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[InstSimplify] Canonicalize predicates into strict versions in simplifyICmpInst #151642

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[InstSimplify] Canonicalize predicates into strict versions in simplifyICmpInst #151642

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ace8b12
[InstSimplify] Canonicalize `X uge 1` to `X ne 0` and `X sle -1` to `…
el-ev Aug 1, 2025
2e74abd
invert non-strict predicates
el-ev Aug 7, 2025
387267a
update test
el-ev Aug 12, 2025
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13 changes: 10 additions & 3 deletions llvm/lib/Analysis/InstructionSimplify.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3025,9 +3025,6 @@ static Value *simplifyICmpWithConstant(CmpPredicate Pred, Value *LHS,
*MulC != 0 && C->srem(*MulC) != 0)))
return ConstantInt::get(ITy, Pred == ICmpInst::ICMP_NE);

if (Pred == ICmpInst::ICMP_UGE && C->isOne() && isKnownNonZero(LHS, Q))
return ConstantInt::getTrue(ITy);

return nullptr;
}

Expand Down Expand Up @@ -3774,6 +3771,16 @@ static Value *simplifyICmpInst(CmpPredicate Pred, Value *LHS, Value *RHS,
if (Value *V = simplifyICmpOfBools(Pred, LHS, RHS, Q))
return V;

const APInt *C;
if (match(RHS, m_APIntAllowPoison(C)) &&
ICmpInst::isNonStrictPredicate(Pred) && !C->isZero()) {
if (auto Flipped = getFlippedStrictnessPredicateAndConstant(
Pred, ConstantInt::get(LHS->getType(), *C))) {
Pred = Flipped->first;
RHS = Flipped->second;
}
}
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Can we use getFlippedStrictnessPredicateAndConstant() to generalize this?

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It seems that the current implementation provides special handling only for comparisons against zero. I'm not sure if flipping other constant and predicate pairs is profitable. For instance, converting x sgt 0 to x sge -1 causes regressions.

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What regressions do you see?

The non-strict predicates are the canonical form, so I'd generally expect code to handle them.

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What regressions do you see?

el-ev@ee94499

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What regressions do you see without the !isZero() check?

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Transforms/Attributor/lvi-after-jumpthreading.ll
Transforms/InstSimplify/abs_intrinsic.ll
Transforms/InstSimplify/compare.ll


// TODO: Sink/common this with other potentially expensive calls that use
// ValueTracking? See comment below for isKnownNonEqual().
if (Value *V = simplifyICmpWithZero(Pred, LHS, RHS, Q))
Expand Down
4 changes: 3 additions & 1 deletion llvm/test/Transforms/InstSimplify/compare.ll
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Expand Up @@ -1895,7 +1895,9 @@ define <2 x i1> @icmp_shl_1_ule_signmask_poison(<2 x i8> %V) {

define <2 x i1> @icmp_shl_1_ule_signmask_poison2(<2 x i8> %V) {
; CHECK-LABEL: @icmp_shl_1_ule_signmask_poison2(
; CHECK-NEXT: ret <2 x i1> splat (i1 true)
; CHECK-NEXT: [[SHL:%.*]] = shl <2 x i8> <i8 1, i8 poison>, [[V:%.*]]
; CHECK-NEXT: [[CMP:%.*]] = icmp ule <2 x i8> [[SHL]], <i8 poison, i8 -128>
; CHECK-NEXT: ret <2 x i1> [[CMP]]
;
%shl = shl <2 x i8> <i8 1, i8 poison>, %V
%cmp = icmp ule <2 x i8> %shl, <i8 poison, i8 128>
Expand Down
6 changes: 1 addition & 5 deletions llvm/test/Transforms/InstSimplify/select-icmp.ll
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@@ -1,19 +1,15 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s -passes=instsimplify -S | FileCheck %s

; TODO: https://alive2.llvm.org/ce/z/3ybZRl
define i32 @pr54735_slt(i32 %x, i32 %y) {
; CHECK-LABEL: @pr54735_slt(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: br i1 [[CMP]], label [[COND_TRUE:%.*]], label [[COND_END:%.*]]
; CHECK: cond.true:
; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 [[X]], [[Y]]
; CHECK-NEXT: [[ADD:%.*]] = add nsw i32 [[SUB]], 1
; CHECK-NEXT: [[NEG:%.*]] = xor i32 [[SUB]], -1
; CHECK-NEXT: [[ABSCOND:%.*]] = icmp sle i32 [[SUB]], -1
; CHECK-NEXT: [[ABS:%.*]] = select i1 [[ABSCOND]], i32 [[NEG]], i32 [[ADD]]
; CHECK-NEXT: ret i32 [[ABS]]
; CHECK-NEXT: ret i32 [[NEG]]
; CHECK: cond.end:
; CHECK-NEXT: ret i32 0
;
Expand Down