[InstCombine] Drop NSW when converting shl X, BW - 1 back into mul
#121633
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@llvm/pr-subscribers-llvm-transforms Author: Yingwei Zheng (dtcxzyw) Changes
Full diff: https://github.com/llvm/llvm-project/pull/121633.diff 2 Files Affected:
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
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nikic
approved these changes
Jan 4, 2025
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LGTM, though I think it would be good to test more cases (you currently test the flag only on one operand, for one of the cases).
dtcxzyw
commented
Jan 4, 2025
| ret i8 %r | ||
| } | ||
|
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||
| define i8 @srem_XY_XZ_with_CY_gt_CZ_drop_nsw_commuted(i8 noundef %X) { |
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The original transform is valid: https://alive2.llvm.org/ce/z/htzvgR
But I don't know how to generalize this.
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Think its okay if the shl is the rhs of the rem: https://alive2.llvm.org/ce/z/SoTDqo
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So I think you can drop Op1PreserveNSW
nikic
approved these changes
Jan 4, 2025
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/175/builds/10907 Here is the relevant piece of the build log for the reference |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/108/builds/7725 Here is the relevant piece of the build log for the reference |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/199/builds/643 Here is the relevant piece of the build log for the reference |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/64/builds/1877 Here is the relevant piece of the build log for the reference |
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X <<s BW - 1andX *s INT_MINare not equivalent.Alive2: https://alive2.llvm.org/ce/z/MKKFrj
Closes #121584