[CodeGenPrepare] Drop nsw flags in `optimizeLoadExt` (#118180)

Alive2: https://alive2.llvm.org/ce/z/pMcD7q
Closes https://github.com/llvm/llvm-project/issues/118172.

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index ead8aa6..83c6ecd 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -7138,6 +7138,7 @@ bool CodeGenPrepare::optimizeLoadExt(LoadInst *Load) {
   SmallVector<Instruction *, 8> WorkList;
   SmallPtrSet<Instruction *, 16> Visited;
   SmallVector<Instruction *, 8> AndsToMaybeRemove;
+  SmallVector<Instruction *, 8> DropFlags;
   for (auto *U : Load->users())
     WorkList.push_back(cast<Instruction>(U));
 
@@ -7185,6 +7186,7 @@ bool CodeGenPrepare::optimizeLoadExt(LoadInst *Load) {
         return false;
       uint64_t ShiftAmt = ShlC->getLimitedValue(BitWidth - 1);
       DemandBits.setLowBits(BitWidth - ShiftAmt);
+      DropFlags.push_back(I);
       break;
     }
 
@@ -7192,6 +7194,7 @@ bool CodeGenPrepare::optimizeLoadExt(LoadInst *Load) {
       EVT TruncVT = TLI->getValueType(*DL, I->getType());
       unsigned TruncBitWidth = TruncVT.getSizeInBits();
       DemandBits.setLowBits(TruncBitWidth);
+      DropFlags.push_back(I);
       break;
     }
 
@@ -7249,6 +7252,10 @@ bool CodeGenPrepare::optimizeLoadExt(LoadInst *Load) {
       ++NumAndUses;
     }
 
+  // NSW flags may not longer hold.
+  for (auto *Inst : DropFlags)
+    Inst->setHasNoSignedWrap(false);
+
   ++NumAndsAdded;
   return true;
 }