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[InstCombine] fold unsigned predicates on srem result #122520

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Merged
merged 2 commits into from
Jan 18, 2025
Merged

[InstCombine] fold unsigned predicates on srem result #122520

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f9ddc44
[InstCombine][NFC] precommit tests for signed floor division
jacobly0 Jan 12, 2025
d13aae7
[InstCombine] fold unsigned predicates to signed on srem result
jacobly0 Jan 12, 2025
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33 changes: 32 additions & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2674,10 +2674,41 @@ Instruction *InstCombinerImpl::foldICmpShrConstant(ICmpInst &Cmp,
Instruction *InstCombinerImpl::foldICmpSRemConstant(ICmpInst &Cmp,
BinaryOperator *SRem,
const APInt &C) {
const ICmpInst::Predicate Pred = Cmp.getPredicate();
if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULT) {
// Canonicalize unsigned predicates to signed:
// (X s% DivisorC) u> C -> (X s% DivisorC) s< 0
// iff (C s< 0 ? ~C : C) u>= abs(DivisorC)-1
// (X s% DivisorC) u< C+1 -> (X s% DivisorC) s> -1
// iff (C+1 s< 0 ? ~C : C) u>= abs(DivisorC)-1

const APInt *DivisorC;
if (!match(SRem->getOperand(1), m_APInt(DivisorC)))
return nullptr;

APInt NormalizedC = C;
if (Pred == ICmpInst::ICMP_ULT) {
assert(!NormalizedC.isZero() &&
"ult X, 0 should have been simplified already.");
--NormalizedC;
}
if (C.isNegative())
NormalizedC.flipAllBits();
assert(!DivisorC->isZero() &&
"srem X, 0 should have been simplified already.");
if (!NormalizedC.uge(DivisorC->abs() - 1))
return nullptr;

Type *Ty = SRem->getType();
if (Pred == ICmpInst::ICMP_UGT)
return new ICmpInst(ICmpInst::ICMP_SLT, SRem,
ConstantInt::getNullValue(Ty));
return new ICmpInst(ICmpInst::ICMP_SGT, SRem,
ConstantInt::getAllOnesValue(Ty));
}
// Match an 'is positive' or 'is negative' comparison of remainder by a
// constant power-of-2 value:
// (X % pow2C) sgt/slt 0
const ICmpInst::Predicate Pred = Cmp.getPredicate();
if (Pred != ICmpInst::ICMP_SGT && Pred != ICmpInst::ICMP_SLT &&
Pred != ICmpInst::ICMP_EQ && Pred != ICmpInst::ICMP_NE)
return nullptr;
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26 changes: 26 additions & 0 deletions llvm/test/Transforms/InstCombine/add.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3018,6 +3018,32 @@ define i32 @floor_sdiv_wrong_op(i32 %x, i32 %y) {
ret i32 %r
}

define i32 @floor_sdiv_using_srem_by_8(i32 %x) {
; CHECK-LABEL: @floor_sdiv_using_srem_by_8(
; CHECK-NEXT: [[F:%.*]] = ashr i32 [[X:%.*]], 3
; CHECK-NEXT: ret i32 [[F]]
;
%d = sdiv i32 %x, 8
%r = srem i32 %x, 8
%i = icmp ugt i32 %r, -2147483648
%s = sext i1 %i to i32
%f = add i32 %d, %s
ret i32 %f
}

define i32 @floor_sdiv_using_srem_by_2(i32 %x) {
; CHECK-LABEL: @floor_sdiv_using_srem_by_2(
; CHECK-NEXT: [[F:%.*]] = ashr i32 [[X:%.*]], 1
; CHECK-NEXT: ret i32 [[F]]
;
%d = sdiv i32 %x, 2
%r = srem i32 %x, 2
%i = icmp ugt i32 %r, -2147483648
%s = sext i1 %i to i32
%f = add i32 %d, %s
ret i32 %f
}

; (X s>> (BW - 1)) + (zext (X s> 0)) --> (X s>> (BW - 1)) | (zext (X != 0))

define i8 @signum_i8_i8(i8 %x) {
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