[InstCombine] Preserve NSW flags for `lshr (mul nuw X, C1), C2 -> mul nuw nsw X, (C1 >> C2)` (#72625)

Alive2: https://alive2.llvm.org/ce/z/TU_V9M

This missed optimization is discovered with the help of
https://github.com/AliveToolkit/alive2/pull/962.

1 files changed, 4 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 9d4a2cc..f72546d 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1435,12 +1435,13 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
       if (Op0->hasOneUse()) {
         APInt NewMulC = MulC->lshr(ShAmtC);
         // if c is divisible by (1 << ShAmtC):
-        // lshr (mul nuw x, MulC), ShAmtC -> mul nuw x, (MulC >> ShAmtC)
+        // lshr (mul nuw x, MulC), ShAmtC -> mul nuw nsw x, (MulC >> ShAmtC)
         if (MulC->eq(NewMulC.shl(ShAmtC))) {
           auto *NewMul =
               BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
-          BinaryOperator *OrigMul = cast<BinaryOperator>(Op0);
-          NewMul->setHasNoSignedWrap(OrigMul->hasNoSignedWrap());
+          assert(ShAmtC != 0 &&
+                 "lshr X, 0 should be handled by simplifyLShrInst.");
+          NewMul->setHasNoSignedWrap(true);
           return NewMul;
         }
       }