Reduce ONIG_NREGION from 10 to 4: power of 2 and testing revealed most pattern matches are less than or equal to 4 results

Closes: https://github.com/ruby/ruby/pull/2135

benchmark/match_gt4.rb

@@ -0,0 +1 @@
+1000000.times { /(.)(.)(\d+)(\d)/.match("THX1138.") }

benchmark/match_small.rb

@@ -0,0 +1 @@
+1000000.times { 'haystack'.match(/hay/) }

debug_counter.h

@@ -177,6 +177,9 @@ RB_DEBUG_COUNTER(gc_isptr_maybe)
  *   * hash_under4:     has under 4 entries
  *   * hash_ge4:        has n entries (4<=n<8)
  *   * hash_ge8:        has n entries (8<=n)
+ *   * match_under4:    has under 4 oniguruma regions allocated
+ *   * match_ge4:       has n regions allocated (4<=n<8)
+ *   * match_ge8:       has n regions allocated (8<=n)
  *   * data_empty: T_DATA but no memory free.
  *   * data_xfree:        free'ed by xfree().
  *   * data_imm_free:     free'ed immediately.
@@ -225,6 +228,9 @@ RB_DEBUG_COUNTER(obj_data_xfree)
 RB_DEBUG_COUNTER(obj_data_imm_free)
 RB_DEBUG_COUNTER(obj_data_zombie)
 
+RB_DEBUG_COUNTER(obj_match_under4)
+RB_DEBUG_COUNTER(obj_match_ge4)
+RB_DEBUG_COUNTER(obj_match_ge8)
 RB_DEBUG_COUNTER(obj_match_ptr)
 RB_DEBUG_COUNTER(obj_file_ptr)
 RB_DEBUG_COUNTER(obj_bignum_ptr)

gc.c

@@ -2436,6 +2436,17 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
       case T_MATCH:
 	if (RANY(obj)->as.match.rmatch) {
             struct rmatch *rm = RANY(obj)->as.match.rmatch;
+#if USE_DEBUG_COUNTER
+            if (rm->regs.num_regs >= 8) {
+                RB_DEBUG_COUNTER_INC(obj_match_ge8);
+            }
+            else if (rm->regs.num_regs >= 4) {
+                RB_DEBUG_COUNTER_INC(obj_match_ge4);
+            }
+            else if (rm->regs.num_regs >= 1) {
+                RB_DEBUG_COUNTER_INC(obj_match_under4);
+            }
+#endif
 	    onig_region_free(&rm->regs, 0);
             if (rm->char_offset)
 		xfree(rm->char_offset);

include/ruby/onigmo.h

@@ -434,7 +434,7 @@ int onigenc_str_bytelen_null(OnigEncoding enc, const OnigUChar* p);
 /* PART: regular expression */
 
 /* config parameters */
-#define ONIG_NREGION                          10
+#define ONIG_NREGION                          4
 #define ONIG_MAX_CAPTURE_GROUP_NUM         32767
 #define ONIG_MAX_BACKREF_NUM                1000
 #define ONIG_MAX_REPEAT_NUM               100000