Skip casing for high code points

As discussed in the previous commit, most code points in Unicode
don't change if upper-, or lower-cased, etc.  In fact as of Unicode
v8.0, 93% of the available code points are above the highest one that
does change.

This commit skips trying to case these 93%.  A regen/ script keeps track
of the max changing one in the current Unicode release, and skips casing
for the higher ones.  Thus currently, casing emoji will be skipped.

Together with the previous commits that dealt with casing, the potential
for huge memory requirements for the swash hashes for casing are
severely limited.

If the following command is run on a perl compiled with -O2 and no
DEBUGGING:

    blead Porting/bench.pl --raw --perlargs="-Ilib -X" --benchfile=plane1_case_perf /path_to_prior_perl=before_this_commit /path_to_new_perl=after

and the file 'plane1_case_perf' contains

    [
        'string::casing::emoji' => {
            desc    => 'yes swash vs no swash',
            setup   => 'my $a = "\x{1F570}"',  # MANTELPIECE CLOCK
            code    => 'uc($a)'
        },
    ];

the following results are obtained:

The numbers represent raw counts per loop iteration.

string::casing::emoji
yes swash vs no swash

       before_this_commit    after
       ------------------ --------
    Ir              981.0    306.0
    Dr              228.0     94.0
    Dw              100.0     45.0
  COND              137.0     49.0
   IND                7.0      4.0

COND_m                5.5      0.0
 IND_m                4.0      2.0

 Ir_m1                0.1     -0.1
 Dr_m1                0.0      0.0
 Dw_m1                0.0      0.0

 Ir_mm                0.0      0.0
 Dr_mm                0.0      0.0
 Dw_mm                0.0      0.0

3 files changed, 29 insertions, 0 deletions

diff --git a/regen/unicode_constants.pl b/regen/unicode_constants.pl
index acd1f91e73..baf25f1258 100644
--- a/regen/unicode_constants.pl
+++ b/regen/unicode_constants.pl
@@ -158,6 +158,22 @@ printf $out_fh "\n/* The number of code points not matching \\pC */\n"
              . "#define NON_OTHER_COUNT_FOR_USE_ONLY_BY_REGCOMP_DOT_C  %d\n",
             0x110000 - $count;
 
+# If this release has both the CWCM and CWCF properties, find the highest code
+# point which changes under any case change.  We can use this to short-circuit
+# code
+my @cwcm = prop_invlist('CWCM');
+if (@cwcm) {
+    my @cwcf = prop_invlist('CWCF');
+    if (@cwcf) {
+        my $max = ($cwcm[-1] < $cwcf[-1])
+                  ? $cwcf[-1]
+                  : $cwcm[-1];
+        printf $out_fh "\n/* The highest code point that has any type of case change */\n"
+             . "#define HIGHEST_CASE_CHANGING_CP_FOR_USE_ONLY_BY_UTF8_DOT_C  0x%X\n",
+            $max - 1;
+    }
+}
+
 print $out_fh "\n#endif /* H_UNICODE_CONSTANTS */\n";
 
 read_only_bottom_close_and_rename($out_fh);
diff --git a/unicode_constants.h b/unicode_constants.h
index 71755de7f6..1384873f19 100644
--- a/unicode_constants.h
+++ b/unicode_constants.h
@@ -182,6 +182,9 @@
 /* The number of code points not matching \pC */
 #define NON_OTHER_COUNT_FOR_USE_ONLY_BY_REGCOMP_DOT_C  120522
 
+/* The highest code point that has any type of case change */
+#define HIGHEST_CASE_CHANGING_CP_FOR_USE_ONLY_BY_UTF8_DOT_C  0x118DF
+
 #endif /* H_UNICODE_CONSTANTS */
 
 /* ex: set ro: */
diff --git a/utf8.c b/utf8.c
index 4c43bdec2f..fa1439bdfc 100644
--- a/utf8.c
+++ b/utf8.c
@@ -1997,6 +1997,16 @@ S__to_utf8_case(pTHX_ const UV uv1, const U8 *p, U8* ustrp, STRLEN *lenp,
                     }
                     goto cases_to_self;
                 }
+#ifdef HIGHEST_CASE_CHANGING_CP_FOR_USE_ONLY_BY_UTF8_DOT_C
+                if (UNLIKELY(uv1
+                    > HIGHEST_CASE_CHANGING_CP_FOR_USE_ONLY_BY_UTF8_DOT_C))
+                {
+
+                    /* As of this writing, this means we avoid swash creation
+                     * for anything beyond low Plane 1 */
+                    goto cases_to_self;
+                }
+#endif
             }
         }