[llvm] [GlobalISel] Preserve volatile and atomic undef stores (PR #200099)

[via llvm-commits]

llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-globalisel

Author: Justin Lebar (jlebar)

<details>
<summary>Changes</summary>

It's not correct to elide a volatile or atomic store, even if the stored
value is undef.

This bug was found by a large run of Opus 4.7 looking for bugs in LLVM.


---
Full diff: https://github.com/llvm/llvm-project/pull/200099.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp (+4-2) 
- (modified) llvm/test/CodeGen/X86/GlobalISel/undef.ll (+25) 


``````````diff
diff --git a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
index a910a900e775f..64f6a93b722ee 100644
--- a/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
@@ -2880,8 +2880,10 @@ bool CombinerHelper::matchUndefShuffleVectorMask(MachineInstr &MI) const {
 
 bool CombinerHelper::matchUndefStore(MachineInstr &MI) const {
   assert(MI.getOpcode() == TargetOpcode::G_STORE);
-  return getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, MI.getOperand(0).getReg(),
-                      MRI);
+  auto &Store = cast<GStore>(MI);
+  if (!Store.isSimple())
+    return false;
+  return getOpcodeDef(TargetOpcode::G_IMPLICIT_DEF, Store.getValueReg(), MRI);
 }
 
 bool CombinerHelper::matchUndefSelectCmp(MachineInstr &MI) const {
diff --git a/llvm/test/CodeGen/X86/GlobalISel/undef.ll b/llvm/test/CodeGen/X86/GlobalISel/undef.ll
index 34ada8a20999d..3fd706a01944c 100644
--- a/llvm/test/CodeGen/X86/GlobalISel/undef.ll
+++ b/llvm/test/CodeGen/X86/GlobalISel/undef.ll
@@ -56,3 +56,28 @@ define float @test4(float %a) {
   ret float %r
 }
 
+define void @plain_undef_store(ptr %p) {
+; ALL-LABEL: plain_undef_store:
+; ALL:       # %bb.0:
+; ALL-NEXT:    retq
+  store i32 undef, ptr %p, align 4
+  ret void
+}
+
+define void @volatile_undef_store(ptr %p) {
+; ALL-LABEL: volatile_undef_store:
+; ALL:       # %bb.0:
+; ALL-NEXT:    movl %eax, (%rdi)
+; ALL-NEXT:    retq
+  store volatile i32 undef, ptr %p, align 4
+  ret void
+}
+
+define void @seq_cst_atomic_undef_store(ptr %p) {
+; ALL-LABEL: seq_cst_atomic_undef_store:
+; ALL:       # %bb.0:
+; ALL-NEXT:    movl %eax, (%rdi)
+; ALL-NEXT:    retq
+  store atomic i32 undef, ptr %p seq_cst, align 4
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/200099