Stop allocating unused backref strings at `defined?`

internal/re.h

@@ -23,5 +23,6 @@ void rb_backref_set_string(VALUE string, long pos, long len);
 void rb_match_unbusy(VALUE);
 int rb_match_count(VALUE match);
 VALUE rb_reg_new_ary(VALUE ary, int options);
+VALUE rb_reg_last_defined(VALUE match);
 
 #endif /* INTERNAL_RE_H */

re.c

@@ -1935,21 +1935,37 @@ rb_reg_match_post(VALUE match)
     return str;
 }
 
-VALUE
-rb_reg_match_last(VALUE match)
+static int
+match_last_index(VALUE match)
 {
     int i;
     struct re_registers *regs;
 
-    if (NIL_P(match)) return Qnil;
+    if (NIL_P(match)) return -1;
     match_check(match);
     regs = RMATCH_REGS(match);
-    if (BEG(0) == -1) return Qnil;
+    if (BEG(0) == -1) return -1;
 
     for (i=regs->num_regs-1; BEG(i) == -1 && i > 0; i--)
         ;
-    if (i == 0) return Qnil;
-    return rb_reg_nth_match(i, match);
+    return i;
+}
+
+VALUE
+rb_reg_match_last(VALUE match)
+{
+    int i = match_last_index(match);
+    if (i <= 0) return Qnil;
+    struct re_registers *regs = RMATCH_REGS(match);
+    return rb_str_subseq(RMATCH(match)->str, BEG(i), END(i) - BEG(i));
+}
+
+VALUE
+rb_reg_last_defined(VALUE match)
+{
+    int i = match_last_index(match);
+    if (i < 0) return Qnil;
+    return RBOOL(i);
 }
 
 static VALUE

vm_insnhelper.c

@@ -672,6 +672,30 @@ vm_getspecial(const rb_execution_context_t *ec, const VALUE *lep, rb_num_t key,
     return val;
 }
 
+static inline VALUE
+vm_backref_defined(const rb_execution_context_t *ec, const VALUE *lep, rb_num_t type)
+{
+    VALUE backref = lep_svar_get(ec, lep, VM_SVAR_BACKREF);
+    int nth = 0;
+
+    if (type & 0x01) {
+        switch (type >> 1) {
+          case '&':
+          case '`':
+          case '\'':
+            break;
+          case '+':
+            return rb_reg_last_defined(backref);
+          default:
+            rb_bug("unexpected back-ref");
+        }
+    }
+    else {
+        nth = (int)(type >> 1);
+    }
+    return rb_reg_nth_defined(nth, backref);
+}
+
 PUREFUNC(static rb_callable_method_entry_t *check_method_entry(VALUE obj, int can_be_svar));
 static rb_callable_method_entry_t *
 check_method_entry(VALUE obj, int can_be_svar)
@@ -5064,10 +5088,8 @@ vm_defined(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, rb_num_t op_
             }
         }
         break;
-      case DEFINED_REF:{
-        return vm_getspecial(ec, GET_LEP(), Qfalse, FIX2INT(obj)) != Qnil;
-        break;
-      }
+      case DEFINED_REF:
+        return RTEST(vm_backref_defined(ec, GET_LEP(), FIX2INT(obj)));
       default:
         rb_bug("unimplemented defined? type (VM)");
         break;