[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) {

[nikic]

The code below is still only replacing one of the uses. We should be replacing all of them.

You kind of get away with it because we'll later try to freeze other uses in a separate transform, but this needs to be individually correct.

[nikic]

You should probably switch this code from working on Use* to working on Value*.