[InstCombine] Treat identical operands as one in pushFreezeToPreventPoisonFromPropagating

[c-rhodes]

To push a freeze through an instruction, only one operand may produce
poison. However, this currently fails for identical operands which are
treated as separate. This patch fixes this by treating them as a single
operand.

Thanks to @david-arm for help with this.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Cullen Rhodes (c-rhodes)

Changes

To push a freeze through an instruction, only one operand may produce
poison. However, this currently fails for identical operands which are
treated as separate. This patch fixes this by treating them as a single
operand.

Thanks to @david-arm for help with this.

---

Full diff: https://github.com/llvm/llvm-project/pull/145348.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+3-1)
• (modified) llvm/test/Transforms/InstCombine/freeze.ll (+11)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index ce42029261359..e33cf699395f0 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -4774,7 +4774,9 @@ InstCombinerImpl::pushFreezeToPreventPoisonFromPropagating(FreezeInst &OrigFI) {
   Use *MaybePoisonOperand = nullptr;
   for (Use &U : OrigOpInst->operands()) {
     if (isa<MetadataAsValue>(U.get()) ||
-        isGuaranteedNotToBeUndefOrPoison(U.get()))
+        isGuaranteedNotToBeUndefOrPoison(U.get()) ||
+        // Treat identical operands as a single operand.
+        (MaybePoisonOperand && MaybePoisonOperand->get() == U.get()))
       continue;
     if (!MaybePoisonOperand)
       MaybePoisonOperand = &U;
diff --git a/llvm/test/Transforms/InstCombine/freeze.ll b/llvm/test/Transforms/InstCombine/freeze.ll
index 8875ce1c566f3..abe7117b27f8c 100644
--- a/llvm/test/Transforms/InstCombine/freeze.ll
+++ b/llvm/test/Transforms/InstCombine/freeze.ll
@@ -142,6 +142,17 @@ define i32 @early_freeze_test3(i32 %v1) {
   ret i32 %v4.fr
 }
 
+define i32 @early_freeze_test4(i32 %v1) {
+; CHECK-LABEL: @early_freeze_test4(
+; CHECK-NEXT:    [[V2_FR:%.*]] = freeze i32 [[V2:%.*]]
+; CHECK-NEXT:    [[V3:%.*]] = mul i32 [[V2_FR]], [[V2_FR]]
+; CHECK-NEXT:    ret i32 [[V3]]
+;
+  %v2 = mul i32 %v1, %v1
+  %v2.fr = freeze i32 %v2
+  ret i32 %v2.fr
+}
+
 ; If replace all dominated uses of v to freeze(v).
 
 define void @freeze_dominated_uses_test1(i32 %v) {