[llvm] [InstCombine] Avoid self-replacing in `getUndefReplacement` (PR #161500)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/161500

Self-replacing has a different meaning in InstCombine. It will replace all uses with poison.
Closes https://github.com/llvm/llvm-project/issues/161492.


>From b3adfafd91f586f571963cb2469dcdf17338ca98 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 17:49:59 +0800
Subject: [PATCH 1/2] [InstCombine] Add pre-commit tests. NFC.

---
 llvm/test/Transforms/InstCombine/freeze.ll | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/llvm/test/Transforms/InstCombine/freeze.ll b/llvm/test/Transforms/InstCombine/freeze.ll
index af5cb0c75537b..0e9b3cb42104b 100644
--- a/llvm/test/Transforms/InstCombine/freeze.ll
+++ b/llvm/test/Transforms/InstCombine/freeze.ll
@@ -1464,6 +1464,27 @@ define ptr @freeze_ptrmask_nonnull(ptr %p, i64 noundef %m) {
   ret ptr %fr
 }
 
+define i64 @pr161492_1(i1 %cond) {
+; CHECK-LABEL: define i64 @pr161492_1(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:    ret i64 poison
+;
+  %fr1 = freeze i64 poison
+  %fr2 = freeze i64 poison
+  %ret = select i1 %cond, i64 %fr1, i64 %fr2
+  ret i64 %ret
+}
+
+define i64 @pr161492_2(i1 %cond) {
+; CHECK-LABEL: define i64 @pr161492_2(
+; CHECK-SAME: i1 [[COND:%.*]]) {
+; CHECK-NEXT:    ret i64 poison
+;
+  %fr = freeze i64 poison
+  %ret = select i1 %cond, i64 %fr, i64 %fr
+  ret i64 %ret
+}
+
 !0 = !{}
 !1 = !{i64 4}
 !2 = !{i32 0, i32 100}

>From 8ba650ae97dae11bb1b857036aeab3433cd51390 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 1 Oct 2025 17:52:10 +0800
Subject: [PATCH 2/2] [InstCombine] Avoid self-replacing in
 `getUndefReplacement`

---
 llvm/lib/Transforms/InstCombine/InstructionCombining.cpp | 3 ++-
 llvm/test/Transforms/InstCombine/freeze.ll               | 4 ++--
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index ff063f929347f..5d2d79e420931 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -5212,7 +5212,7 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
       else if (match(U, m_Select(m_Specific(&I), m_Constant(), m_Value())))
         V = ConstantInt::getTrue(Ty);
       else if (match(U, m_c_Select(m_Specific(&I), m_Value(V)))) {
-        if (!isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
+        if (V == &I || !isGuaranteedNotToBeUndefOrPoison(V, &AC, &I, &DT))
           V = NullValue;
       } else if (auto *PHI = dyn_cast<PHINode>(U)) {
         if (Value *MaybeV = pickCommonConstantFromPHI(*PHI))
@@ -5225,6 +5225,7 @@ Instruction *InstCombinerImpl::visitFreeze(FreezeInst &I) {
         BestValue = NullValue;
     }
     assert(BestValue && "Must have at least one use");
+    assert(BestValue != &I && "Cannot replace with itself");
     return BestValue;
   };
 
diff --git a/llvm/test/Transforms/InstCombine/freeze.ll b/llvm/test/Transforms/InstCombine/freeze.ll
index 0e9b3cb42104b..ac7d65c2a3c6a 100644
--- a/llvm/test/Transforms/InstCombine/freeze.ll
+++ b/llvm/test/Transforms/InstCombine/freeze.ll
@@ -1467,7 +1467,7 @@ define ptr @freeze_ptrmask_nonnull(ptr %p, i64 noundef %m) {
 define i64 @pr161492_1(i1 %cond) {
 ; CHECK-LABEL: define i64 @pr161492_1(
 ; CHECK-SAME: i1 [[COND:%.*]]) {
-; CHECK-NEXT:    ret i64 poison
+; CHECK-NEXT:    ret i64 0
 ;
   %fr1 = freeze i64 poison
   %fr2 = freeze i64 poison
@@ -1478,7 +1478,7 @@ define i64 @pr161492_1(i1 %cond) {
 define i64 @pr161492_2(i1 %cond) {
 ; CHECK-LABEL: define i64 @pr161492_2(
 ; CHECK-SAME: i1 [[COND:%.*]]) {
-; CHECK-NEXT:    ret i64 poison
+; CHECK-NEXT:    ret i64 0
 ;
   %fr = freeze i64 poison
   %ret = select i1 %cond, i64 %fr, i64 %fr