From 035550b4f720cd44f63b7046c25c160c0b0ab877 Mon Sep 17 00:00:00 2001
From: Jude Melton-Houghton <jwmhjwmh@gmail.com>
Date: Mon, 19 Sep 2022 14:30:26 -0400
Subject: [PATCH] Use constant metatable if it already exists

This helps in the case of

   o:a()
   o.a(o)

But not in the case of

   o.a(o)
   o:a()
---
 src/lj_record.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/lj_record.c b/src/lj_record.c
index fbaafead..5b475efa 100644
--- a/src/lj_record.c
+++ b/src/lj_record.c
@@ -1060,6 +1060,7 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
 {
   RecordIndex mix;
   GCtab *mt;
+  TRef kmt;
   if (tref_istab(ix->tab)) {
     mt = tabref(tabV(&ix->tabv)->metatable);
     mix.tab = emitir(IRT(IR_FLOAD, IRT_TAB), ix->tab, IRFL_TAB_META);
@@ -1104,8 +1105,22 @@ int lj_record_mm_lookup(jit_State *J, RecordIndex *ix, MMS mm)
       GG_OFS(g.gcroot[GCROOT_BASEMT+itypemap(&ix->tabv)]));
     goto nocheck;
   }
-  if (ix->mtspec && mt) {
-    TRef kmt = lj_ir_ktab(J, mt);
+  kmt = 0;
+  if (mt) {
+    /* Constify metatable or try to find existing constant. */
+    if (ix->mtspec) {
+      kmt = lj_ir_ktab(J, mt);
+    } else {
+      IRRef ref;
+      for (ref = J->chain[IR_KGC]; ref; ref = J->cur.ir[ref].prev) {
+        if (ir_kgc(&J->cur.ir[ref]) == obj2gco(mt)) {
+          kmt = TREF(ref, IRT_TAB);
+          break;
+        }
+      }
+    }
+  }
+  if (kmt) {
     emitir(IRTG(IR_EQ, IRT_TAB), mix.tab, kmt);
     mix.tab = kmt;
     ix->mt = kmt;