regcomp.c: Do some extra folding

Generally we have to wait until runtime to do folding for regnodes that
are locale dependent, because we don't know what the locale at runtime
will be, and hence what the folds will be.

But UTF-8 locales all have the same folding behavior, no matter what the
locale is, with the exception of two fold pairs in Turkish.  (Lithuanian
too, but Perl doesn't support that language's special folding rules.)
UTF-8 is the only locale type that Perl supports that can represent code
points above 255.  Therefore we do know at compile time what the
above-255 folds are (again excepting the two in Turkish), and so we can
do the folding then.  But only if both the components are above 255.
There are a few folds that cross the 255/256 boundary, and they must be
deferred.

However, there are two instances where there are three characters that
fold together in which two of them are above 255, and the third isn't.
That the two high ones are equivalent under /i is known at compile time,
and so that equivalence can be stated then.

1 files changed, 19 insertions, 4 deletions

diff --git a/regcomp.c b/regcomp.c
index 8b90579ddc..72d788f975 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -14547,6 +14547,16 @@ S_regatom(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth)
                      * things */
                     maybe_exactfu = FALSE;
 
+                    /* Although these two characters have folds that are
+                     * locale-problematic, they also have folds to above Latin1
+                     * that aren't a problem.  Doing these now helps at
+                     * runtime. */
+                    if (UNLIKELY(   ender == GREEK_CAPITAL_LETTER_MU
+                                 || ender == LATIN_CAPITAL_LETTER_SHARP_S))
+                    {
+                        goto fold_anyway;
+                    }
+
                     /* Here, we are adding a problematic fold character.
                      * "Problematic" in this context means that its fold isn't
                      * known until runtime.  (The non-problematic code points
@@ -14600,15 +14610,20 @@ S_regatom(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth)
                             *(s)++ = (U8) toFOLD(ender);
                         }
                         else {
-                            UV folded = _to_uni_fold_flags(
+                            UV folded;
+
+                          fold_anyway:
+                            folded = _to_uni_fold_flags(
                                     ender,
                                     (U8 *) s,  /* We have allocated extra space
                                                   in 's' so can't run off the
                                                   end */
                                     &added_len,
-                                    FOLD_FLAGS_FULL | ((ASCII_FOLD_RESTRICTED)
-                                                    ? FOLD_FLAGS_NOMIX_ASCII
-                                                    : 0));
+                                    FOLD_FLAGS_FULL
+                                  | ((   ASCII_FOLD_RESTRICTED
+                                      || node_type == EXACTFL)
+                                    ? FOLD_FLAGS_NOMIX_ASCII
+                                    : 0));
                             if (UNLIKELY(len + added_len > max_string_len)) {
                                 overflowed = TRUE;
                                 break;