[llvm-branch-commits] [llvm] release/18.x: [GVN] Drop nsw/nuw flags when replacing the result of a with.overflow intrinsic with a overflowing binary operator (#82935) (PR #82965)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (llvmbot)

<details>
<summary>Changes</summary>

Backport 892b4beeac50920e630f10905b2916295e2eb6d8

Requested by: @<!-- -->dtcxzyw

---
Full diff: https://github.com/llvm/llvm-project/pull/82965.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/Local.cpp (+7-1) 
- (added) llvm/test/Transforms/GVN/pr82884.ll (+21) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 459e3d98059283..a1c6bbc52fd05e 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -3369,11 +3369,17 @@ void llvm::patchReplacementInstruction(Instruction *I, Value *Repl) {
 
   // Patch the replacement so that it is not more restrictive than the value
   // being replaced.
+  WithOverflowInst *UnusedWO;
+  // When replacing the result of a llvm.*.with.overflow intrinsic with a
+  // overflowing binary operator, nuw/nsw flags may no longer hold.
+  if (isa<OverflowingBinaryOperator>(ReplInst) &&
+      match(I, m_ExtractValue<0>(m_WithOverflowInst(UnusedWO))))
+    ReplInst->dropPoisonGeneratingFlags();
   // Note that if 'I' is a load being replaced by some operation,
   // for example, by an arithmetic operation, then andIRFlags()
   // would just erase all math flags from the original arithmetic
   // operation, which is clearly not wanted and not needed.
-  if (!isa<LoadInst>(I))
+  else if (!isa<LoadInst>(I))
     ReplInst->andIRFlags(I);
 
   // FIXME: If both the original and replacement value are part of the
diff --git a/llvm/test/Transforms/GVN/pr82884.ll b/llvm/test/Transforms/GVN/pr82884.ll
new file mode 100644
index 00000000000000..71abafda60d93d
--- /dev/null
+++ b/llvm/test/Transforms/GVN/pr82884.ll
@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes=gvn < %s | FileCheck %s
+
+; Make sure nsw/nuw flags are dropped.
+
+define i32 @pr82884(i32 %x) {
+; CHECK-LABEL: define i32 @pr82884(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT:    [[MUL:%.*]] = mul i32 [[X]], [[X]]
+; CHECK-NEXT:    call void @use(i32 [[MUL]])
+; CHECK-NEXT:    [[MUL2:%.*]] = call { i32, i1 } @llvm.smul.with.overflow.i32(i32 [[X]], i32 [[X]])
+; CHECK-NEXT:    ret i32 [[MUL]]
+;
+  %mul = mul nsw nuw i32 %x, %x
+  call void @use(i32 %mul)
+  %mul2 = call { i32, i1 } @llvm.smul.with.overflow.i32(i32 %x, i32 %x)
+  %ret = extractvalue { i32, i1 } %mul2, 0
+  ret i32 %ret
+}
+
+declare void @use(i32)

``````````

</details>


https://github.com/llvm/llvm-project/pull/82965