[llvm] [X86] Fix store MMO offset in breakBlockedCopies (PR #202904)

[via llvm-commits]

https://github.com/adalal-2441 created https://github.com/llvm/llvm-project/pull/202904

The final buildCopies call in breakBlockedCopies passed LMMOffset for both the load and store MachineMemOperand offsets. The store argument should be SMMOffset. This is NFC today since LMMOffset and SMMOffset values were anyways equal before, but the duplication was a latent bug.

Found via @jlebar's X86 LLVM bug hunt / FuzzX effort: https://github.com/SemiAnalysisAI/FuzzX/blob/master/x86/bugs/042-sfb-buildcopies-wrong-mmo-offset/NOTES.md

cc @jlebar

>From 6fa06212fe4ebd7b2dea0a5395df01bbb5dfa86a Mon Sep 17 00:00:00 2001
From: Akshat <adalal at amd.com>
Date: Wed, 10 Jun 2026 14:21:10 +0530
Subject: [PATCH] [X86] Fix store MMO offset in breakBlockedCopies tail copy

The final buildCopies call in breakBlockedCopies passed LMMOffset for both
the load and store MachineMemOperand offsets. The store argument should be
SMMOffset. This is NFC today since LMMOffset and SMMOffset are advanced in
lockstep and are always equal, but the duplication was a latent bug.
---
 llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp b/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
index 044a951bc7c3e..e3b3066747a84 100644
--- a/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
+++ b/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
@@ -612,7 +612,7 @@ void X86AvoidSFBImpl::breakBlockedCopies(
   }
   unsigned Size3 = (LdDispImm + getRegSizeInBytes(LoadInst)) - LdDisp1;
   buildCopies(Size3, LoadInst, LdDisp1, StoreInst, StDisp1, LMMOffset,
-              LMMOffset);
+              SMMOffset);
 }
 
 static bool hasSameBaseOpValue(MachineInstr *LoadInst,