[llvm] [InstCombine] Drop NSW when converting `shl X, BW - 1` back into mul (PR #121633)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

`X <<s BW - 1` and `X *s INT_MIN` are not equivalent.
Alive2: https://alive2.llvm.org/ce/z/MKKFrj
Closes https://github.com/llvm/llvm-project/issues/121584


---
Full diff: https://github.com/llvm/llvm-project/pull/121633.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (+11-5) 
- (modified) llvm/test/Transforms/InstCombine/rem-mul-shl.ll (+13) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
index f85a3c93651353..97a765ecfb6bd5 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
@@ -2066,14 +2066,18 @@ static Instruction *simplifyIRemMulShl(BinaryOperator &I,
   bool ShiftByX = false;
 
   // If V is not nullptr, it will be matched using m_Specific.
-  auto MatchShiftOrMulXC = [](Value *Op, Value *&V, APInt &C) -> bool {
+  auto MatchShiftOrMulXC = [](Value *Op, Value *&V, APInt &C,
+                              bool &PreserveNSW) -> bool {
     const APInt *Tmp = nullptr;
     if ((!V && match(Op, m_Mul(m_Value(V), m_APInt(Tmp)))) ||
         (V && match(Op, m_Mul(m_Specific(V), m_APInt(Tmp)))))
       C = *Tmp;
     else if ((!V && match(Op, m_Shl(m_Value(V), m_APInt(Tmp)))) ||
-             (V && match(Op, m_Shl(m_Specific(V), m_APInt(Tmp)))))
+             (V && match(Op, m_Shl(m_Specific(V), m_APInt(Tmp))))) {
       C = APInt(Tmp->getBitWidth(), 1) << *Tmp;
+      // We cannot preserve NSW when shifting by BW - 1.
+      PreserveNSW = Tmp->ult(Tmp->getBitWidth() - 1);
+    }
     if (Tmp != nullptr)
       return true;
 
@@ -2095,7 +2099,9 @@ static Instruction *simplifyIRemMulShl(BinaryOperator &I,
     return false;
   };
 
-  if (MatchShiftOrMulXC(Op0, X, Y) && MatchShiftOrMulXC(Op1, X, Z)) {
+  bool Op0PreserveNSW = true, Op1PreserveNSW = true;
+  if (MatchShiftOrMulXC(Op0, X, Y, Op0PreserveNSW) &&
+      MatchShiftOrMulXC(Op1, X, Z, Op1PreserveNSW)) {
     // pass
   } else if (MatchShiftCX(Op0, Y, X) && MatchShiftCX(Op1, Z, X)) {
     ShiftByX = true;
@@ -2108,7 +2114,7 @@ static Instruction *simplifyIRemMulShl(BinaryOperator &I,
   OverflowingBinaryOperator *BO0 = cast<OverflowingBinaryOperator>(Op0);
   // TODO: We may be able to deduce more about nsw/nuw of BO0/BO1 based on Y >=
   // Z or Z >= Y.
-  bool BO0HasNSW = BO0->hasNoSignedWrap();
+  bool BO0HasNSW = Op0PreserveNSW && BO0->hasNoSignedWrap();
   bool BO0HasNUW = BO0->hasNoUnsignedWrap();
   bool BO0NoWrap = IsSRem ? BO0HasNSW : BO0HasNUW;
 
@@ -2131,7 +2137,7 @@ static Instruction *simplifyIRemMulShl(BinaryOperator &I,
   };
 
   OverflowingBinaryOperator *BO1 = cast<OverflowingBinaryOperator>(Op1);
-  bool BO1HasNSW = BO1->hasNoSignedWrap();
+  bool BO1HasNSW = Op1PreserveNSW && BO1->hasNoSignedWrap();
   bool BO1HasNUW = BO1->hasNoUnsignedWrap();
   bool BO1NoWrap = IsSRem ? BO1HasNSW : BO1HasNUW;
   // (rem (mul X, Y), (mul nuw/nsw X, Z))
diff --git a/llvm/test/Transforms/InstCombine/rem-mul-shl.ll b/llvm/test/Transforms/InstCombine/rem-mul-shl.ll
index e7d6cc7102c713..aa087129e45d35 100644
--- a/llvm/test/Transforms/InstCombine/rem-mul-shl.ll
+++ b/llvm/test/Transforms/InstCombine/rem-mul-shl.ll
@@ -372,6 +372,19 @@ define <2 x i8> @srem_XY_XZ_with_CY_gt_CZ_no_nuw_out(<2 x i8> %X) {
   ret <2 x i8> %r
 }
 
+define i8 @srem_XY_XZ_with_CY_gt_CZ_drop_nsw(i8 noundef %X) {
+; CHECK-LABEL: @srem_XY_XZ_with_CY_gt_CZ_drop_nsw(
+; CHECK-NEXT:    [[BO0:%.*]] = mul nsw i8 [[X:%.*]], 127
+; CHECK-NEXT:    [[BO1:%.*]] = shl nsw i8 [[X]], 7
+; CHECK-NEXT:    [[R:%.*]] = srem i8 [[BO1]], [[BO0]]
+; CHECK-NEXT:    ret i8 [[R]]
+;
+  %BO0 = mul nsw i8 %X, 127
+  %BO1 = shl nsw i8 %X, 7
+  %r = srem i8 %BO1, %BO0
+  ret i8 %r
+}
+
 define i8 @srem_XY_XZ_with_CY_gt_CZ_fail_missing_flag1(i8 %X) {
 ; CHECK-LABEL: @srem_XY_XZ_with_CY_gt_CZ_fail_missing_flag1(
 ; CHECK-NEXT:    [[BO0:%.*]] = mul nuw nsw i8 [[X:%.*]], 10

``````````

</details>


https://github.com/llvm/llvm-project/pull/121633