[llvm] [InstCombine] Drop noundef in `foldSelectCttzCtlz` (PR #121692)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Close https://github.com/llvm/llvm-project/issues/121428


---
Full diff: https://github.com/llvm/llvm-project/pull/121692.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+5-1) 
- (modified) llvm/test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll (+13) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index e7a8e947705f8d..a18b927678efd8 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1225,8 +1225,12 @@ static Value *foldSelectCttzCtlz(ICmpInst *ICI, Value *TrueVal, Value *FalseVal,
   // zext/trunc) have one use (ending at the select), the cttz/ctlz result will
   // not be used if the input is zero. Relax to 'zero is poison' for that case.
   if (II->hasOneUse() && SelectArg->hasOneUse() &&
-      !match(II->getArgOperand(1), m_One()))
+      !match(II->getArgOperand(1), m_One())) {
     II->setArgOperand(1, ConstantInt::getTrue(II->getContext()));
+    // noundef attribute on the intrinsic may no longer be valid.
+    II->dropUBImplyingAttrsAndMetadata();
+    IC.addToWorklist(II);
+  }
 
   return nullptr;
 }
diff --git a/llvm/test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll b/llvm/test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll
index 35b4087d767a73..2cb70e85f435f6 100644
--- a/llvm/test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll
+++ b/llvm/test/Transforms/InstCombine/select-cmp-cttz-ctlz.ll
@@ -495,6 +495,19 @@ define i32 @test_cttz_not_bw(i32 %x) {
   ret i32 %res
 }
 
+define i32 @test_cttz_not_bw_noundef(i32 %x) {
+; CHECK-LABEL: @test_cttz_not_bw_noundef(
+; CHECK-NEXT:    [[CT:%.*]] = tail call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 true)
+; CHECK-NEXT:    [[CMP_NOT:%.*]] = icmp eq i32 [[X]], 0
+; CHECK-NEXT:    [[RES:%.*]] = select i1 [[CMP_NOT]], i32 123, i32 [[CT]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %ct = tail call noundef i32 @llvm.cttz.i32(i32 %x, i1 false)
+  %cmp = icmp ne i32 %x, 0
+  %res = select i1 %cmp, i32 %ct, i32 123
+  ret i32 %res
+}
+
 define i32 @test_cttz_not_bw_multiuse(i32 %x) {
 ; CHECK-LABEL: @test_cttz_not_bw_multiuse(
 ; CHECK-NEXT:    [[CT:%.*]] = tail call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[X:%.*]], i1 false)

``````````

</details>


https://github.com/llvm/llvm-project/pull/121692