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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

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.


---
Full diff: https://github.com/llvm/llvm-project/pull/72625.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp (+2-3) 
- (modified) llvm/test/Transforms/InstCombine/shift-logic.ll (+2-2) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 9d4a2cc08cca30c..12dc6215e4405a3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1435,12 +1435,11 @@ 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());
+          NewMul->setHasNoSignedWrap(true);
           return NewMul;
         }
       }
diff --git a/llvm/test/Transforms/InstCombine/shift-logic.ll b/llvm/test/Transforms/InstCombine/shift-logic.ll
index e0fbe819666683e..544694d398431e3 100644
--- a/llvm/test/Transforms/InstCombine/shift-logic.ll
+++ b/llvm/test/Transforms/InstCombine/shift-logic.ll
@@ -259,7 +259,7 @@ define i32 @PR44028(i32 %x) {
 
 define i64 @lshr_mul(i64 %0) {
 ; CHECK-LABEL: @lshr_mul(
-; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw i64 [[TMP0:%.*]], 13
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw nsw i64 [[TMP0:%.*]], 13
 ; CHECK-NEXT:    ret i64 [[TMP2]]
 ;
   %2 = mul nuw i64 %0, 52
@@ -279,7 +279,7 @@ define i64 @lshr_mul_nuw_nsw(i64 %0) {
 
 define <4 x i32> @lshr_mul_vector(<4 x i32> %0) {
 ; CHECK-LABEL: @lshr_mul_vector(
-; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw <4 x i32> [[TMP0:%.*]], <i32 13, i32 13, i32 13, i32 13>
+; CHECK-NEXT:    [[TMP2:%.*]] = mul nuw nsw <4 x i32> [[TMP0:%.*]], <i32 13, i32 13, i32 13, i32 13>
 ; CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 ;
   %2 = mul nuw <4 x i32> %0, <i32 52, i32 52, i32 52, i32 52>

``````````

</details>


https://github.com/llvm/llvm-project/pull/72625