From c92d0cb19263e7e302b4740ba6617a32c201c613 Mon Sep 17 00:00:00 2001
From: Mike Pall <mike>
Date: Thu, 24 Jul 2025 15:38:45 +0200
Subject: [PATCH] x86/x64: Don't use undefined MUL/IMUL zero flag.

Reported by VrIgHtEr. #1376
---
 src/lj_asm_x86.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/lj_asm_x86.h b/src/lj_asm_x86.h
index 239066d4..8b6ce479 100644
--- a/src/lj_asm_x86.h
+++ b/src/lj_asm_x86.h
@@ -1841,7 +1841,8 @@ static void asm_intarith(ASMState *as, IRIns *ir, x86Arith xa)
   RegSet allow = RSET_GPR;
   Reg dest, right;
   int32_t k = 0;
-  if (as->flagmcp == as->mcp) {  /* Drop test r,r instruction. */
+  if (as->flagmcp == as->mcp && xa != XOg_X_IMUL) {
+    /* Drop test r,r instruction. */
     MCode *p = as->mcp + ((LJ_64 && *as->mcp < XI_TESTb) ? 3 : 2);
     MCode *q = p[0] == 0x0f ? p+1 : p;
     if ((*q & 15) < 14) {
-- 
2.47.2