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[InstComb] Fold shr ([x]or x, y), x -> shr y, x #165866

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[InstComb] Fold shr ([x]or x, y), x -> shr y, x #165866

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abb1953
[InstComb] Pre-commit (l|a)shr-[x]or test
artagnon Oct 31, 2025
65a88fa
[InstComb] Fold shr ([x]or x, y), x -> shr y, x
artagnon Oct 31, 2025
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34 changes: 21 additions & 13 deletions llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -512,19 +512,27 @@ Instruction *InstCombinerImpl::commonShiftTransforms(BinaryOperator &I) {
if (match(Op1, m_Or(m_Value(), m_SpecificInt(BitWidth - 1))))
return replaceOperand(I, 1, ConstantInt::get(Ty, BitWidth - 1));

Instruction *CmpIntr;
if ((I.getOpcode() == Instruction::LShr ||
I.getOpcode() == Instruction::AShr) &&
match(Op0, m_OneUse(m_Instruction(CmpIntr))) &&
isa<CmpIntrinsic>(CmpIntr) &&
match(Op1, m_SpecificInt(Ty->getScalarSizeInBits() - 1))) {
Value *Cmp =
Builder.CreateICmp(cast<CmpIntrinsic>(CmpIntr)->getLTPredicate(),
CmpIntr->getOperand(0), CmpIntr->getOperand(1));
return CastInst::Create(I.getOpcode() == Instruction::LShr
? Instruction::ZExt
: Instruction::SExt,
Cmp, Ty);
if (I.getOpcode() == Instruction::LShr ||
I.getOpcode() == Instruction::AShr) {
// (l|a)shr ([x]or x, y), x --> (l|a)shr y, x
Value *X;
if (match(Op0, m_CombineOr(m_Xor(m_Value(X), m_Value(Y)),
m_Or(m_Value(X), m_Value(Y)))) &&
Op1 == X)
return BinaryOperator::Create(I.getOpcode(), Y, X);

Instruction *CmpIntr;
if (match(Op0, m_OneUse(m_Instruction(CmpIntr))) &&
isa<CmpIntrinsic>(CmpIntr) &&
match(Op1, m_SpecificInt(Ty->getScalarSizeInBits() - 1))) {
Value *Cmp =
Builder.CreateICmp(cast<CmpIntrinsic>(CmpIntr)->getLTPredicate(),
CmpIntr->getOperand(0), CmpIntr->getOperand(1));
return CastInst::Create(I.getOpcode() == Instruction::LShr
? Instruction::ZExt
: Instruction::SExt,
Cmp, Ty);
}
}

return nullptr;
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98 changes: 98 additions & 0 deletions llvm/test/Transforms/InstCombine/shift-logic.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -186,6 +186,7 @@ define i32 @ashr_xor(i32 %x, i32 %py) {
ret i32 %sh1
}


define i32 @shr_mismatch_xor(i32 %x, i32 %y) {
; CHECK-LABEL: @shr_mismatch_xor(
; CHECK-NEXT: [[SH0:%.*]] = ashr i32 [[X:%.*]], 5
Expand Down Expand Up @@ -546,3 +547,100 @@ define <2 x i64> @lshr_sub_poison(<2 x i64> %x, <2 x i64> %py) {
%sh1 = lshr <2 x i64> %r, <i64 7, i64 poison>
ret <2 x i64> %sh1
}

define i32 @ashr_xor_operand_match(i32 %x, i32 %y) {
; CHECK-LABEL: @ashr_xor_operand_match(
; CHECK-NEXT: [[RET:%.*]] = ashr i32 [[R:%.*]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = xor i32 %x, %y
%ret = ashr i32 %r, %x
ret i32 %ret
}

define i32 @ashr_xor_operand_mismtach(i32 %x, i32 %y) {
; CHECK-LABEL: @ashr_xor_operand_mismtach(
; CHECK-NEXT: [[R:%.*]] = xor i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[RET:%.*]] = ashr i32 [[R]], [[Y]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = xor i32 %x, %y
%ret = ashr i32 %r, %y
ret i32 %ret
}

define i32 @lshr_xor_operand_match(i32 %x, i32 %y) {
; CHECK-LABEL: @lshr_xor_operand_match(
; CHECK-NEXT: [[RET:%.*]] = lshr i32 [[R:%.*]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = xor i32 %x, %y
%ret = lshr i32 %r, %x
ret i32 %ret
}

define i32 @lshr_xor_operand_mismtach(i32 %x, i32 %y) {
; CHECK-LABEL: @lshr_xor_operand_mismtach(
; CHECK-NEXT: [[R:%.*]] = xor i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[RET:%.*]] = lshr i32 [[R]], [[Y]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = xor i32 %x, %y
%ret = lshr i32 %r, %y
ret i32 %ret
}

define i32 @ashr_or_operand_match(i32 %x, i32 %y) {
; CHECK-LABEL: @ashr_or_operand_match(
; CHECK-NEXT: [[RET:%.*]] = ashr i32 [[R:%.*]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = or i32 %x, %y
%ret = ashr i32 %r, %x
ret i32 %ret
}

define i32 @ashr_or_operand_mismtach(i32 %x, i32 %y) {
; CHECK-LABEL: @ashr_or_operand_mismtach(
; CHECK-NEXT: [[R:%.*]] = or i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[RET:%.*]] = ashr i32 [[R]], [[Y]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = or i32 %x, %y
%ret = ashr i32 %r, %y
ret i32 %ret
}

define i32 @lshr_or_operand_match(i32 %x, i32 %y) {
; CHECK-LABEL: @lshr_or_operand_match(
; CHECK-NEXT: [[RET:%.*]] = lshr i32 [[R:%.*]], [[X:%.*]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = or i32 %x, %y
%ret = lshr i32 %r, %x
ret i32 %ret
}

define i32 @lshr_or_operand_mismtach(i32 %x, i32 %y) {
; CHECK-LABEL: @lshr_or_operand_mismtach(
; CHECK-NEXT: [[R:%.*]] = or i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[RET:%.*]] = lshr i32 [[R]], [[Y]]
; CHECK-NEXT: ret i32 [[RET]]
;
%r = or i32 %x, %y
%ret = lshr i32 %r, %y
ret i32 %ret
}

define i32 @ashr_xor_operand_match_multiuse(i32 %x, i32 %y) {
; CHECK-LABEL: @ashr_xor_operand_match_multiuse(
; CHECK-NEXT: [[Q:%.*]] = ashr i32 [[R:%.*]], [[X:%.*]]
; CHECK-NEXT: [[RET:%.*]] = xor i32 [[R]], [[Q]]
; CHECK-NEXT: [[RET1:%.*]] = xor i32 [[RET]], [[X]]
; CHECK-NEXT: ret i32 [[RET1]]
;
%r = xor i32 %x, %y
%q = ashr i32 %r, %x
%ret = xor i32 %r, %q
ret i32 %ret
}
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