S_invlist_trim - don't SvPV_renew if SvLEN(invlist) is < PTRSIZE larger

Currently, `S_invlist_trim` always calls `SvPV_renew(invlist, <size>)`,
which is a macro wrapping a call to `safesysrealloc()`.

However, `SvLEN(invlist)` is often already exactly the desired size, or
it is larger by less than the size of a pointer. With this commit, such
cases just return without calling `SvPV_renew()`.

Author: richardleach

regcomp_invlist.c

@@ -247,13 +247,26 @@ S_invlist_trim(SV* invlist)
 
     /* But don't free up the space needed for the 0 UV that is always at the
      * beginning of the list, nor the trailing NUL */
-    const UV min_size = TO_INTERNAL_SIZE(1) + 1;
+    const STRLEN want_size = MAX(
+        TO_INTERNAL_SIZE(1) + 1, /* minimum length needed */
+        SvCUR(invlist) + 1       /* SvCUR(invlist) can be 0,
+                                  * often is TO_INTERNAL_SIZE(1) or larger */
+        );
 
     PERL_ARGS_ASSERT_INVLIST_TRIM;
 
     assert(is_invlist(invlist));
 
-    SvPV_renew(invlist, MAX(min_size, SvCUR(invlist) + 1));
+    /* It's very common for SvLEN(invlist) to be equal to:
+     *     TO_INTERNAL_SIZE(1) + 1
+     *     SvCUR(invlist) + 2
+     *     SvCUR(invlist) + "a few"
+     * In such cases there's zero or negligible benefit to
+     * calling SvPV_renew and thereby safesysrealloc().
+     * So check if that really seems warranted. */
+    if ( SvLEN(invlist) < want_size || SvLEN(invlist) > want_size + PTRSIZE ) {
+        SvPV_renew(invlist, want_size);
+    }
 }
 
 PERL_STATIC_INLINE void