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locale.c: Not so aggressive collation memory use guess
[perl5.git] / locale.c
index fd46a77..48b9184 100644 (file)
--- a/locale.c
+++ b/locale.c
 
 #include "reentr.h"
 
+/* If the environment says to, we can output debugging information during
+ * initialization.  This is done before option parsing, and before any thread
+ * creation, so can be a file-level static */
+#ifdef DEBUGGING
+static bool debug_initialization = FALSE;
+#endif
+
 #ifdef USE_LOCALE
 
 /*
@@ -119,13 +126,17 @@ Perl_set_numeric_radix(pTHX)
     else
        PL_numeric_radix_sv = NULL;
 
-    DEBUG_L(PerlIO_printf(Perl_debug_log, "Locale radix is '%s', ?UTF-8=%d\n",
+#ifdef DEBUGGING
+    if (DEBUG_L_TEST || debug_initialization) {
+        PerlIO_printf(Perl_debug_log, "Locale radix is '%s', ?UTF-8=%d\n",
                                           (PL_numeric_radix_sv)
                                            ? SvPVX(PL_numeric_radix_sv)
                                            : "NULL",
                                           (PL_numeric_radix_sv)
                                            ? cBOOL(SvUTF8(PL_numeric_radix_sv))
-                                           : 0));
+                                           : 0);
+    }
+#endif
 
 # endif /* HAS_LOCALECONV */
 #endif /* USE_LOCALE_NUMERIC */
@@ -230,8 +241,12 @@ Perl_set_numeric_standard(pTHX)
     PL_numeric_standard = TRUE;
     PL_numeric_local = isNAME_C_OR_POSIX(PL_numeric_name);
     set_numeric_radix();
-    DEBUG_L(PerlIO_printf(Perl_debug_log,
-                          "Underlying LC_NUMERIC locale now is C\n"));
+#ifdef DEBUGGING
+    if (DEBUG_L_TEST || debug_initialization) {
+        PerlIO_printf(Perl_debug_log,
+                          "Underlying LC_NUMERIC locale now is C\n");
+    }
+#endif
 
 #endif /* USE_LOCALE_NUMERIC */
 }
@@ -250,9 +265,13 @@ Perl_set_numeric_local(pTHX)
     PL_numeric_standard = isNAME_C_OR_POSIX(PL_numeric_name);
     PL_numeric_local = TRUE;
     set_numeric_radix();
-    DEBUG_L(PerlIO_printf(Perl_debug_log,
+#ifdef DEBUGGING
+    if (DEBUG_L_TEST || debug_initialization) {
+        PerlIO_printf(Perl_debug_log,
                           "Underlying LC_NUMERIC locale now is %s\n",
-                          PL_numeric_name));
+                          PL_numeric_name);
+    }
+#endif
 
 #endif /* USE_LOCALE_NUMERIC */
 }
@@ -487,6 +506,7 @@ Perl_new_collate(pTHX_ const char *newcoll)
        PL_collxfrm_mult = 2;
         PL_in_utf8_COLLATE_locale = FALSE;
         *PL_strxfrm_min_char = '\0';
+        PL_strxfrm_max_cp = 0;
        return;
     }
 
@@ -502,6 +522,7 @@ Perl_new_collate(pTHX_ const char *newcoll)
 
         PL_in_utf8_COLLATE_locale = _is_cur_LC_category_utf8(LC_COLLATE);
         *PL_strxfrm_min_char = '\0';
+        PL_strxfrm_max_cp = 0;
 
         /* A locale collation definition includes primary, secondary, tertiary,
          * etc. weights for each character.  To sort, the primary weights are
@@ -545,18 +566,119 @@ Perl_new_collate(pTHX_ const char *newcoll)
          * transformations. */
 
        {
-         /*  2: at most so many chars ('a', 'b'). */
-         /* 50: surely no system expands a char more. */
-#define XFRMBUFSIZE  (2 * 50)
-         char xbuf[XFRMBUFSIZE];
-         const Size_t fa = strxfrm(xbuf, "a",  XFRMBUFSIZE);
-         const Size_t fb = strxfrm(xbuf, "ab", XFRMBUFSIZE);
-         const SSize_t mult = fb - fa;
-         if (mult < 1 && !(fa == 0 && fb == 0))
-             Perl_croak(aTHX_ "panic: strxfrm() gets absurd - a => %"UVuf", ab => %"UVuf,
-                        (UV) fa, (UV) fb);
-         PL_collxfrm_base = (fa > (Size_t)mult) ? (fa - mult) : 0;
-         PL_collxfrm_mult = mult;
+            /* We use the string below to find how long the tranformation of it
+             * is.  Almost all locales are supersets of ASCII, or at least the
+             * ASCII letters.  We use all of them, half upper half lower,
+             * because if we used fewer, we might hit just the ones that are
+             * outliers in a particular locale.  Most of the strings being
+             * collated will contain a preponderance of letters, and even if
+             * they are above-ASCII, they are likely to have the same number of
+             * weight levels as the ASCII ones.  It turns out that digits tend
+             * to have fewer levels, and some punctuation has more, but those
+             * are relatively sparse in text, and khw believes this gives a
+             * reasonable result, but it could be changed if experience so
+             * dictates. */
+            const char longer[] = "ABCDEFGHIJKLMnopqrstuvwxyz";
+            char * x_longer;        /* Transformed 'longer' */
+            Size_t x_len_longer;    /* Length of 'x_longer' */
+
+            char * x_shorter;   /* We also transform a substring of 'longer' */
+            Size_t x_len_shorter;
+
+            /* _mem_collxfrm() is used get the transformation (though here we
+             * are interested only in its length).  It is used because it has
+             * the intelligence to handle all cases, but to work, it needs some
+             * values of 'm' and 'b' to get it started.  For the purposes of
+             * this calculation we use a very conservative estimate of 'm' and
+             * 'b'.  This assumes a weight can be multiple bytes, enough to
+             * hold any UV on the platform, and there are 5 levels, 4 weight
+             * bytes, and a trailing NUL.  */
+            PL_collxfrm_base = 5;
+            PL_collxfrm_mult = 5 * sizeof(UV);
+
+            /* Find out how long the transformation really is */
+            x_longer = _mem_collxfrm(longer,
+                                     sizeof(longer) - 1,
+                                     &x_len_longer,
+
+                                     /* We avoid converting to UTF-8 in the
+                                      * called function by telling it the
+                                      * string is in UTF-8 if the locale is a
+                                      * UTF-8 one.  Since the string passed
+                                      * here is invariant under UTF-8, we can
+                                      * claim it's UTF-8 even though it isn't.
+                                      * */
+                                     PL_in_utf8_COLLATE_locale);
+            Safefree(x_longer);
+
+            /* Find out how long the transformation of a substring of 'longer'
+             * is.  Together the lengths of these transformations are
+             * sufficient to calculate 'm' and 'b'.  The substring is all of
+             * 'longer' except the first character.  This minimizes the chances
+             * of being swayed by outliers */
+            x_shorter = _mem_collxfrm(longer + 1,
+                                      sizeof(longer) - 2,
+                                      &x_len_shorter,
+                                      PL_in_utf8_COLLATE_locale);
+            Safefree(x_shorter);
+
+            /* If the results are nonsensical for this simple test, the whole
+             * locale definition is suspect.  Mark it so that locale collation
+             * is not active at all for it.  XXX Should we warn? */
+            if (   x_len_shorter == 0
+                || x_len_longer == 0
+                || x_len_shorter >= x_len_longer)
+            {
+                PL_collxfrm_mult = 0;
+                PL_collxfrm_base = 0;
+            }
+            else {
+                SSize_t base;       /* Temporary */
+
+                /* We have both:    m * strlen(longer)  + b = x_len_longer
+                 *                  m * strlen(shorter) + b = x_len_shorter;
+                 * subtracting yields:
+                 *          m * (strlen(longer) - strlen(shorter))
+                 *                             = x_len_longer - x_len_shorter
+                 * But we have set things up so that 'shorter' is 1 byte smaller
+                 * than 'longer'.  Hence:
+                 *          m = x_len_longer - x_len_shorter
+                 *
+                 * But if something went wrong, make sure the multiplier is at
+                 * least 1.
+                 */
+                if (x_len_longer > x_len_shorter) {
+                    PL_collxfrm_mult = (STRLEN) x_len_longer - x_len_shorter;
+                }
+                else {
+                    PL_collxfrm_mult = 1;
+                }
+
+                /*     mx + b = len
+                 * so:      b = len - mx
+                 * but in case something has gone wrong, make sure it is
+                 * non-negative */
+                base = x_len_longer - PL_collxfrm_mult * (sizeof(longer) - 1);
+                if (base < 0) {
+                    base = 0;
+                }
+
+                /* Add 1 for the trailing NUL */
+                PL_collxfrm_base = base + 1;
+            }
+
+#ifdef DEBUGGING
+            if (DEBUG_L_TEST || debug_initialization) {
+                PerlIO_printf(Perl_debug_log,
+                    "%s:%d: ?UTF-8 locale=%d; x_len_shorter=%"UVuf", "
+                    "x_len_longer=%"UVuf","
+                    " collate multipler=%"UVuf", collate base=%"UVuf"\n",
+                    __FILE__, __LINE__,
+                    PL_in_utf8_COLLATE_locale,
+                    x_len_shorter, x_len_longer,
+                    PL_collxfrm_mult, PL_collxfrm_base);
+            }
+#endif
        }
     }
 
@@ -794,24 +916,6 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
     const char * const setlocale_init = (PerlEnv_getenv("PERL_SKIP_LOCALE_INIT"))
                                         ? NULL
                                         : "";
-#ifdef DEBUGGING
-    const bool debug = (PerlEnv_getenv("PERL_DEBUG_LOCALE_INIT"))
-                       ? TRUE
-                       : FALSE;
-#   define DEBUG_LOCALE_INIT(category, locale, result)                      \
-       STMT_START {                                                        \
-               if (debug) {                                                \
-                    PerlIO_printf(Perl_debug_log,                           \
-                                  "%s:%d: %s\n",                            \
-                                  __FILE__, __LINE__,                       \
-                                  _setlocale_debug_string(category,         \
-                                                          locale,           \
-                                                          result));         \
-                }                                                           \
-       } STMT_END
-#else
-#   define DEBUG_LOCALE_INIT(a,b,c)
-#endif
     const char* trial_locales[5];   /* 5 = 1 each for "", LC_ALL, LANG, "", C */
     unsigned int trial_locales_count;
     const char * const lc_all     = savepv(PerlEnv_getenv("LC_ALL"));
@@ -842,6 +946,25 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
     const char *system_default_locale = NULL;
 #endif
 
+#ifdef DEBUGGING
+    debug_initialization = (PerlEnv_getenv("PERL_DEBUG_LOCALE_INIT"))
+                           ? TRUE
+                           : FALSE;
+#   define DEBUG_LOCALE_INIT(category, locale, result)                      \
+       STMT_START {                                                        \
+               if (debug_initialization) {                                 \
+                    PerlIO_printf(Perl_debug_log,                           \
+                                  "%s:%d: %s\n",                            \
+                                  __FILE__, __LINE__,                       \
+                                  _setlocale_debug_string(category,         \
+                                                          locale,           \
+                                                          result));         \
+                }                                                           \
+       } STMT_END
+#else
+#   define DEBUG_LOCALE_INIT(a,b,c)
+#endif
+
 #ifndef LOCALE_ENVIRON_REQUIRED
     PERL_UNUSED_VAR(done);
     PERL_UNUSED_VAR(locale_param);
@@ -1280,34 +1403,50 @@ Perl_init_i18nl10n(pTHX_ int printwarn)
     PERL_UNUSED_ARG(printwarn);
 #endif /* USE_LOCALE */
 
+#ifdef DEBUGGING
+    /* So won't continue to output stuff */
+    debug_initialization = FALSE;
+#endif
+
     return ok;
 }
 
-
 #ifdef USE_LOCALE_COLLATE
 
-/*
- * mem_collxfrm() is a bit like strxfrm() but with two important
- * differences. First, it handles embedded NULs. Second, it allocates
- * a bit more memory than needed for the transformed data itself.
- * The real transformed data begins at offset sizeof(collationix).
- * *xlen is set to the length of that, and doesn't include the collation index
- * size.
- * Please see sv_collxfrm() to see how this is used.
- */
-
 char *
-Perl_mem_collxfrm(pTHX_ const char *input_string,
-                         STRLEN len,
-                         STRLEN *xlen
+Perl__mem_collxfrm(pTHX_ const char *input_string,
+                         STRLEN len,    /* Length of 'input_string' */
+                         STRLEN *xlen,  /* Set to length of returned string
+                                           (not including the collation index
+                                           prefix) */
+                         bool utf8      /* Is the input in UTF-8? */
                    )
 {
+
+    /* _mem_collxfrm() is a bit like strxfrm() but with two important
+     * differences. First, it handles embedded NULs. Second, it allocates a bit
+     * more memory than needed for the transformed data itself.  The real
+     * transformed data begins at offset COLLXFRM_HDR_LEN.  *xlen is set to
+     * the length of that, and doesn't include the collation index size.
+     * Please see sv_collxfrm() to see how this is used. */
+
+#define COLLXFRM_HDR_LEN    sizeof(PL_collation_ix)
+
     char * s = (char *) input_string;
     STRLEN s_strlen = strlen(input_string);
-    char *xbuf;
-    STRLEN xAlloc, xout; /* xalloc is a reserved word in VC */
+    char *xbuf = NULL;
+    STRLEN xAlloc;          /* xalloc is a reserved word in VC */
+    bool first_time = TRUE; /* Cleared after first loop iteration */
+
+    PERL_ARGS_ASSERT__MEM_COLLXFRM;
+
+    /* Must be NUL-terminated */
+    assert(*(input_string + len) == '\0');
 
-    PERL_ARGS_ASSERT_MEM_COLLXFRM;
+    /* If this locale has defective collation, skip */
+    if (PL_collxfrm_base == 0 && PL_collxfrm_mult == 0) {
+        goto bad;
+    }
 
     /* Replace any embedded NULs with the control that sorts before any others.
      * This will give as good as possible results on strings that don't
@@ -1334,7 +1473,10 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
          * locale, find it */
         if (*PL_strxfrm_min_char == '\0') {
             int j;
-            char * cur_min_x = NULL;    /* Cur cp's xfrm, (except it also
+#ifdef DEBUGGING
+            U8     cur_min_cp = 1;  /* The code point that sorts lowest, so far */
+#endif
+            char * cur_min_x = NULL;    /* And its xfrm, (except it also
                                            includes the collation index
                                            prefixed. */
 
@@ -1355,7 +1497,9 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
 
                     /* If needs to be 2 bytes, find them */
                     if (! UVCHR_IS_INVARIANT(j)) {
-                        continue;  /* Can't handle variants yet */
+                        char * d = cur_source;
+                        append_utf8_from_native_byte((U8) j, (U8 **) &d);
+                        trial_len = 2;
                     }
                 }
                 else if (! isCNTRL_LC(j)) {
@@ -1363,12 +1507,13 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
                 }
 
                 /* Then transform it */
-                x = mem_collxfrm(cur_source, trial_len, &x_len);
+                x = _mem_collxfrm(cur_source, trial_len, &x_len,
+                                  PL_in_utf8_COLLATE_locale);
 
                 /* If something went wrong (which it shouldn't), just
                  * ignore this code point */
                 if (   x_len == 0
-                    || strlen(x + sizeof(PL_collation_ix)) < x_len)
+                    || strlen(x + COLLXFRM_HDR_LEN) < x_len)
                 {
                     continue;
                 }
@@ -1376,11 +1521,14 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
                 /* If this character's transformation is lower than
                  * the current lowest, this one becomes the lowest */
                 if (   cur_min_x == NULL
-                    || strLT(x         + sizeof(PL_collation_ix),
-                             cur_min_x + sizeof(PL_collation_ix)))
+                    || strLT(x         + COLLXFRM_HDR_LEN,
+                             cur_min_x + COLLXFRM_HDR_LEN))
                 {
                     strcpy(PL_strxfrm_min_char, cur_source);
                     cur_min_x = x;
+#ifdef DEBUGGING
+                    cur_min_cp = j;
+#endif
                 }
                 else {
                     Safefree(x);
@@ -1391,10 +1539,26 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
              * locale, arbitrarily use \001 */
             if (cur_min_x == NULL) {
                 STRLEN x_len;   /* temporary */
-                cur_min_x = mem_collxfrm("\001", 1, &x_len);
+                cur_min_x = _mem_collxfrm("\001", 1, &x_len,
+                                          PL_in_utf8_COLLATE_locale);
                 /* cur_min_cp was already initialized to 1 */
             }
 
+            DEBUG_L(PerlIO_printf(Perl_debug_log,
+                    "_mem_collxfrm: lowest collating control in the 0-255 "
+                    "range in locale %s is 0x%02X\n",
+                    PL_collation_name,
+                    cur_min_cp));
+            if (DEBUG_Lv_TEST) {
+                unsigned i;
+                PerlIO_printf(Perl_debug_log, "Its xfrm is");
+                for (i = 0; i < strlen(cur_min_x + COLLXFRM_HDR_LEN); i ++) {
+                    PerlIO_printf(Perl_debug_log, " %02x",
+                                (U8) *(cur_min_x + COLLXFRM_HDR_LEN + i));
+                }
+                PerlIO_printf(Perl_debug_log, "\n");
+            }
+
             Safefree(cur_min_x);
         }
 
@@ -1427,46 +1591,235 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
         len = strlen(s);
     }
 
+    /* Make sure the UTF8ness of the string and locale match */
+    if (utf8 != PL_in_utf8_COLLATE_locale) {
+        const char * const t = s;   /* Temporary so we can later find where the
+                                       input was */
+
+        /* Here they don't match.  Change the string's to be what the locale is
+         * expecting */
+
+        if (! utf8) { /* locale is UTF-8, but input isn't; upgrade the input */
+            s = (char *) bytes_to_utf8((const U8 *) s, &len);
+            utf8 = TRUE;
+        }
+        else {   /* locale is not UTF-8; but input is; downgrade the input */
+
+            s = (char *) bytes_from_utf8((const U8 *) s, &len, &utf8);
+
+            /* If the downgrade was successful we are done, but if the input
+             * contains things that require UTF-8 to represent, have to do
+             * damage control ... */
+            if (UNLIKELY(utf8)) {
+
+                /* What we do is construct a non-UTF-8 string with
+                 *  1) the characters representable by a single byte converted
+                 *     to be so (if necessary);
+                 *  2) and the rest converted to collate the same as the
+                 *     highest collating representable character.  That makes
+                 *     them collate at the end.  This is similar to how we
+                 *     handle embedded NULs, but we use the highest collating
+                 *     code point instead of the smallest.  Like the NUL case,
+                 *     this isn't perfect, but is the best we can reasonably
+                 *     do.  Every above-255 code point will sort the same as
+                 *     the highest-sorting 0-255 code point.  If that code
+                 *     point can combine in a sequence with some other code
+                 *     points for weight calculations, us changing something to
+                 *     be it can adversely affect the results.  But in most
+                 *     cases, it should work reasonably.  And note that this is
+                 *     really an illegal situation: using code points above 255
+                 *     on a locale where only 0-255 are valid.  If two strings
+                 *     sort entirely equal, then the sort order for the
+                 *     above-255 code points will be in code point order. */
+
+                utf8 = FALSE;
+
+                /* If we haven't calculated the code point with the maximum
+                 * collating order for this locale, do so now */
+                if (! PL_strxfrm_max_cp) {
+                    int j;
+
+                    /* The current transformed string that collates the
+                     * highest (except it also includes the prefixed collation
+                     * index. */
+                    char * cur_max_x = NULL;
+
+                    /* Look through all legal code points (NUL isn't) */
+                    for (j = 1; j < 256; j++) {
+                        char * x;
+                        STRLEN x_len;
+
+                        /* Create a 1-char string of the current code point. */
+                        char cur_source[] = { (char) j, '\0' };
+
+                        /* Then transform it */
+                        x = _mem_collxfrm(cur_source, 1, &x_len, FALSE);
+
+                        /* If something went wrong (which it shouldn't), just
+                         * ignore this code point */
+                        if (x_len == 0) {
+                            Safefree(x);
+                            continue;
+                        }
+
+                        /* If this character's transformation is higher than
+                         * the current highest, this one becomes the highest */
+                        if (   cur_max_x == NULL
+                            || strGT(x         + COLLXFRM_HDR_LEN,
+                                     cur_max_x + COLLXFRM_HDR_LEN))
+                        {
+                            PL_strxfrm_max_cp = j;
+                            cur_max_x = x;
+                        }
+                        else {
+                            Safefree(x);
+                        }
+                    }
+
+                    DEBUG_L(PerlIO_printf(Perl_debug_log,
+                            "_mem_collxfrm: highest 1-byte collating character"
+                            " in locale %s is 0x%02X\n",
+                            PL_collation_name,
+                            PL_strxfrm_max_cp));
+                    if (DEBUG_Lv_TEST) {
+                        unsigned i;
+                        PerlIO_printf(Perl_debug_log, "Its xfrm is ");
+                        for (i = 0;
+                             i < strlen(cur_max_x + COLLXFRM_HDR_LEN);
+                             i++)
+                        {
+                            PerlIO_printf(Perl_debug_log, " %02x",
+                                        (U8) cur_max_x[i + COLLXFRM_HDR_LEN]);
+                        }
+                        PerlIO_printf(Perl_debug_log, "\n");
+                    }
+
+                    Safefree(cur_max_x);
+                }
+
+                /* Here we know which legal code point collates the highest.
+                 * We are ready to construct the non-UTF-8 string.  The length
+                 * will be at least 1 byte smaller than the input string
+                 * (because we changed at least one 2-byte character into a
+                 * single byte), but that is eaten up by the trailing NUL */
+                Newx(s, len, char);
+
+                {
+                    STRLEN i;
+                    STRLEN d= 0;
+
+                    for (i = 0; i < len; i+= UTF8SKIP(t + i)) {
+                        U8 cur_char = t[i];
+                        if (UTF8_IS_INVARIANT(cur_char)) {
+                            s[d++] = cur_char;
+                        }
+                        else if (UTF8_IS_DOWNGRADEABLE_START(cur_char)) {
+                            s[d++] = EIGHT_BIT_UTF8_TO_NATIVE(cur_char, t[i+1]);
+                        }
+                        else {  /* Replace illegal cp with highest collating
+                                   one */
+                            s[d++] = PL_strxfrm_max_cp;
+                        }
+                    }
+                    s[d++] = '\0';
+                    Renew(s, d, char);   /* Free up unused space */
+                }
+            }
+        }
+
+        /* Here, we have constructed a modified version of the input.  It could
+         * be that we already had a modified copy before we did this version.
+         * If so, that copy is no longer needed */
+        if (t != input_string) {
+            Safefree(t);
+        }
+    }
+
     /* The first element in the output is the collation id, used by
      * sv_collxfrm(); then comes the space for the transformed string.  The
      * equation should give us a good estimate as to how much is needed */
-    xAlloc = sizeof(PL_collation_ix) + PL_collxfrm_base + (PL_collxfrm_mult * len) + 1;
+    xAlloc = COLLXFRM_HDR_LEN
+           + PL_collxfrm_base
+           + (PL_collxfrm_mult * ((utf8)
+                                 ? utf8_length((U8 *) s, (U8 *) s + len)
+                                 : len));
     Newx(xbuf, xAlloc, char);
     if (UNLIKELY(! xbuf))
        goto bad;
 
     /* Store the collation id */
     *(U32*)xbuf = PL_collation_ix;
-    xout = sizeof(PL_collation_ix);
 
     /* Then the transformation of the input.  We loop until successful, or we
      * give up */
     for (;;) {
-        STRLEN xused = strxfrm(xbuf + xout, s, xAlloc - xout);
+        *xlen = strxfrm(xbuf + COLLXFRM_HDR_LEN, s, xAlloc - COLLXFRM_HDR_LEN);
 
         /* If the transformed string occupies less space than we told strxfrm()
          * was available, it means it successfully transformed the whole
          * string. */
-        if (xused < xAlloc - xout) {
-            xout += xused;
+        if (*xlen < xAlloc - COLLXFRM_HDR_LEN) {
             break;
         }
 
-        if (UNLIKELY(xused >= PERL_INT_MAX))
+        if (UNLIKELY(*xlen >= PERL_INT_MAX))
             goto bad;
 
-        /* Otherwise it should be that the transformation stopped in the middle
-         * because it ran out of space.  Malloc more, and try again.  */
-        xAlloc = (2 * xAlloc) + 1;
+        /* A well-behaved strxfrm() returns exactly how much space it needs
+         * (not including the trailing NUL) when it fails due to not enough
+         * space being provided.  Assume that this is the case unless it's been
+         * proven otherwise */
+        if (LIKELY(PL_strxfrm_is_behaved) && first_time) {
+            xAlloc = *xlen + COLLXFRM_HDR_LEN + 1;
+        }
+        else { /* Here, either:
+                *  1)  The strxfrm() has previously shown bad behavior; or
+                *  2)  It isn't the first time through the loop, which means
+                *      that the strxfrm() is now showing bad behavior, because
+                *      we gave it what it said was needed in the previous
+                *      iteration, and it came back saying it needed still more.
+                *      (Many versions of cygwin fit this.  When the buffer size
+                *      isn't sufficient, they return the input size instead of
+                *      how much is needed.)
+                * Increase the buffer size by a fixed percentage and try again. */
+            xAlloc += (xAlloc / 4) + 1;
+            PL_strxfrm_is_behaved = FALSE;
+
+#ifdef DEBUGGING
+            if (DEBUG_Lv_TEST || debug_initialization) {
+                PerlIO_printf(Perl_debug_log,
+                "_mem_collxfrm required more space than previously calculated"
+                " for locale %s, trying again with new guess=%d+%"UVuf"\n",
+                PL_collation_name, (int) COLLXFRM_HDR_LEN,
+                (UV) xAlloc - COLLXFRM_HDR_LEN);
+            }
+#endif
+        }
+
         Renew(xbuf, xAlloc, char);
         if (UNLIKELY(! xbuf))
             goto bad;
+
+        first_time = FALSE;
     }
 
-    *xlen = xout - sizeof(PL_collation_ix);
+
+#ifdef DEBUGGING
+    if (DEBUG_Lv_TEST || debug_initialization) {
+        unsigned i;
+        PerlIO_printf(Perl_debug_log,
+            "_mem_collxfrm[%d]: returning %"UVuf" for locale %s '%s'\n",
+            PL_collation_ix, *xlen, PL_collation_name, input_string);
+        PerlIO_printf(Perl_debug_log, "Its xfrm is");
+        for (i = COLLXFRM_HDR_LEN; i < *xlen + COLLXFRM_HDR_LEN; i++) {
+            PerlIO_printf(Perl_debug_log, " %02x", (U8) xbuf[i]);
+        }
+        PerlIO_printf(Perl_debug_log, "\n");
+    }
+#endif
 
     /* Free up unneeded space; retain ehough for trailing NUL */
-    Renew(xbuf, xout + 1, char);
+    Renew(xbuf, COLLXFRM_HDR_LEN + *xlen + 1, char);
 
     if (s != input_string) {
         Safefree(s);
@@ -1480,6 +1833,12 @@ Perl_mem_collxfrm(pTHX_ const char *input_string,
         Safefree(s);
     }
     *xlen = 0;
+#ifdef DEBUGGING
+    if (DEBUG_Lv_TEST || debug_initialization) {
+        PerlIO_printf(Perl_debug_log, "_mem_collxfrm[%d] returning NULL\n",
+                                      PL_collation_ix);
+    }
+#endif
     return NULL;
 }