 
            RETURN; 
        if(op_type == OP_DOR)
            --SP;
        RETURNOP(cLOGOP->op_other);
    }
    /* assuming OP_DEFINED */
    if(defined) 
        RETPUSHYES;
    RETPUSHNO;
}



PP(pp_add)
{
    dSP; dATARGET; bool useleft; SV *svl, *svr;

    tryAMAGICbin_MG(add_amg, AMGf_assign|AMGf_numeric);
    svr = TOPs;
    svl = TOPm1s;

#ifdef PERL_PRESERVE_IVUV

    /* special-case some simple common cases */
    if (!((svl->sv_flags|svr->sv_flags) & (SVf_IVisUV|SVs_GMG))) {
        IV il, ir;
        U32 flags = (svl->sv_flags & svr->sv_flags);
        if (flags & SVf_IOK) {
            /* both args are simple IVs */
            UV topl, topr;
            il = SvIVX(svl);
            ir = SvIVX(svr);
          do_iv:
            topl = ((UV)il) >> (UVSIZE * 8 - 2);
            topr = ((UV)ir) >> (UVSIZE * 8 - 2);

            /* if both are in a range that can't under/overflow, do a
             * simple integer add: if the top of both numbers
             * are 00  or 11, then it's safe */
            if (!( ((topl+1) | (topr+1)) & 2)) {
                SP--;
                TARGi(il + ir, 0); /* args not GMG, so can't be tainted */
                SETs(TARG);
                RETURN;
            }
            goto generic;
        }
        else if (flags & SVf_NOK) {
            /* both args are NVs */
            NV nl = SvNVX(svl);
            NV nr = SvNVX(svr);

            if (
#if defined(NAN_COMPARE_BROKEN) && defined(Perl_isnan)
                !Perl_isnan(nl) && nl == (NV)(il = (IV)nl)
                && !Perl_isnan(nr) && nr == (NV)(ir = (IV)nr)
#else
                nl == (NV)(il = (IV)nl) && nr == (NV)(ir = (IV)nr)
#endif
                )
                /* nothing was lost by converting to IVs */
                goto do_iv;
            SP--;
            TARGn(nl + nr, 0); /* args not GMG, so can't be tainted */
            SETs(TARG);
            RETURN;
        }
    }

  generic:

    useleft = USE_LEFT(svl);
    /* We must see if we can perform the addition with integers if possible,
       as the integer code detects overflow while the NV code doesn't.
       If either argument hasn't had a numeric conversion yet attempt to get
       the IV. It's important to do this now, rather than just assuming that
       it's not IOK as a PV of "9223372036854775806" may not take well to NV
       addition, and an SV which is NOK, NV=6.0 ought to be coerced to
       integer in case the second argument is IV=9223372036854775806
       We can (now) rely on sv_2iv to do the right thing, only setting the
       public IOK flag if the value in the NV (or PV) slot is truly integer.

       A side effect is that this also aggressively prefers integer maths over
       fp maths for integer values.

       How to detect overflow?

       C 99 section 6.2.6.1 says

       The range of nonnegative values of a signed integer type is a subrange
       of the corresponding unsigned integer type, and the representation of
       the same value in each type is the same. A computation involving
       unsigned operands can never overflow, because a result that cannot be
       represented by the resulting unsigned integer type is reduced modulo
       the number that is one greater than the largest value that can be
       represented by the resulting type.

       (the 9th paragraph)

       which I read as "unsigned ints wrap."

       signed integer overflow seems to be classed as "exception condition"

       If an exceptional condition occurs during the evaluation of an
       expression (that is, if the result is not mathematically defined or not
       in the range of representable values for its type), the behavior is
       undefined.

       (6.5, the 5th paragraph)

       I had assumed that on 2s complement machines signed arithmetic would
       wrap, hence coded pp_add and pp_subtract on the assumption that
       everything perl builds on would be happy.  After much wailing and
       gnashing of teeth it would seem that irix64 knows its ANSI spec well,
       knows that it doesn't need to, and doesn't.  Bah.  Anyway, the all-
       unsigned code below is actually shorter than the old code. :-)
    */

    if (SvIV_please_nomg(svr)) {
	/* Unless the left argument is integer in range we are going to have to
	   use NV maths. Hence only attempt to coerce the right argument if
	   we know the left is integer.  */
	UV auv = 0;
	bool auvok = FALSE;
	bool a_valid = 0;

	if (!useleft) {
	    auv = 0;
	    a_valid = auvok = 1;
	    /* left operand is undef, treat as zero. + 0 is identity,
	       Could SETi or SETu right now, but space optimise by not adding
	       lots of code to speed up what is probably a rarish case.  */
	} else {
	    /* Left operand is defined, so is it IV? */
	    if (SvIV_please_nomg(svl)) {
		if ((auvok = SvUOK(svl)))
		    auv = SvUVX(svl);
		else {
		    const IV aiv = SvIVX(svl);
		    if (aiv >= 0) {
			auv = aiv;
			auvok = 1;	/* Now acting as a sign flag.  */
		    } else {
			auv = (aiv == IV_MIN) ? (UV)aiv : (UV)(-aiv);
		    }
		}
		a_valid = 1;
	    }
	}
	if (a_valid) {
	    bool result_good = 0;
	    UV result;
	    UV buv;
	    bool buvok = SvUOK(svr);
	
	    if (buvok)
		buv = SvUVX(svr);
	    else {
		const IV biv = SvIVX(svr);
		if (biv >= 0) {
		    buv = biv;
		    buvok = 1;
		} else
                    buv = (biv == IV_MIN) ? (UV)biv : (UV)(-biv);
	    }
	    /* ?uvok if value is >= 0. basically, flagged as UV if it's +ve,
	       else "IV" now, independent of how it came in.
	       if a, b represents positive, A, B negative, a maps to -A etc
	       a + b =>  (a + b)
	       A + b => -(a - b)
	       a + B =>  (a - b)
	       A + B => -(a + b)
	       all UV maths. negate result if A negative.
	       add if signs same, subtract if signs differ. */

	    if (auvok ^ buvok) {
		/* Signs differ.  */
		if (auv >= buv) {
		    result = auv - buv;
		    /* Must get smaller */
		    if (result <= auv)
			result_good = 1;
		} else {
		    result = buv - auv;
		    if (result <= buv) {
			/* result really should be -(auv-buv). as its negation
			   of true value, need to swap our result flag  */
			auvok = !auvok;
			result_good = 1;
		    }
		}
	    } else {
		/* Signs same */
		result = auv + buv;
		if (result >= auv)
		    result_good = 1;
	    }
	    if (result_good) {
		SP--;
		if (auvok)
		    SETu( result );
		else {
		    /* Negate result */
		    if (result <= (UV)IV_MIN)
                        SETi(result == (UV)IV_MIN
                                ? IV_MIN : -(IV)result);
		    else {
			/* result valid, but out of range for IV.  */
			SETn( -(NV)result );
		    }
		}
		RETURN;
	    } /* Overflow, drop through to NVs.  */
	}
    }

#else
    useleft = USE_LEFT(svl);
#endif

    {
	NV value = SvNV_nomg(svr);
	(void)POPs;
	if (!useleft) {
	    /* left operand is undef, treat as zero. + 0.0 is identity. */
	    SETn(value);
	    RETURN;
	}
	SETn( value + SvNV_nomg(svl) );
	RETURN;
    }
}


/* also used for: pp_aelemfast_lex() */

PP(pp_aelemfast)
{
    dSP;
    AV * const av = PL_op->op_type == OP_AELEMFAST_LEX
	? MUTABLE_AV(PAD_SV(PL_op->op_targ)) : GvAVn(cGVOP_gv);
    const U32 lval = PL_op->op_flags & OPf_MOD;
    const I8 key   = (I8)PL_op->op_private;
    SV** svp;
    SV *sv;

    assert(SvTYPE(av) == SVt_PVAV);

    EXTEND(SP, 1);

    /* inlined av_fetch() for simple cases ... */
    if (!SvRMAGICAL(av) && key >= 0 && key <= AvFILLp(av)) {
        sv = AvARRAY(av)[key];
        if (sv) {
            PUSHs(sv);
            RETURN;
        }
    }

    /* ... else do it the hard way */
    svp = av_fetch(av, key, lval);
    sv = (svp ? *svp : &PL_sv_undef);

    if (UNLIKELY(!svp && lval))
        DIE(aTHX_ PL_no_aelem, (int)key);

    if (!lval && SvRMAGICAL(av) && SvGMAGICAL(sv)) /* see note in pp_helem() */
	mg_get(sv);
    PUSHs(sv);
    RETURN;
}

PP(pp_join)
{
    dSP; dMARK; dTARGET;
    MARK++;
    do_join(TARG, *MARK, MARK, SP);
    SP = MARK;
    SETs(TARG);
    RETURN;
}

/* Oversized hot code. */

/* also used for: pp_say() */

PP(pp_print)
{
    dSP; dMARK; dORIGMARK;
    PerlIO *fp;
    MAGIC *mg;
    GV * const gv
	= (PL_op->op_flags & OPf_STACKED) ? MUTABLE_GV(*++MARK) : PL_defoutgv;
    IO *io = GvIO(gv);

    if (io
	&& (mg = SvTIED_mg((const SV *)io, PERL_MAGIC_tiedscalar)))
    {
      had_magic:
	if (MARK == ORIGMARK) {
	    /* If using default handle then we need to make space to
	     * pass object as 1st arg, so move other args up ...
	     */
	    MEXTEND(SP, 1);
	    ++MARK;
	    Move(MARK, MARK + 1, (SP - MARK) + 1, SV*);
	    ++SP;
	}
	return Perl_tied_method(aTHX_ SV_CONST(PRINT), mark - 1, MUTABLE_SV(io),
				mg,
				(G_SCALAR | TIED_METHOD_ARGUMENTS_ON_STACK
				 | (PL_op->op_type == OP_SAY
				    ? TIED_METHOD_SAY : 0)), sp - mark);
    }
    if (!io) {
        if ( gv && GvEGVx(gv) && (io = GvIO(GvEGV(gv)))
	    && (mg = SvTIED_mg((const SV *)io, PERL_MAGIC_tiedscalar)))
            goto had_magic;
	report_evil_fh(gv);
	SETERRNO(EBADF,RMS_IFI);
	goto just_say_no;
    }
    else if (!(fp = IoOFP(io))) {
	if (IoIFP(io))
	    report_wrongway_fh(gv, '<');
	else
	    report_evil_fh(gv);
	SETERRNO(EBADF,IoIFP(io)?RMS_FAC:RMS_IFI);
	goto just_say_no;
    }
    else {
	SV * const ofs = GvSV(PL_ofsgv); /* $, */
	MARK++;
	if (ofs && (SvGMAGICAL(ofs) || SvOK(ofs))) {
	    while (MARK <= SP) {
		if (!do_print(*MARK, fp))
		    break;
		MARK++;
		if (MARK <= SP) {
		    /* don't use 'ofs' here - it may be invalidated by magic callbacks */
		    if (!do_print(GvSV(PL_ofsgv), fp)) {
			MARK--;
			break;
		    }
		}
	    }
	}
	else {
	    while (MARK <= SP) {
		if (!do_print(*MARK, fp))
		    break;
		MARK++;
	    }
	}
	if (MARK <= SP)
	    goto just_say_no;
	else {
	    if (PL_op->op_type == OP_SAY) {
		if (PerlIO_write(fp, "\n", 1) == 0 || PerlIO_error(fp))
		    goto just_say_no;
	    }
            else if (PL_ors_sv && SvOK(PL_ors_sv))
		if (!do_print(PL_ors_sv, fp)) /* $\ */
		    goto just_say_no;

	    if (IoFLAGS(io) & IOf_FLUSH)
		if (PerlIO_flush(fp) == EOF)
		    goto just_say_no;
	}
    }
    SP = ORIGMARK;
    XPUSHs(&PL_sv_yes);
    RETURN;

  just_say_no:
    SP = ORIGMARK;
    XPUSHs(&PL_sv_undef);
    RETURN;
}


/* do the common parts of pp_padhv() and pp_rv2hv()
 * It assumes the caller has done EXTEND(SP, 1) or equivalent.
 * 'is_keys' indicates the OPpPADHV_ISKEYS/OPpRV2HV_ISKEYS flag is set.
 * 'has_targ' indicates that the op has a target - this should
 * be a compile-time constant so that the code can constant-folded as
 * appropriate
 * */

PERL_STATIC_INLINE OP*
S_padhv_rv2hv_common(pTHX_ HV *hv, U8 gimme, bool is_keys, bool has_targ)
{
    bool tied;
    dSP;

    assert(PL_op->op_type == OP_PADHV || PL_op->op_type == OP_RV2HV);

    if (gimme == G_ARRAY) {
        hv_pushkv(hv);
        return NORMAL;
    }

    if (is_keys)
        /* 'keys %h' masquerading as '%h': reset iterator */
        (void)hv_iterinit(hv);

    tied = SvRMAGICAL(hv) && mg_find(MUTABLE_SV(hv), PERL_MAGIC_tied);

    if (  (  PL_op->op_private & OPpTRUEBOOL
	  || (  PL_op->op_private & OPpMAYBE_TRUEBOOL
	     && block_gimme() == G_VOID)
          )
	  && !tied
    )
	PUSHs(HvUSEDKEYS(hv) ? &PL_sv_yes : &PL_sv_zero);
    else if (gimme == G_SCALAR) {
        if (is_keys) {
            IV i;
            if (tied) {
                i = 0;
                while (hv_iternext(hv))
                    i++;
            }
            else
                i = HvUSEDKEYS(hv);
            if (has_targ) {
                dTARGET;
                PUSHi(i);
            }
            else
                mPUSHi(i);
        }
        else
            PUSHs(Perl_hv_scalar(aTHX_ hv));
    }

    PUTBACK;
    return NORMAL;
}


/* This is also called directly by pp_lvavref.  */
PP(pp_padav)
{
    dSP; dTARGET;
    U8 gimme;
    assert(SvTYPE(TARG) == SVt_PVAV);
    if (UNLIKELY( PL_op->op_private & OPpLVAL_INTRO ))
	if (LIKELY( !(PL_op->op_private & OPpPAD_STATE) ))
	    SAVECLEARSV(PAD_SVl(PL_op->op_targ));
    EXTEND(SP, 1);

    if (PL_op->op_flags & OPf_REF) {
	PUSHs(TARG);
	RETURN;
    }
    else if (PL_op->op_private & OPpMAYBE_LVSUB) {
        const I32 flags = is_lvalue_sub();
        if (flags && !(flags & OPpENTERSUB_INARGS)) {
	    if (GIMME_V == G_SCALAR)
                /* diag_listed_as: Can't return %s to lvalue scalar context */
                Perl_croak(aTHX_ "Can't return array to lvalue scalar context");
            PUSHs(TARG);
            RETURN;
       }
    }

    gimme = GIMME_V;
    if (gimme == G_ARRAY) {
        /* XXX see also S_pushav in pp_hot.c */
	const SSize_t maxarg = AvFILL(MUTABLE_AV(TARG)) + 1;
	EXTEND(SP, maxarg);
	if (SvRMAGICAL(TARG)) {
	    SSize_t i;
	    for (i=0; i < maxarg; i++) {
		SV * const * const svp = av_fetch(MUTABLE_AV(TARG), i, FALSE);
		SP[i+1] = (svp) ? *svp : &PL_sv_undef;
	    }
	}
	else {
	    SSize_t i;
	    for (i=0; i < maxarg; i++) {
		SV * const sv = AvARRAY((const AV *)TARG)[i];
		SP[i+1] = sv ? sv : &PL_sv_undef;
	    }
	}
	SP += maxarg;
    }
    else if (gimme == G_SCALAR) {
	const SSize_t maxarg = AvFILL(MUTABLE_AV(TARG)) + 1;
        if (!maxarg)
            PUSHs(&PL_sv_zero);
        else if (PL_op->op_private & OPpTRUEBOOL)
            PUSHs(&PL_sv_yes);
        else
            mPUSHi(maxarg);
    }
    RETURN;
}


PP(pp_padhv)
{
    dSP; dTARGET;
    U8 gimme;

    assert(SvTYPE(TARG) == SVt_PVHV);
    if (UNLIKELY( PL_op->op_private & OPpLVAL_INTRO ))
	if (LIKELY( !(PL_op->op_private & OPpPAD_STATE) ))
	    SAVECLEARSV(PAD_SVl(PL_op->op_targ));

    EXTEND(SP, 1);

    if (PL_op->op_flags & OPf_REF) {
        PUSHs(TARG);
	RETURN;
    }
    else if (PL_op->op_private & OPpMAYBE_LVSUB) {
        const I32 flags = is_lvalue_sub();
        if (flags && !(flags & OPpENTERSUB_INARGS)) {
            if (GIMME_V == G_SCALAR)
                /* diag_listed_as: Can't return %s to lvalue scalar context */
                Perl_croak(aTHX_ "Can't return hash to lvalue scalar context");
            PUSHs(TARG);
            RETURN;
        }
    }

    gimme = GIMME_V;

    return S_padhv_rv2hv_common(aTHX_ (HV*)TARG, gimme,
                        cBOOL(PL_op->op_private & OPpPADHV_ISKEYS),
                        0 /* has_targ*/);
}


/* also used for: pp_rv2hv() */
/* also called directly by pp_lvavref */

PP(pp_rv2av)
{
    dSP; dTOPss;
    const U8 gimme = GIMME_V;
    static const char an_array[] = "an ARRAY";
    static const char a_hash[] = "a HASH";
    const bool is_pp_rv2av = PL_op->op_type == OP_RV2AV
			  || PL_op->op_type == OP_LVAVREF;
    const svtype type = is_pp_rv2av ? SVt_PVAV : SVt_PVHV;

    SvGETMAGIC(sv);
    if (SvROK(sv)) {
	if (UNLIKELY(SvAMAGIC(sv))) {
	    sv = amagic_deref_call(sv, is_pp_rv2av ? to_av_amg : to_hv_amg);
	}
	sv = SvRV(sv);
	if (UNLIKELY(SvTYPE(sv) != type))
	    /* diag_listed_as: Not an ARRAY reference */
	    DIE(aTHX_ "Not %s reference", is_pp_rv2av ? an_array : a_hash);
	else if (UNLIKELY(PL_op->op_flags & OPf_MOD
		&& PL_op->op_private & OPpLVAL_INTRO))
	    Perl_croak(aTHX_ "%s", PL_no_localize_ref);
    }
    else if (UNLIKELY(SvTYPE(sv) != type)) {
	    GV *gv;
	
	    if (!isGV_with_GP(sv)) {
		gv = Perl_softref2xv(aTHX_ sv, is_pp_rv2av ? an_array : a_hash,
				     type, &sp);
		if (!gv)
		    RETURN;
	    }
	    else {
		gv = MUTABLE_GV(sv);
	    }
	    sv = is_pp_rv2av ? MUTABLE_SV(GvAVn(gv)) : MUTABLE_SV(GvHVn(gv));
	    if (PL_op->op_private & OPpLVAL_INTRO)
		sv = is_pp_rv2av ? MUTABLE_SV(save_ary(gv)) : MUTABLE_SV(save_hash(gv));
    }
    if (PL_op->op_flags & OPf_REF) {
		SETs(sv);
		RETURN;
    }
    else if (UNLIKELY(PL_op->op_private & OPpMAYBE_LVSUB)) {
	      const I32 flags = is_lvalue_sub();
	      if (flags && !(flags & OPpENTERSUB_INARGS)) {
		if (gimme != G_ARRAY)
		    goto croak_cant_return;
		SETs(sv);
		RETURN;
	      }
    }

    if (is_pp_rv2av) {
	AV *const av = MUTABLE_AV(sv);
	/* The guts of pp_rv2av  */
	if (gimme == G_ARRAY) {
            SP--;
            PUTBACK;
            S_pushav(aTHX_ av);
            SPAGAIN;
	}
	else if (gimme == G_SCALAR) {
	    const SSize_t maxarg = AvFILL(av) + 1;
            if (PL_op->op_private & OPpTRUEBOOL)
                SETs(maxarg ? &PL_sv_yes : &PL_sv_zero);
            else {
                dTARGET;
                SETi(maxarg);
            }
	}
    }
    else {
        SP--; PUTBACK;
        return S_padhv_rv2hv_common(aTHX_ (HV*)sv, gimme,
                        cBOOL(PL_op->op_private & OPpRV2HV_ISKEYS),
                        1 /* has_targ*/);
    }
    RETURN;

 croak_cant_return:
    Perl_croak(aTHX_ "Can't return %s to lvalue scalar context",
	       is_pp_rv2av ? "array" : "hash");
    RETURN;
}

STATIC void
S_do_oddball(pTHX_ SV **oddkey, SV **firstkey)
{
    PERL_ARGS_ASSERT_DO_ODDBALL;

    if (*oddkey) {
        if (ckWARN(WARN_MISC)) {
	    const char *err;
	    if (oddkey == firstkey &&
		SvROK(*oddkey) &&
		(SvTYPE(SvRV(*oddkey)) == SVt_PVAV ||
		 SvTYPE(SvRV(*oddkey)) == SVt_PVHV))
	    {
		err = "Reference found where even-sized list expected";
	    }
	    else
		err = "Odd number of elements in hash assignment";
	    Perl_warner(aTHX_ packWARN(WARN_MISC), "%s", err);
	}

    }
}


/* Do a mark and sweep with the SVf_BREAK flag to detect elements which
 * are common to both the LHS and RHS of an aassign, and replace them
 * with copies. All these copies are made before the actual list assign is
 * done.
 *
 * For example in ($a,$b) = ($b,$a), assigning the value of the first RHS
 * element ($b) to the first LH element ($a), modifies $a; when the
 * second assignment is done, the second RH element now has the wrong
 * value. So we initially replace the RHS with ($b, mortalcopy($a)).
 * Note that we don't need to make a mortal copy of $b.
 *
 * The algorithm below works by, for every RHS element, mark the
 * corresponding LHS target element with SVf_BREAK. Then if the RHS
 * element is found with SVf_BREAK set, it means it would have been
 * modified, so make a copy.
 * Note that by scanning both LHS and RHS in lockstep, we avoid
 * unnecessary copies (like $b above) compared with a naive
 * "mark all LHS; copy all marked RHS; unmark all LHS".
 *
 * If the LHS element is a 'my' declaration' and has a refcount of 1, then
 * it can't be common and can be skipped.
 *
 * On DEBUGGING builds it takes an extra boolean, fake. If true, it means
 * that we thought we didn't need to call S_aassign_copy_common(), but we
 * have anyway for sanity checking. If we find we need to copy, then panic.
 */

PERL_STATIC_INLINE void
S_aassign_copy_common(pTHX_ SV **firstlelem, SV **lastlelem,
        SV **firstrelem, SV **lastrelem
#ifdef DEBUGGING
        , bool fake
#endif
)
{
    dVAR;
    SV **relem;
    SV **lelem;
    SSize_t lcount = lastlelem - firstlelem + 1;
    bool marked = FALSE; /* have we marked any LHS with SVf_BREAK ? */
    bool const do_rc1 = cBOOL(PL_op->op_private & OPpASSIGN_COMMON_RC1);
    bool copy_all = FALSE;

    assert(!PL_in_clean_all); /* SVf_BREAK not already in use */
    assert(firstlelem < lastlelem); /* at least 2 LH elements */
    assert(firstrelem < lastrelem); /* at least 2 RH elements */


    lelem = firstlelem;
    /* we never have to copy the first RH element; it can't be corrupted
     * by assigning something to the corresponding first LH element.
     * So this scan does in a loop: mark LHS[N]; test RHS[N+1]
     */
    relem = firstrelem + 1;

    for (; relem <= lastrelem; relem++) {
        SV *svr;

        /* mark next LH element */

        if (--lcount >= 0) {
            SV *svl = *lelem++;

            if (UNLIKELY(!svl)) {/* skip AV alias marker */
                assert (lelem <= lastlelem);
                svl = *lelem++;
                lcount--;
            }

            assert(svl);
            if (SvSMAGICAL(svl)) {
                copy_all = TRUE;
            }
            if (SvTYPE(svl) == SVt_PVAV || SvTYPE(svl) == SVt_PVHV) {
                if (!marked)
                    return;
                /* this LH element will consume all further args;
                 * no need to mark any further LH elements (if any).
                 * But we still need to scan any remaining RHS elements;
                 * set lcount negative to distinguish from  lcount == 0,
                 * so the loop condition continues being true
                 */
                lcount = -1;
                lelem--; /* no need to unmark this element */
            }
            else if (!(do_rc1 && SvREFCNT(svl) == 1) && !SvIMMORTAL(svl)) {
                SvFLAGS(svl) |= SVf_BREAK;
                marked = TRUE;
            }
            else if (!marked) {
                /* don't check RH element if no SVf_BREAK flags set yet */
                if (!lcount)
                    break;
                continue;
            }
        }

        /* see if corresponding RH element needs copying */

        assert(marked);
        svr = *relem;
        assert(svr);

        if (UNLIKELY(SvFLAGS(svr) & (SVf_BREAK|SVs_GMG) || copy_all)) {
            U32 brk = (SvFLAGS(svr) & SVf_BREAK);

#ifdef DEBUGGING
            if (fake) {
                /* op_dump(PL_op); */
                Perl_croak(aTHX_
                    "panic: aassign skipped needed copy of common RH elem %"
                        UVuf, (UV)(relem - firstrelem));
            }
#endif

            TAINT_NOT;	/* Each item is independent */

            /* Dear TODO test in t/op/sort.t, I love you.
               (It's relying on a panic, not a "semi-panic" from newSVsv()
               and then an assertion failure below.)  */
            if (UNLIKELY(SvIS_FREED(svr))) {
                Perl_croak(aTHX_ "panic: attempt to copy freed scalar %p",
                           (void*)svr);
            }
            /* avoid break flag while copying; otherwise COW etc
             * disabled... */
            SvFLAGS(svr) &= ~SVf_BREAK;
            /* Not newSVsv(), as it does not allow copy-on-write,
               resulting in wasteful copies.
               Also, we use SV_NOSTEAL in case the SV is used more than
               once, e.g.  (...) = (f())[0,0]
               Where the same SV appears twice on the RHS without a ref
               count bump.  (Although I suspect that the SV won't be
               stealable here anyway - DAPM).
               */
            *relem = sv_mortalcopy_flags(svr,
                                SV_GMAGIC|SV_DO_COW_SVSETSV|SV_NOSTEAL);
            /* ... but restore afterwards in case it's needed again,
             * e.g. ($a,$b,$c) = (1,$a,$a)
             */
            SvFLAGS(svr) |= brk;
        }

        if (!lcount)
            break;
    }

    if (!marked)
        return;

    /*unmark LHS */

    while (lelem > firstlelem) {
        SV * const svl = *(--lelem);
        if (svl)
            SvFLAGS(svl) &= ~SVf_BREAK;
    }
}



PP(pp_aassign)
{
    dVAR; dSP;
    SV **lastlelem = PL_stack_sp;
    SV **lastrelem = PL_stack_base + POPMARK;
    SV **firstrelem = PL_stack_base + POPMARK + 1;
    SV **firstlelem = lastrelem + 1;

    SV **relem;
    SV **lelem;
    U8 gimme;
    /* PL_delaymagic is restored by JUMPENV_POP on dieing, so we
     * only need to save locally, not on the save stack */
    U16 old_delaymagic = PL_delaymagic;
#ifdef DEBUGGING
    bool fake = 0;
#endif

    PL_delaymagic = DM_DELAY;		/* catch simultaneous items */

    /* If there's a common identifier on both sides we have to take
     * special care that assigning the identifier on the left doesn't
     * clobber a value on the right that's used later in the list.
     */

    /* at least 2 LH and RH elements, or commonality isn't an issue */
    if (firstlelem < lastlelem && firstrelem < lastrelem) {
        for (relem = firstrelem+1; relem <= lastrelem; relem++) {
            if (SvGMAGICAL(*relem))
                goto do_scan;
        }
        for (lelem = firstlelem; lelem <= lastlelem; lelem++) {
            if (*lelem && SvSMAGICAL(*lelem))
                goto do_scan;
        }
        if ( PL_op->op_private & (OPpASSIGN_COMMON_SCALAR|OPpASSIGN_COMMON_RC1) ) {
            if (PL_op->op_private & OPpASSIGN_COMMON_RC1) {
                /* skip the scan if all scalars have a ref count of 1 */
                for (lelem = firstlelem; lelem <= lastlelem; lelem++) {
                    SV *sv = *lelem;
                    if (!sv || SvREFCNT(sv) == 1)
                        continue;
                    if (SvTYPE(sv) != SVt_PVAV && SvTYPE(sv) != SVt_PVAV)
                        goto do_scan;
                    break;
                }
            }
            else {
            do_scan:
                S_aassign_copy_common(aTHX_
                                      firstlelem, lastlelem, firstrelem, lastrelem
#ifdef DEBUGGING
                    , fake
#endif
                );
            }
        }
    }
#ifdef DEBUGGING
    else {
        /* on debugging builds, do the scan even if we've concluded we
         * don't need to, then panic if we find commonality. Note that the
         * scanner assumes at least 2 elements */
        if (firstlelem < lastlelem && firstrelem < lastrelem) {
            fake = 1;
            goto do_scan;
        }
    }
#endif

    gimme = GIMME_V;
    relem = firstrelem;
    lelem = firstlelem;

    if (relem > lastrelem)
        goto no_relems;

    /* first lelem loop while there are still relems */
    while (LIKELY(lelem <= lastlelem)) {
	bool alias = FALSE;
	SV *lsv = *lelem++;

        TAINT_NOT; /* Each item stands on its own, taintwise. */

        assert(relem <= lastrelem);
	if (UNLIKELY(!lsv)) {
	    alias = TRUE;
	    lsv = *lelem++;
	    ASSUME(SvTYPE(lsv) == SVt_PVAV);
	}

	switch (SvTYPE(lsv)) {
	case SVt_PVAV: {
            SV **svp;
            SSize_t i;
            SSize_t tmps_base;
            SSize_t nelems = lastrelem - relem + 1;
            AV *ary = MUTABLE_AV(lsv);

            /* Assigning to an aggregate is tricky. First there is the
             * issue of commonality, e.g. @a = ($a[0]). Since the
             * stack isn't refcounted, clearing @a prior to storing
             * elements will free $a[0]. Similarly with
             *    sub FETCH { $status[$_[1]] } @status = @tied[0,1];
             *
             * The way to avoid these issues is to make the copy of each
             * SV (and we normally store a *copy* in the array) *before*
             * clearing the array. But this has a problem in that
             * if the code croaks during copying, the not-yet-stored copies
             * could leak. One way to avoid this is to make all the copies
             * mortal, but that's quite expensive.
             *
             * The current solution to these issues is to use a chunk
             * of the tmps stack as a temporary refcounted-stack. SVs
             * will be put on there during processing to avoid leaks,
             * but will be removed again before the end of this block,
             * so free_tmps() is never normally called. Also, the
             * sv_refcnt of the SVs doesn't have to be manipulated, since
             * the ownership of 1 reference count is transferred directly
             * from the tmps stack to the AV when the SV is stored.
             *
             * We disarm slots in the temps stack by storing PL_sv_undef
             * there: it doesn't matter if that SV's refcount is
             * repeatedly decremented during a croak. But usually this is
             * only an interim measure. By the end of this code block
             * we try where possible to not leave any PL_sv_undef's on the
             * tmps stack e.g. by shuffling newer entries down.
             *
             * There is one case where we don't copy: non-magical
             * SvTEMP(sv)'s with a ref count of 1. The only owner of these
             * is on the tmps stack, so its safe to directly steal the SV
             * rather than copying. This is common in things like function
             * returns, map etc, which all return a list of such SVs.
             *
             * Note however something like @a = (f())[0,0], where there is
             * a danger of the same SV being shared:  this avoided because
             * when the SV is stored as $a[0], its ref count gets bumped,
             * so the RC==1 test fails and the second element is copied
             * instead.
             *
             * We also use one slot in the tmps stack to hold an extra
             * ref to the array, to ensure it doesn't get prematurely
             * freed. Again, this is removed before the end of this block.
             *
             * Note that OPpASSIGN_COMMON_AGG is used to flag a possible
             * @a = ($a[0]) case, but the current implementation uses the
             * same algorithm regardless, so ignores that flag. (It *is*
             * used in the hash branch below, however).
            */

            /* Reserve slots for ary, plus the elems we're about to copy,
             * then protect ary and temporarily void the remaining slots
             * with &PL_sv_undef */
            EXTEND_MORTAL(nelems + 1);
            PL_tmps_stack[++PL_tmps_ix] = SvREFCNT_inc_simple_NN(ary);
            tmps_base = PL_tmps_ix + 1;
            for (i = 0; i < nelems; i++)
                PL_tmps_stack[tmps_base + i] = &PL_sv_undef;
            PL_tmps_ix += nelems;

            /* Make a copy of each RHS elem and save on the tmps_stack
             * (or pass through where we can optimise away the copy) */

            if (UNLIKELY(alias)) {
                U32 lval = (gimme == G_ARRAY)
                                ? (PL_op->op_flags & OPf_MOD || LVRET) : 0;
                for (svp = relem; svp <= lastrelem; svp++) {
                    SV *rsv = *svp;

                    SvGETMAGIC(rsv);
                    if (!SvROK(rsv))
                        DIE(aTHX_ "Assigned value is not a reference");
                    if (SvTYPE(SvRV(rsv)) > SVt_PVLV)
                   /* diag_listed_as: Assigned value is not %s reference */
                        DIE(aTHX_
                           "Assigned value is not a SCALAR reference");
                    if (lval)
                        *svp = rsv = sv_mortalcopy(rsv);
                    /* XXX else check for weak refs?  */
                    rsv = SvREFCNT_inc_NN(SvRV(rsv));
                    assert(tmps_base <= PL_tmps_max);
                    PL_tmps_stack[tmps_base++] = rsv;
                }
            }
            else {
                for (svp = relem; svp <= lastrelem; svp++) {
                    SV *rsv = *svp;

                    if (SvTEMP(rsv) && !SvGMAGICAL(rsv) && SvREFCNT(rsv) == 1) {
                        /* can skip the copy */
                        SvREFCNT_inc_simple_void_NN(rsv);
                        SvTEMP_off(rsv);
                    }
                    else {
                        SV *nsv;
                        /* do get before newSV, in case it dies and leaks */
                        SvGETMAGIC(rsv);
                        nsv = newSV(0);
                        /* see comment in S_aassign_copy_common about
                         * SV_NOSTEAL */
                        sv_setsv_flags(nsv, rsv,
                                (SV_DO_COW_SVSETSV|SV_NOSTEAL));
                        rsv = *svp = nsv;
                    }

                    assert(tmps_base <= PL_tmps_max);
                    PL_tmps_stack[tmps_base++] = rsv;
                }
            }

            if (SvRMAGICAL(ary) || AvFILLp(ary) >= 0) /* may be non-empty */
                av_clear(ary);

            /* store in the array, the SVs that are in the tmps stack */

            tmps_base -= nelems;

            if (SvMAGICAL(ary) || SvREADONLY(ary) || !AvREAL(ary)) {
                /* for arrays we can't cheat with, use the official API */
                av_extend(ary, nelems - 1);
                for (i = 0; i < nelems; i++) {
                    SV **svp = &(PL_tmps_stack[tmps_base + i]);
                    SV *rsv = *svp;
                    /* A tied store won't take ownership of rsv, so keep
                     * the 1 refcnt on the tmps stack; otherwise disarm
                     * the tmps stack entry */
                    if (av_store(ary, i, rsv))
                        *svp = &PL_sv_undef;
                    /* av_store() may have added set magic to rsv */;
                    SvSETMAGIC(rsv);
                }
                /* disarm ary refcount: see comments below about leak */
                PL_tmps_stack[tmps_base - 1] = &PL_sv_undef;
            }
            else {
                /* directly access/set the guts of the AV */
                SSize_t fill = nelems - 1;
                if (fill > AvMAX(ary))
                    av_extend_guts(ary, fill, &AvMAX(ary), &AvALLOC(ary),
                                    &AvARRAY(ary));
                AvFILLp(ary) = fill;
                Copy(&(PL_tmps_stack[tmps_base]), AvARRAY(ary), nelems, SV*);
                /* Quietly remove all the SVs from the tmps stack slots,
                 * since ary has now taken ownership of the refcnt.
                 * Also remove ary: which will now leak if we die before
                 * the SvREFCNT_dec_NN(ary) below */
                if (UNLIKELY(PL_tmps_ix >= tmps_base + nelems))
                    Move(&PL_tmps_stack[tmps_base + nelems],
                         &PL_tmps_stack[tmps_base - 1],
                         PL_tmps_ix - (tmps_base + nelems) + 1,
                         SV*);
                PL_tmps_ix -= (nelems + 1);
            }

	    if (UNLIKELY(PL_delaymagic & DM_ARRAY_ISA))
                /* its assumed @ISA set magic can't die and leak ary */
		SvSETMAGIC(MUTABLE_SV(ary));
            SvREFCNT_dec_NN(ary);

            relem = lastrelem + 1;
	    goto no_relems;
        }

	case SVt_PVHV: {				/* normal hash */

            SV **svp;
            bool dirty_tmps;
            SSize_t i;
            SSize_t tmps_base;
            SSize_t nelems = lastrelem - relem + 1;
            HV *hash = MUTABLE_HV(lsv);

            if (UNLIKELY(nelems & 1)) {
                do_oddball(lastrelem, relem);
                /* we have firstlelem to reuse, it's not needed any more */
                *++lastrelem = &PL_sv_undef;
                nelems++;
            }

            /* See the SVt_PVAV branch above for a long description of
             * how the following all works. The main difference for hashes
             * is that we treat keys and values separately (and have
             * separate loops for them): as for arrays, values are always
             * copied (except for the SvTEMP optimisation), since they
             * need to be stored in the hash; while keys are only
             * processed where they might get prematurely freed or
             * whatever. */

            /* tmps stack slots:
             * * reserve a slot for the hash keepalive;
             * * reserve slots for the hash values we're about to copy;
             * * preallocate for the keys we'll possibly copy or refcount bump
             *   later;
             * then protect hash and temporarily void the remaining
             * value slots with &PL_sv_undef */
            EXTEND_MORTAL(nelems + 1);

             /* convert to number of key/value pairs */
             nelems >>= 1;

            PL_tmps_stack[++PL_tmps_ix] = SvREFCNT_inc_simple_NN(hash);
            tmps_base = PL_tmps_ix + 1;
            for (i = 0; i < nelems; i++)
                PL_tmps_stack[tmps_base + i] = &PL_sv_undef;
            PL_tmps_ix += nelems;

            /* Make a copy of each RHS hash value and save on the tmps_stack
             * (or pass through where we can optimise away the copy) */

            for (svp = relem + 1; svp <= lastrelem; svp += 2) {
                SV *rsv = *svp;

                if (SvTEMP(rsv) && !SvGMAGICAL(rsv) && SvREFCNT(rsv) == 1) {
                    /* can skip the copy */
                    SvREFCNT_inc_simple_void_NN(rsv);
                    SvTEMP_off(rsv);
                }
                else {
                    SV *nsv;
                    /* do get before newSV, in case it dies and leaks */
                    SvGETMAGIC(rsv);
                    nsv = newSV(0);
                    /* see comment in S_aassign_copy_common about
                     * SV_NOSTEAL */
                    sv_setsv_flags(nsv, rsv,
                            (SV_DO_COW_SVSETSV|SV_NOSTEAL));
                    rsv = *svp = nsv;
                }

                assert(tmps_base <= PL_tmps_max);
                PL_tmps_stack[tmps_base++] = rsv;
            }
            tmps_base -= nelems;


            /* possibly protect keys */

            if (UNLIKELY(gimme == G_ARRAY)) {
                /* handle e.g.
                *     @a = ((%h = ($$r, 1)), $r = "x");
                *     $_++ for %h = (1,2,3,4);
                */
                EXTEND_MORTAL(nelems);
                for (svp = relem; svp <= lastrelem; svp += 2)
                    *svp = sv_mortalcopy_flags(*svp,
                                SV_GMAGIC|SV_DO_COW_SVSETSV|SV_NOSTEAL);
            }
            else if (PL_op->op_private & OPpASSIGN_COMMON_AGG) {
                /* for possible commonality, e.g.
                 *       %h = ($h{a},1)
                 * avoid premature freeing RHS keys by mortalising
                 * them.
                 * For a magic element, make a copy so that its magic is
                 * called *before* the hash is emptied (which may affect
                 * a tied value for example).
                 * In theory we should check for magic keys in all
                 * cases, not just under OPpASSIGN_COMMON_AGG, but in
                 * practice, !OPpASSIGN_COMMON_AGG implies only
                 * constants or padtmps on the RHS.
                 */
                EXTEND_MORTAL(nelems);
                for (svp = relem; svp <= lastrelem; svp += 2) {
                    SV *rsv = *svp;
                    if (UNLIKELY(SvGMAGICAL(rsv))) {
                        SSize_t n;
                        *svp = sv_mortalcopy_flags(*svp,
                                SV_GMAGIC|SV_DO_COW_SVSETSV|SV_NOSTEAL);
                        /* allow other branch to continue pushing
                         * onto tmps stack without checking each time */
                        n = (lastrelem - relem) >> 1;
                        EXTEND_MORTAL(n);
                    }
                    else
                        PL_tmps_stack[++PL_tmps_ix] =
                                    SvREFCNT_inc_simple_NN(rsv);
                }
            }

            if (SvRMAGICAL(hash) || HvUSEDKEYS(hash))
                hv_clear(hash);

            /* now assign the keys and values to the hash */

            dirty_tmps = FALSE;

            if (UNLIKELY(gimme == G_ARRAY)) {
                /* @a = (%h = (...)) etc */
                SV **svp;
                SV **topelem = relem;

                for (i = 0, svp = relem; svp <= lastrelem; i++, svp++) {
                    SV *key = *svp++;
                    SV *val = *svp;
                    /* remove duplicates from list we return */
                    if (!hv_exists_ent(hash, key, 0)) {
                        /* copy key back: possibly to an earlier
                         * stack location if we encountered dups earlier,
                         * The values will be updated later
                         */
                        *topelem = key;
                        topelem += 2;
                    }
                    /* A tied store won't take ownership of val, so keep
                     * the 1 refcnt on the tmps stack; otherwise disarm
                     * the tmps stack entry */
                    if (hv_store_ent(hash, key, val, 0))
                        PL_tmps_stack[tmps_base + i] = &PL_sv_undef;
                    else
                        dirty_tmps = TRUE;
                    /* hv_store_ent() may have added set magic to val */;
                    SvSETMAGIC(val);
                }
                if (topelem < svp) {
                    /* at this point we have removed the duplicate key/value
                     * pairs from the stack, but the remaining values may be
                     * wrong; i.e. with (a 1 a 2 b 3) on the stack we've removed
                     * the (a 2), but the stack now probably contains
                     * (a <freed> b 3), because { hv_save(a,1); hv_save(a,2) }
                     * obliterates the earlier key. So refresh all values. */
                    lastrelem = topelem - 1;
                    while (relem < lastrelem) {
                        HE *he;
                        he = hv_fetch_ent(hash, *relem++, 0, 0);
                        *relem++ = (he ? HeVAL(he) : &PL_sv_undef);
                    }
                }
            }
            else {
                SV **svp;
                for (i = 0, svp = relem; svp <= lastrelem; i++, svp++) {
                    SV *key = *svp++;
                    SV *val = *svp;
                    if (hv_store_ent(hash, key, val, 0))
                        PL_tmps_stack[tmps_base + i] = &PL_sv_undef;
                    else
                        dirty_tmps = TRUE;
                    /* hv_store_ent() may have added set magic to val */;
                    SvSETMAGIC(val);
                }
            }

            if (dirty_tmps) {
                /* there are still some 'live' recounts on the tmps stack
                 * - usually caused by storing into a tied hash. So let
                 * free_tmps() do the proper but slow job later.
                 * Just disarm hash refcount: see comments below about leak
                 */
                PL_tmps_stack[tmps_base - 1] = &PL_sv_undef;
            }
            else {
                /* Quietly remove all the SVs from the tmps stack slots,
                 * since hash has now taken ownership of the refcnt.
                 * Also remove hash: which will now leak if we die before
                 * the SvREFCNT_dec_NN(hash) below */
                if (UNLIKELY(PL_tmps_ix >= tmps_base + nelems))
                    Move(&PL_tmps_stack[tmps_base + nelems],
                         &PL_tmps_stack[tmps_base - 1],
                         PL_tmps_ix - (tmps_base + nelems) + 1,
                         SV*);
                PL_tmps_ix -= (nelems + 1);
            }

            SvREFCNT_dec_NN(hash);

            relem = lastrelem + 1;
	    goto no_relems;
	}

	default:
	    if (!SvIMMORTAL(lsv)) {
                SV *ref;

                if (UNLIKELY(
                  SvTEMP(lsv) && !SvSMAGICAL(lsv) && SvREFCNT(lsv) == 1 &&
                  (!isGV_with_GP(lsv) || SvFAKE(lsv)) && ckWARN(WARN_MISC)
                ))
                    Perl_warner(aTHX_
                       packWARN(WARN_MISC),
                      "Useless assignment to a temporary"
                    );

                /* avoid freeing $$lsv if it might be needed for further
                 * elements, e.g. ($ref, $foo) = (1, $$ref) */
                if (   SvROK(lsv)
                    && ( ((ref = SvRV(lsv)), SvREFCNT(ref)) == 1)
                    && lelem <= lastlelem
                ) {
                    SSize_t ix;
                    SvREFCNT_inc_simple_void_NN(ref);
                    /* an unrolled sv_2mortal */
                    ix = ++PL_tmps_ix;
                    if (UNLIKELY(ix >= PL_tmps_max))
                        /* speculatively grow enough to cover other
                         * possible refs */
                         (void)tmps_grow_p(ix + (lastlelem - lelem));
                    PL_tmps_stack[ix] = ref;
                }

                sv_setsv(lsv, *relem);
                *relem = lsv;
                SvSETMAGIC(lsv);
            }
            if (++relem > lastrelem)
                goto no_relems;
	    break;
        } /* switch */
    } /* while */


  no_relems:

    /* simplified lelem loop for when there are no relems left */
    while (LIKELY(lelem <= lastlelem)) {
	SV *lsv = *lelem++;

        TAINT_NOT; /* Each item stands on its own, taintwise. */

	if (UNLIKELY(!lsv)) {
	    lsv = *lelem++;
	    ASSUME(SvTYPE(lsv) == SVt_PVAV);
	}

	switch (SvTYPE(lsv)) {
	case SVt_PVAV:
            if (SvRMAGICAL(lsv) || AvFILLp((SV*)lsv) >= 0) {
                av_clear((AV*)lsv);
                if (UNLIKELY(PL_delaymagic & DM_ARRAY_ISA))
                    SvSETMAGIC(lsv);
            }
            break;

	case SVt_PVHV:
            if (SvRMAGICAL(lsv) || HvUSEDKEYS((HV*)lsv))
                hv_clear((HV*)lsv);
            break;

	default:
	    if (!SvIMMORTAL(lsv)) {
                sv_set_undef(lsv);
                SvSETMAGIC(lsv);
                *relem++ = lsv;
            }
	    break;
        } /* switch */
    } /* while */

    TAINT_NOT; /* result of list assign isn't tainted */

    if (UNLIKELY(PL_delaymagic & ~DM_DELAY)) {
	/* Will be used to set PL_tainting below */
	Uid_t tmp_uid  = PerlProc_getuid();
	Uid_t tmp_euid = PerlProc_geteuid();
	Gid_t tmp_gid  = PerlProc_getgid();
	Gid_t tmp_egid = PerlProc_getegid();

        /* XXX $> et al currently silently ignore failures */
	if (PL_delaymagic & DM_UID) {
#ifdef HAS_SETRESUID
	    PERL_UNUSED_RESULT(
               setresuid((PL_delaymagic & DM_RUID) ? PL_delaymagic_uid  : (Uid_t)-1,
                         (PL_delaymagic & DM_EUID) ? PL_delaymagic_euid : (Uid_t)-1,
                         (Uid_t)-1));
#else
#  ifdef HAS_SETREUID
            PERL_UNUSED_RESULT(
                setreuid((PL_delaymagic & DM_RUID) ? PL_delaymagic_uid  : (Uid_t)-1,
                         (PL_delaymagic & DM_EUID) ? PL_delaymagic_euid : (Uid_t)-1));
#  else
#    ifdef HAS_SETRUID
	    if ((PL_delaymagic & DM_UID) == DM_RUID) {
		PERL_UNUSED_RESULT(setruid(PL_delaymagic_uid));
		PL_delaymagic &= ~DM_RUID;
	    }
#    endif /* HAS_SETRUID */
#    ifdef HAS_SETEUID
	    if ((PL_delaymagic & DM_UID) == DM_EUID) {
		PERL_UNUSED_RESULT(seteuid(PL_delaymagic_euid));
		PL_delaymagic &= ~DM_EUID;
	    }
#    endif /* HAS_SETEUID */
	    if (PL_delaymagic & DM_UID) {
		if (PL_delaymagic_uid != PL_delaymagic_euid)
		    DIE(aTHX_ "No setreuid available");
		PERL_UNUSED_RESULT(PerlProc_setuid(PL_delaymagic_uid));
	    }
#  endif /* HAS_SETREUID */
#endif /* HAS_SETRESUID */

	    tmp_uid  = PerlProc_getuid();
	    tmp_euid = PerlProc_geteuid();
	}
        /* XXX $> et al currently silently ignore failures */
	if (PL_delaymagic & DM_GID) {
#ifdef HAS_SETRESGID
	    PERL_UNUSED_RESULT(
                setresgid((PL_delaymagic & DM_RGID) ? PL_delaymagic_gid  : (Gid_t)-1,
                          (PL_delaymagic & DM_EGID) ? PL_delaymagic_egid : (Gid_t)-1,
                          (Gid_t)-1));
#else
#  ifdef HAS_SETREGID
	    PERL_UNUSED_RESULT(
                setregid((PL_delaymagic & DM_RGID) ? PL_delaymagic_gid  : (Gid_t)-1,
                         (PL_delaymagic & DM_EGID) ? PL_delaymagic_egid : (Gid_t)-1));
#  else
#    ifdef HAS_SETRGID
	    if ((PL_delaymagic & DM_GID) == DM_RGID) {
		PERL_UNUSED_RESULT(setrgid(PL_delaymagic_gid));
		PL_delaymagic &= ~DM_RGID;
	    }
#    endif /* HAS_SETRGID */
#    ifdef HAS_SETEGID
	    if ((PL_delaymagic & DM_GID) == DM_EGID) {
		PERL_UNUSED_RESULT(setegid(PL_delaymagic_egid));
		PL_delaymagic &= ~DM_EGID;
	    }
#    endif /* HAS_SETEGID */
	    if (PL_delaymagic & DM_GID) {
		if (PL_delaymagic_gid != PL_delaymagic_egid)
		    DIE(aTHX_ "No setregid available");
		PERL_UNUSED_RESULT(PerlProc_setgid(PL_delaymagic_gid));
	    }
#  endif /* HAS_SETREGID */
#endif /* HAS_SETRESGID */

	    tmp_gid  = PerlProc_getgid();
	    tmp_egid = PerlProc_getegid();
	}
	TAINTING_set( TAINTING_get | (tmp_uid && (tmp_euid != tmp_uid || tmp_egid != tmp_gid)) );
#ifdef NO_TAINT_SUPPORT
        PERL_UNUSED_VAR(tmp_uid);
        PERL_UNUSED_VAR(tmp_euid);
        PERL_UNUSED_VAR(tmp_gid);
        PERL_UNUSED_VAR(tmp_egid);
#endif
    }
    PL_delaymagic = old_delaymagic;

    if (gimme == G_VOID)
	SP = firstrelem - 1;
    else if (gimme == G_SCALAR) {
	SP = firstrelem;
        EXTEND(SP,1);
        if (PL_op->op_private & OPpASSIGN_TRUEBOOL)
            SETs((firstlelem - firstrelem) ? &PL_sv_yes : &PL_sv_zero);
        else {
            dTARGET;
            SETi(firstlelem - firstrelem);
        }
    }
    else
        SP = relem - 1;

    RETURN;
}

PP(pp_qr)
{
    dSP;
    PMOP * const pm = cPMOP;
    REGEXP * rx = PM_GETRE(pm);
    regexp *prog = ReANY(rx);
    SV * const pkg = RXp_ENGINE(prog)->qr_package(aTHX_ (rx));
    SV * const rv = sv_newmortal();
    CV **cvp;
    CV *cv;

    SvUPGRADE(rv, SVt_IV);
    /* For a subroutine describing itself as "This is a hacky workaround" I'm
       loathe to use it here, but it seems to be the right fix. Or close.
       The key part appears to be that it's essential for pp_qr to return a new
       object (SV), which implies that there needs to be an effective way to
       generate a new SV from the existing SV that is pre-compiled in the
       optree.  */
    SvRV_set(rv, MUTABLE_SV(reg_temp_copy(NULL, rx)));
    SvROK_on(rv);

    cvp = &( ReANY((REGEXP *)SvRV(rv))->qr_anoncv);
    if (UNLIKELY((cv = *cvp) && CvCLONE(*cvp))) {
	*cvp = cv_clone(cv);
	SvREFCNT_dec_NN(cv);
    }

    if (pkg) {
	HV *const stash = gv_stashsv(pkg, GV_ADD);
	SvREFCNT_dec_NN(pkg);
	(void)sv_bless(rv, stash);
    }

    if (UNLIKELY(RXp_ISTAINTED(prog))) {
        SvTAINTED_on(rv);
        SvTAINTED_on(SvRV(rv));
    }
    XPUSHs(rv);
    RETURN;
}

PP(pp_match)
{
    dSP; dTARG;
    PMOP *pm = cPMOP;
    PMOP *dynpm = pm;
    const char *s;
    const char *strend;
    SSize_t curpos = 0; /* initial pos() or current $+[0] */
    I32 global;
    U8 r_flags = 0;
    const char *truebase;			/* Start of string  */
    REGEXP *rx = PM_GETRE(pm);
    regexp *prog = ReANY(rx);
    bool rxtainted;
    const U8 gimme = GIMME_V;
    STRLEN len;
    const I32 oldsave = PL_savestack_ix;
    I32 had_zerolen = 0;
    MAGIC *mg = NULL;

    if (PL_op->op_flags & OPf_STACKED)
	TARG = POPs;
    else {
        if (ARGTARG)
            GETTARGET;
        else {
            TARG = DEFSV;
        }
	EXTEND(SP,1);
    }

    PUTBACK;				/* EVAL blocks need stack_sp. */
    /* Skip get-magic if this is a qr// clone, because regcomp has
       already done it. */
    truebase = prog->mother_re
	 ? SvPV_nomg_const(TARG, len)
	 : SvPV_const(TARG, len);
    if (!truebase)
	DIE(aTHX_ "panic: pp_match");
    strend = truebase + len;
    rxtainted = (RXp_ISTAINTED(prog) ||
		 (TAINT_get && (pm->op_pmflags & PMf_RETAINT)));
    TAINT_NOT;

    /* We need to know this in case we fail out early - pos() must be reset */
    global = dynpm->op_pmflags & PMf_GLOBAL;

    /* PMdf_USED is set after a ?? matches once */
    if (
#ifdef USE_ITHREADS
        SvREADONLY(PL_regex_pad[pm->op_pmoffset])
#else
        pm->op_pmflags & PMf_USED
#endif
    ) {
        DEBUG_r(PerlIO_printf(Perl_debug_log, "?? already matched once"));
	goto nope;
    }

    /* handle the empty pattern */
    if (!RX_PRELEN(rx) && PL_curpm && !prog->mother_re) {
        if (PL_curpm == PL_reg_curpm) {
            if (PL_curpm_under) {
                if (PL_curpm_under == PL_reg_curpm) {
                    Perl_croak(aTHX_ "Infinite recursion via empty pattern");
                } else {
                    pm = PL_curpm_under;
                }
            }
        } else {
            pm = PL_curpm;
        }
        rx = PM_GETRE(pm);
        prog = ReANY(rx);
    }

    if (RXp_MINLEN(prog) >= 0 && (STRLEN)RXp_MINLEN(prog) > len) {
        DEBUG_r(PerlIO_printf(Perl_debug_log, "String shorter than min possible regex match (%"
                                              UVuf " < %" IVdf ")\n",
                                              (UV)len, (IV)RXp_MINLEN(prog)));
	goto nope;
    }

    /* get pos() if //g */
    if (global) {
        mg = mg_find_mglob(TARG);
        if (mg && mg->mg_len >= 0) {
            curpos = MgBYTEPOS(mg, TARG, truebase, len);
            /* last time pos() was set, it was zero-length match */
            if (mg->mg_flags & MGf_MINMATCH)
                had_zerolen = 1;
        }
    }

#ifdef PERL_SAWAMPERSAND
    if (       RXp_NPARENS(prog)
            || PL_sawampersand
            || (RXp_EXTFLAGS(prog) & (RXf_EVAL_SEEN|RXf_PMf_KEEPCOPY))
            || (dynpm->op_pmflags & PMf_KEEPCOPY)
    )
#endif
    {
	r_flags |= (REXEC_COPY_STR|REXEC_COPY_SKIP_PRE);
        /* in @a =~ /(.)/g, we iterate multiple times, but copy the buffer
         * only on the first iteration. Therefore we need to copy $' as well
         * as $&, to make the rest of the string available for captures in
         * subsequent iterations */
        if (! (global && gimme == G_ARRAY))
            r_flags |= REXEC_COPY_SKIP_POST;
    };
#ifdef PERL_SAWAMPERSAND
    if (dynpm->op_pmflags & PMf_KEEPCOPY)
        /* handle KEEPCOPY in pmop but not rx, eg $r=qr/a/; /$r/p */
        r_flags &= ~(REXEC_COPY_SKIP_PRE|REXEC_COPY_SKIP_POST);
#endif

    s = truebase;

  play_it_again:
    if (global)
	s = truebase + curpos;

    if (!CALLREGEXEC(rx, (char*)s, (char *)strend, (char*)truebase,
		     had_zerolen, TARG, NULL, r_flags))
	goto nope;

    PL_curpm = pm;
    if (dynpm->op_pmflags & PMf_ONCE)
#ifdef USE_ITHREADS
	SvREADONLY_on(PL_regex_pad[dynpm->op_pmoffset]);
#else
	dynpm->op_pmflags |= PMf_USED;
#endif

    if (rxtainted)
	RXp_MATCH_TAINTED_on(prog);
    TAINT_IF(RXp_MATCH_TAINTED(prog));

    /* update pos */

    if (global && (gimme != G_ARRAY || (dynpm->op_pmflags & PMf_CONTINUE))) {
        if (!mg)
            mg = sv_magicext_mglob(TARG);
        MgBYTEPOS_set(mg, TARG, truebase, RXp_OFFS(prog)[0].end);
        if (RXp_ZERO_LEN(prog))
            mg->mg_flags |= MGf_MINMATCH;
        else
            mg->mg_flags &= ~MGf_MINMATCH;
    }

    if ((!RXp_NPARENS(prog) && !global) || gimme != G_ARRAY) {
	LEAVE_SCOPE(oldsave);
	RETPUSHYES;
    }

    /* push captures on stack */

    {
	const I32 nparens = RXp_NPARENS(prog);
	I32 i = (global && !nparens) ? 1 : 0;

	SPAGAIN;			/* EVAL blocks could move the stack. */
	EXTEND(SP, nparens + i);
	EXTEND_MORTAL(nparens + i);
	for (i = !i; i <= nparens; i++) {
	    PUSHs(sv_newmortal());
	    if (LIKELY((RXp_OFFS(prog)[i].start != -1)
                     && RXp_OFFS(prog)[i].end   != -1 ))
            {
		const I32 len = RXp_OFFS(prog)[i].end - RXp_OFFS(prog)[i].start;
		const char * const s = RXp_OFFS(prog)[i].start + truebase;
	        if (UNLIKELY(  RXp_OFFS(prog)[i].end   < 0
                            || RXp_OFFS(prog)[i].start < 0
                            || len < 0
                            || len > strend - s)
                )
		    DIE(aTHX_ "panic: pp_match start/end pointers, i=%ld, "
			"start=%ld, end=%ld, s=%p, strend=%p, len=%" UVuf,
			(long) i, (long) RXp_OFFS(prog)[i].start,
			(long)RXp_OFFS(prog)[i].end, s, strend, (UV) len);
		sv_setpvn(*SP, s, len);
		if (DO_UTF8(TARG) && is_utf8_string((U8*)s, len))
		    SvUTF8_on(*SP);
	    }
	}
	if (global) {
            curpos = (UV)RXp_OFFS(prog)[0].end;
	    had_zerolen = RXp_ZERO_LEN(prog);
	    PUTBACK;			/* EVAL blocks may use stack */
	    r_flags |= REXEC_IGNOREPOS | REXEC_NOT_FIRST;
	    goto play_it_again;
	}
	LEAVE_SCOPE(oldsave);
	RETURN;
    }
    NOT_REACHED; /* NOTREACHED */

  nope:
    if (global && !(dynpm->op_pmflags & PMf_CONTINUE)) {
        if (!mg)
            mg = mg_find_mglob(TARG);
        if (mg)
            mg->mg_len = -1;
    }
    LEAVE_SCOPE(oldsave);
    if (gimme == G_ARRAY)
	RETURN;
    RETPUSHNO;
}

OP *
Perl_do_readline(pTHX)
{
    dSP; dTARGETSTACKED;
    SV *sv;
    STRLEN tmplen = 0;
    STRLEN offset;
    PerlIO *fp;
    IO * const io = GvIO(PL_last_in_gv);
    const I32 type = PL_op->op_type;
    const U8 gimme = GIMME_V;

    if (io) {
	const MAGIC *const mg = SvTIED_mg((const SV *)io, PERL_MAGIC_tiedscalar);
	if (mg) {
	    Perl_tied_method(aTHX_ SV_CONST(READLINE), SP, MUTABLE_SV(io), mg, gimme, 0);
	    if (gimme == G_SCALAR) {
		SPAGAIN;
		SvSetSV_nosteal(TARG, TOPs);
		SETTARG;
	    }
	    return NORMAL;
	}
    }
    fp = NULL;
    if (io) {
	fp = IoIFP(io);
	if (!fp) {
	    if (IoFLAGS(io) & IOf_ARGV) {
		if (IoFLAGS(io) & IOf_START) {
		    IoLINES(io) = 0;
		    if (av_tindex(GvAVn(PL_last_in_gv)) < 0) {
			IoFLAGS(io) &= ~IOf_START;
			do_open6(PL_last_in_gv, "-", 1, NULL, NULL, 0);
			SvTAINTED_off(GvSVn(PL_last_in_gv)); /* previous tainting irrelevant */
			sv_setpvs(GvSVn(PL_last_in_gv), "-");
			SvSETMAGIC(GvSV(PL_last_in_gv));
			fp = IoIFP(io);
			goto have_fp;
		    }
		}
		fp = nextargv(PL_last_in_gv, PL_op->op_flags & OPf_SPECIAL);
		if (!fp) { /* Note: fp != IoIFP(io) */
		    (void)do_close(PL_last_in_gv, FALSE); /* now it does*/
		}
	    }
	    else if (type == OP_GLOB)
		fp = Perl_start_glob(aTHX_ POPs, io);
	}
	else if (type == OP_GLOB)
	    SP--;
	else if (IoTYPE(io) == IoTYPE_WRONLY) {
	    report_wrongway_fh(PL_last_in_gv, '>');
	}
    }
    if (!fp) {
	if ((!io || !(IoFLAGS(io) & IOf_START))
	    && ckWARN(WARN_CLOSED)
            && type != OP_GLOB)
	{
	    report_evil_fh(PL_last_in_gv);
	}
	if (gimme == G_SCALAR) {
	    /* undef TARG, and push that undefined value */
	    if (type != OP_RCATLINE) {
		sv_set_undef(TARG);
	    }
	    PUSHTARG;
	}
	RETURN;
    }
  have_fp:
    if (gimme == G_SCALAR) {
	sv = TARG;
	if (type == OP_RCATLINE && SvGMAGICAL(sv))
	    mg_get(sv);
	if (SvROK(sv)) {
	    if (type == OP_RCATLINE)
		SvPV_force_nomg_nolen(sv);
	    else
		sv_unref(sv);
	}
	else if (isGV_with_GP(sv)) {
	    SvPV_force_nomg_nolen(sv);
	}
	SvUPGRADE(sv, SVt_PV);
	tmplen = SvLEN(sv);	/* remember if already alloced */
	if (!tmplen && !SvREADONLY(sv) && !SvIsCOW(sv)) {
            /* try short-buffering it. Please update t/op/readline.t
	     * if you change the growth length.
	     */
	    Sv_Grow(sv, 80);
        }
	offset = 0;
	if (type == OP_RCATLINE && SvOK(sv)) {
	    if (!SvPOK(sv)) {
		SvPV_force_nomg_nolen(sv);
	    }
	    offset = SvCUR(sv);
	}
    }
    else {
	sv = sv_2mortal(newSV(80));
	offset = 0;
    }

    /* This should not be marked tainted if the fp is marked clean */
#define MAYBE_TAINT_LINE(io, sv) \
    if (!(IoFLAGS(io) & IOf_UNTAINT)) { \
	TAINT;				\
	SvTAINTED_on(sv);		\
    }

/* delay EOF state for a snarfed empty file */
#define SNARF_EOF(gimme,rs,io,sv) \
    (gimme != G_SCALAR || SvCUR(sv)					\
     || (IoFLAGS(io) & IOf_NOLINE) || !RsSNARF(rs))

    for (;;) {
	PUTBACK;
	if (!sv_gets(sv, fp, offset)
	    && (type == OP_GLOB
		|| SNARF_EOF(gimme, PL_rs, io, sv)
		|| PerlIO_error(fp)))
	{
	    PerlIO_clearerr(fp);
	    if (IoFLAGS(io) & IOf_ARGV) {
		fp = nextargv(PL_last_in_gv, PL_op->op_flags & OPf_SPECIAL);
		if (fp)
		    continue;
		(void)do_close(PL_last_in_gv, FALSE);
	    }
	    else if (type == OP_GLOB) {
		if (!do_close(PL_last_in_gv, FALSE)) {
		    Perl_ck_warner(aTHX_ packWARN(WARN_GLOB),
				   "glob failed (child exited with status %d%s)",
				   (int)(STATUS_CURRENT >> 8),
				   (STATUS_CURRENT & 0x80) ? ", core dumped" : "");
		}
	    }
	    if (gimme == G_SCALAR) {
		if (type != OP_RCATLINE) {
		    SV_CHECK_THINKFIRST_COW_DROP(TARG);
		    SvOK_off(TARG);
		}
		SPAGAIN;
		PUSHTARG;
	    }
	    MAYBE_TAINT_LINE(io, sv);
	    RETURN;
	}
	MAYBE_TAINT_LINE(io, sv);
	IoLINES(io)++;
	IoFLAGS(io) |= IOf_NOLINE;
	SvSETMAGIC(sv);
	SPAGAIN;
	XPUSHs(sv);
	if (type == OP_GLOB) {
	    const char *t1;
	    Stat_t statbuf;

	    if (SvCUR(sv) > 0 && SvCUR(PL_rs) > 0) {
		char * const tmps = SvEND(sv) - 1;
		if (*tmps == *SvPVX_const(PL_rs)) {
		    *tmps = '\0';
		    SvCUR_set(sv, SvCUR(sv) - 1);
		}
	    }
	    for (t1 = SvPVX_const(sv); *t1; t1++)
#ifdef __VMS
		if (strchr("*%?", *t1))
#else
		if (strchr("$&*(){}[]'\";\\|?<>~`", *t1))
#endif
			break;
	    if (*t1 && PerlLIO_lstat(SvPVX_const(sv), &statbuf) < 0) {
		(void)POPs;		/* Unmatched wildcard?  Chuck it... */
		continue;
	    }
	} else if (SvUTF8(sv)) { /* OP_READLINE, OP_RCATLINE */
	     if (ckWARN(WARN_UTF8)) {
		const U8 * const s = (const U8*)SvPVX_const(sv) + offset;
		const STRLEN len = SvCUR(sv) - offset;
		const U8 *f;

		if (!is_utf8_string_loc(s, len, &f))
		    /* Emulate :encoding(utf8) warning in the same case. */
		    Perl_warner(aTHX_ packWARN(WARN_UTF8),
				"utf8 \"\\x%02X\" does not map to Unicode",
				f < (U8*)SvEND(sv) ? *f : 0);
	     }
	}
	if (gimme == G_ARRAY) {
	    if (SvLEN(sv) - SvCUR(sv) > 20) {
		SvPV_shrink_to_cur(sv);
	    }
	    sv = sv_2mortal(newSV(80));
	    continue;
	}
	else if (gimme == G_SCALAR && !tmplen && SvLEN(sv) - SvCUR(sv) > 80) {
	    /* try to reclaim a bit of scalar space (only on 1st alloc) */
	    const STRLEN new_len
		= SvCUR(sv) < 60 ? 80 : SvCUR(sv)+40; /* allow some slop */
	    SvPV_renew(sv, new_len);
	}
	RETURN;
    }
}

PP(pp_helem)
{
    dSP;
    HE* he;
    SV **svp;
    SV * const keysv = POPs;
    HV * const hv = MUTABLE_HV(POPs);
    const U32 lval = PL_op->op_flags & OPf_MOD || LVRET;
    const U32 defer = PL_op->op_private & OPpLVAL_DEFER;
    SV *sv;
    const bool localizing = PL_op->op_private & OPpLVAL_INTRO;
    bool preeminent = TRUE;

    if (SvTYPE(hv) != SVt_PVHV)
	RETPUSHUNDEF;

    if (localizing) {
	MAGIC *mg;
	HV *stash;

	/* If we can determine whether the element exist,
	 * Try to preserve the existenceness of a tied hash
	 * element by using EXISTS and DELETE if possible.
	 * Fallback to FETCH and STORE otherwise. */
	if (SvCANEXISTDELETE(hv))
	    preeminent = hv_exists_ent(hv, keysv, 0);
    }

    he = hv_fetch_ent(hv, keysv, lval && !defer, 0);
    svp = he ? &HeVAL(he) : NULL;
    if (lval) {
	if (!svp || !*svp || *svp == &PL_sv_undef) {
	    SV* lv;
	    SV* key2;
	    if (!defer) {
		DIE(aTHX_ PL_no_helem_sv, SVfARG(keysv));
	    }
	    lv = sv_newmortal();
	    sv_upgrade(lv, SVt_PVLV);
	    LvTYPE(lv) = 'y';
	    sv_magic(lv, key2 = newSVsv(keysv), PERL_MAGIC_defelem, NULL, 0);
	    SvREFCNT_dec_NN(key2);	/* sv_magic() increments refcount */
	    LvTARG(lv) = SvREFCNT_inc_simple_NN(hv);
	    LvTARGLEN(lv) = 1;
	    PUSHs(lv);
	    RETURN;
	}
	if (localizing) {
	    if (HvNAME_get(hv) && isGV(*svp))
		save_gp(MUTABLE_GV(*svp), !(PL_op->op_flags & OPf_SPECIAL));
	    else if (preeminent)
		save_helem_flags(hv, keysv, svp,
		     (PL_op->op_flags & OPf_SPECIAL) ? 0 : SAVEf_SETMAGIC);
	    else
		SAVEHDELETE(hv, keysv);
	}
	else if (PL_op->op_private & OPpDEREF) {
	    PUSHs(vivify_ref(*svp, PL_op->op_private & OPpDEREF));
	    RETURN;
	}
    }
    sv = (svp && *svp ? *svp : &PL_sv_undef);
    /* Originally this did a conditional C<sv = sv_mortalcopy(sv)>; this
     * was to make C<local $tied{foo} = $tied{foo}> possible.
     * However, it seems no longer to be needed for that purpose, and
     * introduced a new bug: stuff like C<while ($hash{taintedval} =~ /.../g>
     * would loop endlessly since the pos magic is getting set on the
     * mortal copy and lost. However, the copy has the effect of
     * triggering the get magic, and losing it altogether made things like
     * c<$tied{foo};> in void context no longer do get magic, which some
     * code relied on. Also, delayed triggering of magic on @+ and friends
     * meant the original regex may be out of scope by now. So as a
     * compromise, do the get magic here. (The MGf_GSKIP flag will stop it
     * being called too many times). */
    if (!lval && SvRMAGICAL(hv) && SvGMAGICAL(sv))
	mg_get(sv);
    PUSHs(sv);
    RETURN;
}


/* a stripped-down version of Perl_softref2xv() for use by
 * pp_multideref(), which doesn't use PL_op->op_flags */

STATIC GV *
S_softref2xv_lite(pTHX_ SV *const sv, const char *const what,
		const svtype type)
{
    if (PL_op->op_private & HINT_STRICT_REFS) {
	if (SvOK(sv))
	    Perl_die(aTHX_ PL_no_symref_sv, sv,
		     (SvPOKp(sv) && SvCUR(sv)>32 ? "..." : ""), what);
	else
	    Perl_die(aTHX_ PL_no_usym, what);
    }
    if (!SvOK(sv))
        Perl_die(aTHX_ PL_no_usym, what);
    return gv_fetchsv_nomg(sv, GV_ADD, type);
}


/* Handle one or more aggregate derefs and array/hash indexings, e.g.
 * $h->{foo}  or  $a[0]{$key}[$i]  or  f()->[1]
 *
 * op_aux points to an array of unions of UV / IV / SV* / PADOFFSET.
 * Each of these either contains a set of actions, or an argument, such as
 * an IV to use as an array index, or a lexical var to retrieve.
 * Several actions re stored per UV; we keep shifting new actions off the
 * one UV, and only reload when it becomes zero.
 */

PP(pp_multideref)
{
    SV *sv = NULL; /* init to avoid spurious 'may be used uninitialized' */
    UNOP_AUX_item *items = cUNOP_AUXx(PL_op)->op_aux;
    UV actions = items->uv;

    assert(actions);
    /* this tells find_uninit_var() where we're up to */
    PL_multideref_pc = items;

    while (1) {
        /* there are three main classes of action; the first retrieve
         * the initial AV or HV from a variable or the stack; the second
         * does the equivalent of an unrolled (/DREFAV, rv2av, aelem),
         * the third an unrolled (/DREFHV, rv2hv, helem).
         */
        switch (actions & MDEREF_ACTION_MASK) {

        case MDEREF_reload:
            actions = (++items)->uv;
            continue;

        case MDEREF_AV_padav_aelem:                 /* $lex[...] */
            sv = PAD_SVl((++items)->pad_offset);
            goto do_AV_aelem;

        case MDEREF_AV_gvav_aelem:                  /* $pkg[...] */
            sv = UNOP_AUX_item_sv(++items);
            assert(isGV_with_GP(sv));
            sv = (SV*)GvAVn((GV*)sv);
            goto do_AV_aelem;

        case MDEREF_AV_pop_rv2av_aelem:             /* expr->[...] */
            {
                dSP;
                sv = POPs;
                PUTBACK;
                goto do_AV_rv2av_aelem;
            }

        case MDEREF_AV_gvsv_vivify_rv2av_aelem:     /* $pkg->[...] */
            sv = UNOP_AUX_item_sv(++items);
            assert(isGV_with_GP(sv));
            sv = GvSVn((GV*)sv);
            goto do_AV_vivify_rv2av_aelem;

        case MDEREF_AV_padsv_vivify_rv2av_aelem:     /* $lex->[...] */
            sv = PAD_SVl((++items)->pad_offset);
            /* FALLTHROUGH */

        do_AV_vivify_rv2av_aelem:
        case MDEREF_AV_vivify_rv2av_aelem:           /* vivify, ->[...] */
            /* this is the OPpDEREF action normally found at the end of
             * ops like aelem, helem, rv2sv */
            sv = vivify_ref(sv, OPpDEREF_AV);
            /* FALLTHROUGH */

        do_AV_rv2av_aelem:
            /* this is basically a copy of pp_rv2av when it just has the
             * sKR/1 flags */
            SvGETMAGIC(sv);
            if (LIKELY(SvROK(sv))) {
                if (UNLIKELY(SvAMAGIC(sv))) {
                    sv = amagic_deref_call(sv, to_av_amg);
                }
                sv = SvRV(sv);
                if (UNLIKELY(SvTYPE(sv) != SVt_PVAV))
                    DIE(aTHX_ "Not an ARRAY reference");
            }
            else if (SvTYPE(sv) != SVt_PVAV) {
                if (!isGV_with_GP(sv))
                    sv = (SV*)S_softref2xv_lite(aTHX_ sv, "an ARRAY", SVt_PVAV);
                sv = MUTABLE_SV(GvAVn((GV*)sv));
            }
            /* FALLTHROUGH */

        do_AV_aelem:
            {
                /* retrieve the key; this may be either a lexical or package
                 * var (whose index/ptr is stored as an item) or a signed
                 * integer constant stored as an item.
                 */
                SV *elemsv;
                IV elem = 0; /* to shut up stupid compiler warnings */


                assert(SvTYPE(sv) == SVt_PVAV);

                switch (actions & MDEREF_INDEX_MASK) {
                case MDEREF_INDEX_none:
                    goto finish;
                case MDEREF_INDEX_const:
                    elem  = (++items)->iv;
                    break;
                case MDEREF_INDEX_padsv:
                    elemsv = PAD_SVl((++items)->pad_offset);
                    goto check_elem;
                case MDEREF_INDEX_gvsv:
                    elemsv = UNOP_AUX_item_sv(++items);
                    assert(isGV_with_GP(elemsv));
                    elemsv = GvSVn((GV*)elemsv);
                check_elem:
                    if (UNLIKELY(SvROK(elemsv) && !SvGAMAGIC(elemsv)
                                            && ckWARN(WARN_MISC)))
                        Perl_warner(aTHX_ packWARN(WARN_MISC),
                                "Use of reference \"%" SVf "\" as array index",
                                SVfARG(elemsv));
                    /* the only time that S_find_uninit_var() needs this
                     * is to determine which index value triggered the
                     * undef warning. So just update it here. Note that
                     * since we don't save and restore this var (e.g. for
                     * tie or overload execution), its value will be
                     * meaningless apart from just here */
                    PL_multideref_pc = items;
                    elem = SvIV(elemsv);
                    break;
                }


                /* this is basically a copy of pp_aelem with OPpDEREF skipped */

                if (!(actions & MDEREF_FLAG_last)) {
                    SV** svp = av_fetch((AV*)sv, elem, 1);
                    if (!svp || ! (sv=*svp))
                        DIE(aTHX_ PL_no_aelem, elem);
                    break;
                }

                if (PL_op->op_private &
                    (OPpMULTIDEREF_EXISTS|OPpMULTIDEREF_DELETE))
                {
                    if (PL_op->op_private & OPpMULTIDEREF_EXISTS) {
                        sv = av_exists((AV*)sv, elem) ? &PL_sv_yes : &PL_sv_no;
                    }
                    else {
                        I32 discard = (GIMME_V == G_VOID) ? G_DISCARD : 0;
                        sv = av_delete((AV*)sv, elem, discard);
                        if (discard)
                            return NORMAL;
                        if (!sv)
                            sv = &PL_sv_undef;
                    }
                }
                else {
                    const U32 lval = PL_op->op_flags & OPf_MOD || LVRET;
                    const U32 defer = PL_op->op_private & OPpLVAL_DEFER;
                    const bool localizing = PL_op->op_private & OPpLVAL_INTRO;
                    bool preeminent = TRUE;
                    AV *const av = (AV*)sv;
                    SV** svp;

                    if (UNLIKELY(localizing)) {
                        MAGIC *mg;
                        HV *stash;

                        /* If we can determine whether the element exist,
                         * Try to preserve the existenceness of a tied array
                         * element by using EXISTS and DELETE if possible.
                         * Fallback to FETCH and STORE otherwise. */
                        if (SvCANEXISTDELETE(av))
                            preeminent = av_exists(av, elem);
                    }

                    svp = av_fetch(av, elem, lval && !defer);

                    if (lval) {
                        if (!svp || !(sv = *svp)) {
                            IV len;
                            if (!defer)
                                DIE(aTHX_ PL_no_aelem, elem);
                            len = av_tindex(av);
                            sv = sv_2mortal(newSVavdefelem(av,
                            /* Resolve a negative index now, unless it points
                             * before the beginning of the array, in which
                             * case record it for error reporting in
                             * magic_setdefelem. */
                                elem < 0 && len + elem >= 0
                                    ? len + elem : elem, 1));
                        }
                        else {
                            if (UNLIKELY(localizing)) {
                                if (preeminent) {
                                    save_aelem(av, elem, svp);
                                    sv = *svp; /* may have changed */
                                }
                                else
                                    SAVEADELETE(av, elem);
                            }
                        }
                    }
                    else {
                        sv = (svp ? *svp : &PL_sv_undef);
                        /* see note in pp_helem() */
                        if (SvRMAGICAL(av) && SvGMAGICAL(sv))
                            mg_get(sv);
                    }
                }

            }
          finish:
            {
                dSP;
                XPUSHs(sv);
                RETURN;
            }
            /* NOTREACHED */




        case MDEREF_HV_padhv_helem:                 /* $lex{...} */
            sv = PAD_SVl((++items)->pad_offset);
            goto do_HV_helem;

        case MDEREF_HV_gvhv_helem:                  /* $pkg{...} */
            sv = UNOP_AUX_item_sv(++items);
            assert(isGV_with_GP(sv));
            sv = (SV*)GvHVn((GV*)sv);
            goto do_HV_helem;

        case MDEREF_HV_pop_rv2hv_helem:             /* expr->{...} */
            {
                dSP;
                sv = POPs;
                PUTBACK;
                goto do_HV_rv2hv_helem;
            }

        case MDEREF_HV_gvsv_vivify_rv2hv_helem:     /* $pkg->{...} */
            sv = UNOP_AUX_item_sv(++items);
            assert(isGV_with_GP(sv));
            sv = GvSVn((GV*)sv);
            goto do_HV_vivify_rv2hv_helem;

        case MDEREF_HV_padsv_vivify_rv2hv_helem:    /* $lex->{...} */
            sv = PAD_SVl((++items)->pad_offset);
            /* FALLTHROUGH */

        do_HV_vivify_rv2hv_helem:
        case MDEREF_HV_vivify_rv2hv_helem:           /* vivify, ->{...} */
            /* this is the OPpDEREF action normally found at the end of
             * ops like aelem, helem, rv2sv */
            sv = vivify_ref(sv, OPpDEREF_HV);
            /* FALLTHROUGH */

        do_HV_rv2hv_helem:
            /* this is basically a copy of pp_rv2hv when it just has the
             * sKR/1 flags (and pp_rv2hv is aliased to pp_rv2av) */

            SvGETMAGIC(sv);
            if (LIKELY(SvROK(sv))) {
                if (UNLIKELY(SvAMAGIC(sv))) {
                    sv = amagic_deref_call(sv, to_hv_amg);
                }
                sv = SvRV(sv);
                if (UNLIKELY(SvTYPE(sv) != SVt_PVHV))
                    DIE(aTHX_ "Not a HASH reference");
            }
            else if (SvTYPE(sv) != SVt_PVHV) {
                if (!isGV_with_GP(sv))
                    sv = (SV*)S_softref2xv_lite(aTHX_ sv, "a HASH", SVt_PVHV);
                sv = MUTABLE_SV(GvHVn((GV*)sv));
            }
            /* FALLTHROUGH */

        do_HV_helem:
            {
                /* retrieve the key; this may be either a lexical / package
                 * var or a string constant, whose index/ptr is stored as an
                 * item
                 */
                SV *keysv = NULL; /* to shut up stupid compiler warnings */

                assert(SvTYPE(sv) == SVt_PVHV);

                switch (actions & MDEREF_INDEX_MASK) {
                case MDEREF_INDEX_none:
                    goto finish;

                case MDEREF_INDEX_const:
                    keysv = UNOP_AUX_item_sv(++items);
                    break;

                case MDEREF_INDEX_padsv:
                    keysv = PAD_SVl((++items)->pad_offset);
                    break;

                case MDEREF_INDEX_gvsv:
                    keysv = UNOP_AUX_item_sv(++items);
                    keysv = GvSVn((GV*)keysv);
                    break;
                }

                /* see comment above about setting this var */
                PL_multideref_pc = items;


                /* ensure that candidate CONSTs have been HEKified */
                assert(   ((actions & MDEREF_INDEX_MASK) != MDEREF_INDEX_const)
                       || SvTYPE(keysv) >= SVt_PVMG
                       || !SvOK(keysv)
                       || SvROK(keysv)
                       || SvIsCOW_shared_hash(keysv));

                /* this is basically a copy of pp_helem with OPpDEREF skipped */

                if (!(actions & MDEREF_FLAG_last)) {
                    HE *he = hv_fetch_ent((HV*)sv, keysv, 1, 0);
                    if (!he || !(sv=HeVAL(he)) || sv == &PL_sv_undef)
                        DIE(aTHX_ PL_no_helem_sv, SVfARG(keysv));
                    break;
                }

                if (PL_op->op_private &
                    (OPpMULTIDEREF_EXISTS|OPpMULTIDEREF_DELETE))
                {
                    if (PL_op->op_private & OPpMULTIDEREF_EXISTS) {
                        sv = hv_exists_ent((HV*)sv, keysv, 0)
                                                ? &PL_sv_yes : &PL_sv_no;
                    }
                    else {
                        I32 discard = (GIMME_V == G_VOID) ? G_DISCARD : 0;
                        sv = hv_delete_ent((HV*)sv, keysv, discard, 0);
                        if (discard)
                            return NORMAL;
                        if (!sv)
                            sv = &PL_sv_undef;
                    }
                }
                else {
                    const U32 lval = PL_op->op_flags & OPf_MOD || LVRET;
                    const U32 defer = PL_op->op_private & OPpLVAL_DEFER;
                    const bool localizing = PL_op->op_private & OPpLVAL_INTRO;
                    bool preeminent = TRUE;
                    SV **svp;
                    HV * const hv = (HV*)sv;
                    HE* he;

                    if (UNLIKELY(localizing)) {
                        MAGIC *mg;
                        HV *stash;

                        /* If we can determine whether the element exist,
                         * Try to preserve the existenceness of a tied hash
                         * element by using EXISTS and DELETE if possible.
                         * Fallback to FETCH and STORE otherwise. */
                        if (SvCANEXISTDELETE(hv))
                            preeminent = hv_exists_ent(hv, keysv, 0);
                    }

                    he = hv_fetch_ent(hv, keysv, lval && !defer, 0);
                    svp = he ? &HeVAL(he) : NULL;


                    if (lval) {
                        if (!svp || !(sv = *svp) || sv == &PL_sv_undef) {
                            SV* lv;
                            SV* key2;
                            if (!defer)
                                DIE(aTHX_ PL_no_helem_sv, SVfARG(keysv));
                            lv = sv_newmortal();
                            sv_upgrade(lv, SVt_PVLV);
                            LvTYPE(lv) = 'y';
                            sv_magic(lv, key2 = newSVsv(keysv),
                                                PERL_MAGIC_defelem, NULL, 0);
                            /* sv_magic() increments refcount */
                            SvREFCNT_dec_NN(key2);
                            LvTARG(lv) = SvREFCNT_inc_simple_NN(hv);
                            LvTARGLEN(lv) = 1;
                            sv = lv;
                        }
                        else {
                            if (localizing) {
                                if (HvNAME_get(hv) && isGV(sv))
                                    save_gp(MUTABLE_GV(sv),
                                        !(PL_op->op_flags & OPf_SPECIAL));
                                else if (preeminent) {
                                    save_helem_flags(hv, keysv, svp,
                                         (PL_op->op_flags & OPf_SPECIAL)
                                            ? 0 : SAVEf_SETMAGIC);
                                    sv = *svp; /* may have changed */
                                }
                                else
                                    SAVEHDELETE(hv, keysv);
                            }
                        }
                    }
                    else {
                        sv = (svp && *svp ? *svp : &PL_sv_undef);
                        /* see note in pp_helem() */
                        if (SvRMAGICAL(hv) && SvGMAGICAL(sv))
                            mg_get(sv);
                    }
                }
                goto finish;
            }

        } /* switch */

        actions >>= MDEREF_SHIFT;
    } /* while */
    /* NOTREACHED */
}


PP(pp_iter)
{
    PERL_CONTEXT *cx;
    SV *oldsv;
    SV **itersvp;

    SV *sv;
    AV *av;
    IV ix;
    IV inc;

    cx = CX_CUR();
    itersvp = CxITERVAR(cx);
    assert(itersvp);

    switch (CxTYPE(cx)) {

    case CXt_LOOP_LAZYSV: /* string increment */
    {
        SV* cur = cx->blk_loop.state_u.lazysv.cur;
        SV *end = cx->blk_loop.state_u.lazysv.end;
        /* If the maximum is !SvOK(), pp_enteriter substitutes PL_sv_no.
           It has SvPVX of "" and SvCUR of 0, which is what we want.  */
        STRLEN maxlen = 0;
        const char *max = SvPV_const(end, maxlen);
        if (DO_UTF8(end) && IN_UNI_8_BIT)
            maxlen = sv_len_utf8_nomg(end);
        if (UNLIKELY(SvNIOK(cur) || SvCUR(cur) > maxlen))
            goto retno;

        oldsv = *itersvp;
        /* NB: on the first iteration, oldsv will have a ref count of at
         * least 2 (one extra from blk_loop.itersave), so the GV or pad
         * slot will get localised; on subsequent iterations the RC==1
         * optimisation may kick in and the SV will be reused. */
         if (oldsv && LIKELY(SvREFCNT(oldsv) == 1 && !SvMAGICAL(oldsv))) {
            /* safe to reuse old SV */
            sv_setsv(oldsv, cur);
        }
        else
        {
            /* we need a fresh SV every time so that loop body sees a
             * completely new SV for closures/references to work as
             * they used to */
            *itersvp = newSVsv(cur);
            SvREFCNT_dec(oldsv);
        }
        if (strEQ(SvPVX_const(cur), max))
            sv_setiv(cur, 0); /* terminate next time */
        else
            sv_inc(cur);
        break;
    }

    case CXt_LOOP_LAZYIV: /* integer increment */
    {
        IV cur = cx->blk_loop.state_u.lazyiv.cur;
	if (UNLIKELY(cur > cx->blk_loop.state_u.lazyiv.end))
	    goto retno;

        oldsv = *itersvp;
	/* see NB comment above */
	if (oldsv && LIKELY(SvREFCNT(oldsv) == 1 && !SvMAGICAL(oldsv))) {
	    /* safe to reuse old SV */

            if (    (SvFLAGS(oldsv) & (SVTYPEMASK|SVf_THINKFIRST|SVf_IVisUV))
                 == SVt_IV)
            {
                /* Cheap SvIOK_only().
                 * Assert that flags which SvIOK_only() would test or
                 * clear can't be set, because we're SVt_IV */
                assert(!(SvFLAGS(oldsv) &
                    (SVf_OOK|SVf_UTF8|(SVf_OK & ~(SVf_IOK|SVp_IOK)))));
                SvFLAGS(oldsv) |= (SVf_IOK|SVp_IOK);
                /* SvIV_set() where sv_any points to head */
                oldsv->sv_u.svu_iv = cur;

            }
            else
                sv_setiv(oldsv, cur);
	}
	else
	{
	    /* we need a fresh SV every time so that loop body sees a
	     * completely new SV for closures/references to work as they
	     * used to */
	    *itersvp = newSViv(cur);
	    SvREFCNT_dec(oldsv);
	}

	if (UNLIKELY(cur == IV_MAX)) {
	    /* Handle end of range at IV_MAX */
	    cx->blk_loop.state_u.lazyiv.end = IV_MIN;
	} else
	    ++cx->blk_loop.state_u.lazyiv.cur;
        break;
    }

    case CXt_LOOP_LIST: /* for (1,2,3) */

        assert(OPpITER_REVERSED == 2); /* so inc becomes -1 or 1 */
        inc = 1 - (PL_op->op_private & OPpITER_REVERSED);
        ix = (cx->blk_loop.state_u.stack.ix += inc);
        if (UNLIKELY(inc > 0
                        ? ix > cx->blk_oldsp
                        : ix <= cx->blk_loop.state_u.stack.basesp)
        )
            goto retno;

        sv = PL_stack_base[ix];
        av = NULL;
        goto loop_ary_common;

    case CXt_LOOP_ARY: /* for (@ary) */

        av = cx->blk_loop.state_u.ary.ary;
        inc = 1 - (PL_op->op_private & OPpITER_REVERSED);
        ix = (cx->blk_loop.state_u.ary.ix += inc);
        if (UNLIKELY(inc > 0
                        ? ix > AvFILL(av)
                        : ix < 0)
        )
            goto retno;

        if (UNLIKELY(SvRMAGICAL(av))) {
            SV * const * const svp = av_fetch(av, ix, FALSE);
            sv = svp ? *svp : NULL;
        }
        else {
            sv = AvARRAY(av)[ix];
        }

      loop_ary_common:

        if (UNLIKELY(cx->cx_type & CXp_FOR_LVREF)) {
            SvSetMagicSV(*itersvp, sv);
            break;
        }

        if (LIKELY(sv)) {
            if (UNLIKELY(SvIS_FREED(sv))) {
                *itersvp = NULL;
                Perl_croak(aTHX_ "Use of freed value in iteration");
            }
            if (SvPADTMP(sv)) {
                sv = newSVsv(sv);
            }
            else {
                SvTEMP_off(sv);
                SvREFCNT_inc_simple_void_NN(sv);
            }
        }
        else if (av) {
            sv = newSVavdefelem(av, ix, 0);
        }
        else
            sv = &PL_sv_undef;

        oldsv = *itersvp;
        *itersvp = sv;
        SvREFCNT_dec(oldsv);
        break;

    default:
	DIE(aTHX_ "panic: pp_iter, type=%u", CxTYPE(cx));
    }

    /* Bypass pushing &PL_sv_yes and calling pp_and(); instead
     * jump straight to the AND op's op_other */
    assert(PL_op->op_next->op_type == OP_AND);
    assert(PL_op->op_next->op_ppaddr == Perl_pp_and);
    return cLOGOPx(PL_op->op_next)->op_other;

  retno:
    /* Bypass pushing &PL_sv_no and calling pp_and(); instead
     * jump straight to the AND op's op_next */
    assert(PL_op->op_next->op_type == OP_AND);
    assert(PL_op->op_next->op_ppaddr == Perl_pp_and);
    /* pp_enteriter should have pre-extended the stack */
    EXTEND_SKIP(PL_stack_sp, 1);
    /* we only need this for the rare case where the OP_AND isn't
     * in void context, e.g. $x = do { for (..) {...} };
     * but its cheaper to just push it rather than testing first
     */
    *++PL_stack_sp = &PL_sv_no;
    return PL_op->op_next->op_next;
}


/*
A description of how taint works in pattern matching and substitution.

This is all conditional on NO_TAINT_SUPPORT not being defined. Under
NO_TAINT_SUPPORT, taint-related operations should become no-ops.

While the pattern is being assembled/concatenated and then compiled,
PL_tainted will get set (via TAINT_set) if any component of the pattern
is tainted, e.g. /.*$tainted/.  At the end of pattern compilation,
the RXf_TAINTED flag is set on the pattern if PL_tainted is set (via
TAINT_get).  It will also be set if any component of the pattern matches
based on locale-dependent behavior.

When the pattern is copied, e.g. $r = qr/..../, the SV holding the ref to
the pattern is marked as tainted. This means that subsequent usage, such
as /x$r/, will set PL_tainted using TAINT_set, and thus RXf_TAINTED,
on the new pattern too.

RXf_TAINTED_SEEN is used post-execution by the get magic code
of $1 et al to indicate whether the returned value should be tainted.
It is the responsibility of the caller of the pattern (i.e. pp_match,
pp_subst etc) to set this flag for any other circumstances where $1 needs
to be tainted.

The taint behaviour of pp_subst (and pp_substcont) is quite complex.

There are three possible sources of taint
    * the source string
    * the pattern (both compile- and run-time, RXf_TAINTED / RXf_TAINTED_SEEN)
    * the replacement string (or expression under /e)
    
There are four destinations of taint and they are affected by the sources
according to the rules below:

    * the return value (not including /r):
	tainted by the source string and pattern, but only for the
	number-of-iterations case; boolean returns aren't tainted;
    * the modified string (or modified copy under /r):
	tainted by the source string, pattern, and replacement strings;
    * $1 et al:
	tainted by the pattern, and under 'use re "taint"', by the source
	string too;
    * PL_taint - i.e. whether subsequent code (e.g. in a /e block) is tainted:
	should always be unset before executing subsequent code.

The overall action of pp_subst is:

    * at the start, set bits in rxtainted indicating the taint status of
	the various sources.

    * After each pattern execution, update the SUBST_TAINT_PAT bit in
	rxtainted if RXf_TAINTED_SEEN has been set, to indicate that the
	pattern has subsequently become tainted via locale ops.

    * If control is being passed to pp_substcont to execute a /e block,
	save rxtainted in the CXt_SUBST block, for future use by
	pp_substcont.

    * Whenever control is being returned to perl code (either by falling
	off the "end" of pp_subst/pp_substcont, or by entering a /e block),
	use the flag bits in rxtainted to make all the appropriate types of
	destination taint visible; e.g. set RXf_TAINTED_SEEN so that $1
	et al will appear tainted.

pp_match is just a simpler version of the above.

*/

PP(pp_subst)
{
    dSP; dTARG;
    PMOP *pm = cPMOP;
    PMOP *rpm = pm;
    char *s;
    char *strend;
    const char *c;
    STRLEN clen;
    SSize_t iters = 0;
    SSize_t maxiters;
    bool once;
    U8 rxtainted = 0; /* holds various SUBST_TAINT_* flag bits.
			See "how taint works" above */
    char *orig;
    U8 r_flags;
    REGEXP *rx = PM_GETRE(pm);
    regexp *prog = ReANY(rx);
    STRLEN len;
    int force_on_match = 0;
    const I32 oldsave = PL_savestack_ix;
    STRLEN slen;
    bool doutf8 = FALSE; /* whether replacement is in utf8 */
#ifdef PERL_ANY_COW
    bool was_cow;
#endif
    SV *nsv = NULL;
    /* known replacement string? */
    SV *dstr = (pm->op_pmflags & PMf_CONST) ? POPs : NULL;

    PERL_ASYNC_CHECK();

    if (PL_op->op_flags & OPf_STACKED)
	TARG = POPs;
    else {
        if (ARGTARG)
            GETTARGET;
        else {
            TARG = DEFSV;
        }
	EXTEND(SP,1);
    }

    SvGETMAGIC(TARG); /* must come before cow check */
#ifdef PERL_ANY_COW
    /* note that a string might get converted to COW during matching */
    was_cow = cBOOL(SvIsCOW(TARG));
#endif
    if (!(rpm->op_pmflags & PMf_NONDESTRUCT)) {
#ifndef PERL_ANY_COW
	if (SvIsCOW(TARG))
	    sv_force_normal_flags(TARG,0);
#endif
	if ((SvREADONLY(TARG)
		|| ( ((SvTYPE(TARG) == SVt_PVGV && isGV_with_GP(TARG))
		      || SvTYPE(TARG) > SVt_PVLV)
		     && !(SvTYPE(TARG) == SVt_PVGV && SvFAKE(TARG)))))
	    Perl_croak_no_modify();
    }
    PUTBACK;

    orig = SvPV_nomg(TARG, len);
    /* note we don't (yet) force the var into being a string; if we fail
     * to match, we leave as-is; on successful match however, we *will*
     * coerce into a string, then repeat the match */
    if (!SvPOKp(TARG) || SvTYPE(TARG) == SVt_PVGV || SvVOK(TARG))
	force_on_match = 1;

    /* only replace once? */
    once = !(rpm->op_pmflags & PMf_GLOBAL);

    /* See "how taint works" above */
    if (TAINTING_get) {
	rxtainted  = (
	    (SvTAINTED(TARG) ? SUBST_TAINT_STR : 0)
	  | (RXp_ISTAINTED(prog) ? SUBST_TAINT_PAT : 0)
	  | ((pm->op_pmflags & PMf_RETAINT) ? SUBST_TAINT_RETAINT : 0)
	  | ((once && !(rpm->op_pmflags & PMf_NONDESTRUCT))
		? SUBST_TAINT_BOOLRET : 0));
	TAINT_NOT;
    }

  force_it:
    if (!pm || !orig)
	DIE(aTHX_ "panic: pp_subst, pm=%p, orig=%p", pm, orig);

    strend = orig + len;
    slen = DO_UTF8(TARG) ? utf8_length((U8*)orig, (U8*)strend) : len;
    maxiters = 2 * slen + 10;	/* We can match twice at each
				   position, once with zero-length,
				   second time with non-zero. */

    /* handle the empty pattern */
    if (!RX_PRELEN(rx) && PL_curpm && !prog->mother_re) {
        if (PL_curpm == PL_reg_curpm) {
            if (PL_curpm_under) {
                if (PL_curpm_under == PL_reg_curpm) {
                    Perl_croak(aTHX_ "Infinite recursion via empty pattern");
                } else {
                    pm = PL_curpm_under;
                }
            }
        } else {
            pm = PL_curpm;
        }
        rx = PM_GETRE(pm);
        prog = ReANY(rx);
    }

#ifdef PERL_SAWAMPERSAND
    r_flags = (    RXp_NPARENS(prog)
                || PL_sawampersand
                || (RXp_EXTFLAGS(prog) & (RXf_EVAL_SEEN|RXf_PMf_KEEPCOPY))
                || (rpm->op_pmflags & PMf_KEEPCOPY)
              )
          ? REXEC_COPY_STR
          : 0;
#else
    r_flags = REXEC_COPY_STR;
#endif

    if (!CALLREGEXEC(rx, orig, strend, orig, 0, TARG, NULL, r_flags))
    {
	SPAGAIN;
	PUSHs(rpm->op_pmflags & PMf_NONDESTRUCT ? TARG : &PL_sv_no);
	LEAVE_SCOPE(oldsave);
	RETURN;
    }
    PL_curpm = pm;

    /* known replacement string? */
    if (dstr) {
	/* replacement needing upgrading? */
	if (DO_UTF8(TARG) && !doutf8) {
	     nsv = sv_newmortal();
	     SvSetSV(nsv, dstr);
	     sv_utf8_upgrade(nsv);
	     c = SvPV_const(nsv, clen);
	     doutf8 = TRUE;
	}
	else {
	    c = SvPV_const(dstr, clen);
	    doutf8 = DO_UTF8(dstr);
	}

	if (SvTAINTED(dstr))
	    rxtainted |= SUBST_TAINT_REPL;
    }
    else {
	c = NULL;
	doutf8 = FALSE;
    }
    
    /* can do inplace substitution? */
    if (c
#ifdef PERL_ANY_COW
	&& !was_cow
#endif
        && (I32)clen <= RXp_MINLENRET(prog)
        && (  once
           || !(r_flags & REXEC_COPY_STR)
           || (!SvGMAGICAL(dstr) && !(RXp_EXTFLAGS(prog) & RXf_EVAL_SEEN))
           )
        && !(RXp_EXTFLAGS(prog) & RXf_NO_INPLACE_SUBST)
	&& (!doutf8 || SvUTF8(TARG))
	&& !(rpm->op_pmflags & PMf_NONDESTRUCT))
    {

#ifdef PERL_ANY_COW
        /* string might have got converted to COW since we set was_cow */
	if (SvIsCOW(TARG)) {
	  if (!force_on_match)
	    goto have_a_cow;
	  assert(SvVOK(TARG));
	}
#endif
	if (force_on_match) {
            /* redo the first match, this time with the orig var
             * forced into being a string */
	    force_on_match = 0;
	    orig = SvPV_force_nomg(TARG, len);
	    goto force_it;
	}

	if (once) {
            char *d, *m;
	    if (RXp_MATCH_TAINTED(prog)) /* run time pattern taint, eg locale */
		rxtainted |= SUBST_TAINT_PAT;
	    m = orig + RXp_OFFS(prog)[0].start;
	    d = orig + RXp_OFFS(prog)[0].end;
	    s = orig;
	    if (m - s > strend - d) {  /* faster to shorten from end */
                I32 i;
		if (clen) {
		    Copy(c, m, clen, char);
		    m += clen;
		}
		i = strend - d;
		if (i > 0) {
		    Move(d, m, i, char);
		    m += i;
		}
		*m = '\0';
		SvCUR_set(TARG, m - s);
	    }
	    else {	/* faster from front */
                I32 i = m - s;
		d -= clen;
                if (i > 0)
                    Move(s, d - i, i, char);
		sv_chop(TARG, d-i);
		if (clen)
		    Copy(c, d, clen, char);
	    }
	    SPAGAIN;
	    PUSHs(&PL_sv_yes);
	}
	else {
            char *d, *m;
            d = s = RXp_OFFS(prog)[0].start + orig;
	    do {
                I32 i;
		if (UNLIKELY(iters++ > maxiters))
		    DIE(aTHX_ "Substitution loop");
                /* run time pattern taint, eg locale */
		if (UNLIKELY(RXp_MATCH_TAINTED(prog)))
		    rxtainted |= SUBST_TAINT_PAT;
		m = RXp_OFFS(prog)[0].start + orig;
		if ((i = m - s)) {
		    if (s != d)
			Move(s, d, i, char);
		    d += i;
		}
		if (clen) {
		    Copy(c, d, clen, char);
		    d += clen;
		}
		s = RXp_OFFS(prog)[0].end + orig;
	    } while (CALLREGEXEC(rx, s, strend, orig,
				 s == m, /* don't match same null twice */
				 TARG, NULL,
                     REXEC_NOT_FIRST|REXEC_IGNOREPOS|REXEC_FAIL_ON_UNDERFLOW));
	    if (s != d) {
                I32 i = strend - s;
		SvCUR_set(TARG, d - SvPVX_const(TARG) + i);
		Move(s, d, i+1, char);		/* include the NUL */
	    }
	    SPAGAIN;
            if (PL_op->op_private & OPpTRUEBOOL)
                PUSHs(iters ? &PL_sv_yes : &PL_sv_zero);
            else
                mPUSHi(iters);
	}
    }
    else {
	bool first;
        char *m;
	SV *repl;
	if (force_on_match) {
            /* redo the first match, this time with the orig var
             * forced into being a string */
	    force_on_match = 0;
	    if (rpm->op_pmflags & PMf_NONDESTRUCT) {
		/* I feel that it should be possible to avoid this mortal copy
		   given that the code below copies into a new destination.
		   However, I suspect it isn't worth the complexity of
		   unravelling the C<goto force_it> for the small number of
		   cases where it would be viable to drop into the copy code. */
		TARG = sv_2mortal(newSVsv(TARG));
	    }
	    orig = SvPV_force_nomg(TARG, len);
	    goto force_it;
	}
#ifdef PERL_ANY_COW
      have_a_cow:
#endif
	if (RXp_MATCH_TAINTED(prog)) /* run time pattern taint, eg locale */
	    rxtainted |= SUBST_TAINT_PAT;
	repl = dstr;
        s = RXp_OFFS(prog)[0].start + orig;
	dstr = newSVpvn_flags(orig, s-orig,
                    SVs_TEMP | (DO_UTF8(TARG) ? SVf_UTF8 : 0));
	if (!c) {
	    PERL_CONTEXT *cx;
	    SPAGAIN;
            m = orig;
	    /* note that a whole bunch of local vars are saved here for
	     * use by pp_substcont: here's a list of them in case you're
	     * searching for places in this sub that uses a particular var:
	     * iters maxiters r_flags oldsave rxtainted orig dstr targ
	     * s m strend rx once */
	    CX_PUSHSUBST(cx);
	    RETURNOP(cPMOP->op_pmreplrootu.op_pmreplroot);
	}
	first = TRUE;
	do {
	    if (UNLIKELY(iters++ > maxiters))
		DIE(aTHX_ "Substitution loop");
	    if (UNLIKELY(RXp_MATCH_TAINTED(prog)))
		rxtainted |= SUBST_TAINT_PAT;
	    if (RXp_MATCH_COPIED(prog) && RXp_SUBBEG(prog) != orig) {
		char *old_s    = s;
		char *old_orig = orig;
                assert(RXp_SUBOFFSET(prog) == 0);

		orig = RXp_SUBBEG(prog);
		s = orig + (old_s - old_orig);
		strend = s + (strend - old_s);
	    }
	    m = RXp_OFFS(prog)[0].start + orig;
	    sv_catpvn_nomg_maybeutf8(dstr, s, m - s, DO_UTF8(TARG));
	    s = RXp_OFFS(prog)[0].end + orig;
	    if (first) {
		/* replacement already stringified */
	      if (clen)
		sv_catpvn_nomg_maybeutf8(dstr, c, clen, doutf8);
	      first = FALSE;
	    }
	    else {
		sv_catsv(dstr, repl);
		if (UNLIKELY(SvTAINTED(repl)))
		    rxtainted |= SUBST_TAINT_REPL;
	    }
	    if (once)
		break;
	} while (CALLREGEXEC(rx, s, strend, orig,
                             s == m,    /* Yields minend of 0 or 1 */
			     TARG, NULL,
                    REXEC_NOT_FIRST|REXEC_IGNOREPOS|REXEC_FAIL_ON_UNDERFLOW));
        assert(strend >= s);
	sv_catpvn_nomg_maybeutf8(dstr, s, strend - s, DO_UTF8(TARG));

	if (rpm->op_pmflags & PMf_NONDESTRUCT) {
	    /* From here on down we're using the copy, and leaving the original
	       untouched.  */
	    TARG = dstr;
	    SPAGAIN;
	    PUSHs(dstr);
	} else {
#ifdef PERL_ANY_COW
	    /* The match may make the string COW. If so, brilliant, because
	       that's just saved us one malloc, copy and free - the regexp has
	       donated the old buffer, and we malloc an entirely new one, rather
	       than the regexp malloc()ing a buffer and copying our original,
	       only for us to throw it away here during the substitution.  */
	    if (SvIsCOW(TARG)) {
		sv_force_normal_flags(TARG, SV_COW_DROP_PV);
	    } else
#endif
	    {
		SvPV_free(TARG);
	    }
	    SvPV_set(TARG, SvPVX(dstr));
	    SvCUR_set(TARG, SvCUR(dstr));
	    SvLEN_set(TARG, SvLEN(dstr));
	    SvFLAGS(TARG) |= SvUTF8(dstr);
	    SvPV_set(dstr, NULL);

	    SPAGAIN;
	    mPUSHi(iters);
	}
    }

    if (!(rpm->op_pmflags & PMf_NONDESTRUCT)) {
	(void)SvPOK_only_UTF8(TARG);
    }

    /* See "how taint works" above */
    if (TAINTING_get) {
	if ((rxtainted & SUBST_TAINT_PAT) ||
	    ((rxtainted & (SUBST_TAINT_STR|SUBST_TAINT_RETAINT)) ==
				(SUBST_TAINT_STR|SUBST_TAINT_RETAINT))
	)
	    (RXp_MATCH_TAINTED_on(prog)); /* taint $1 et al */

	if (!(rxtainted & SUBST_TAINT_BOOLRET)
	    && (rxtainted & (SUBST_TAINT_STR|SUBST_TAINT_PAT))
	)