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[InstCombine] Fold icmp with clamp into unsigned bound check #161303

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merged 7 commits into from
Oct 2, 2025
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[InstCombine] Fold icmp with clamp into unsigned bound check #161303

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cffbd01
[InstCombine] Add baseline tests for icmp with clamp operation
brandonxin Sep 29, 2025
eab6eba
[InstCombine] Fold icmp with clamp to unsigned bound check
brandonxin Sep 30, 2025
68c86f4
[InstCombine] Remove redundant checks
brandonxin Sep 30, 2025
4eefa5e
[InstCombine] Improve header comments
brandonxin Sep 30, 2025
9cde0b2
[InstCombine] Check if Min is actually a Min.
brandonxin Oct 1, 2025
f306854
[InstCombine] Avoid unnecessary addition when offset is zero
brandonxin Oct 1, 2025
af64a82
[InstComine] Use getNonEmpty to handle edge case
brandonxin Oct 1, 2025
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44 changes: 43 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 @@ -5780,6 +5780,44 @@ Instruction *InstCombinerImpl::foldICmpWithMinMax(Instruction &I,
return nullptr;
}

/// Match and fold patterns like:
/// icmp eq/ne X, min(max(X, Lo), Hi)
/// which represents a range check and can be repsented as a ConstantRange.
///
/// For icmp eq, build ConstantRange [Lo, Hi + 1) and convert to:
/// (X - Lo) u< (Hi + 1 - Lo)
/// For icmp ne, build ConstantRange [Hi + 1, Lo) and convert to:
/// (X - (Hi + 1)) u< (Lo - (Hi + 1))
Instruction *InstCombinerImpl::foldICmpWithClamp(ICmpInst &I, Value *X,
MinMaxIntrinsic *Min) {
if (!I.isEquality() || !Min->hasOneUse())
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@nikic nikic Oct 1, 2025

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Shouldn't we also check that this is actually a min and not a max intrinsic?

I guess the max one will get folded away, but better not rely on it due to worklist order issues.

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@brandonxin brandonxin Oct 1, 2025

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Yes you are right. Without those preliminary transforms, there will be a problem here. I added isMin and isMax helper methods to MinMaxIntrinsic and then added a check here. Thanks for pointing that out.

return nullptr;

const APInt *Lo = nullptr, *Hi = nullptr;
if (Min->isSigned()) {
if (!match(Min->getLHS(), m_OneUse(m_SMax(m_Specific(X), m_APInt(Lo)))) ||
!match(Min->getRHS(), m_APInt(Hi)) || !Lo->slt(*Hi))
return nullptr;
} else {
if (!match(Min->getLHS(), m_OneUse(m_UMax(m_Specific(X), m_APInt(Lo)))) ||
!match(Min->getRHS(), m_APInt(Hi)) || !Lo->ult(*Hi))
return nullptr;
}

ConstantRange CR(*Lo, *Hi + 1);
ICmpInst::Predicate Pred;
APInt C, Offset;
if (I.getPredicate() == ICmpInst::ICMP_EQ)
CR.getEquivalentICmp(Pred, C, Offset);
else
CR.inverse().getEquivalentICmp(Pred, C, Offset);

X = Builder.CreateAdd(X, ConstantInt::get(X->getType(), Offset));

return replaceInstUsesWith(
I, Builder.CreateICmp(Pred, X, ConstantInt::get(X->getType(), C)));
}

// Canonicalize checking for a power-of-2-or-zero value:
static Instruction *foldICmpPow2Test(ICmpInst &I,
InstCombiner::BuilderTy &Builder) {
Expand Down Expand Up @@ -7467,10 +7505,14 @@ Instruction *InstCombinerImpl::foldICmpCommutative(CmpPredicate Pred,
if (Instruction *NI = foldSelectICmp(Pred, SI, Op1, CxtI))
return NI;

if (auto *MinMax = dyn_cast<MinMaxIntrinsic>(Op0))
if (auto *MinMax = dyn_cast<MinMaxIntrinsic>(Op0)) {
if (Instruction *Res = foldICmpWithMinMax(CxtI, MinMax, Op1, Pred))
return Res;

if (Instruction *Res = foldICmpWithClamp(CxtI, Op1, MinMax))
return Res;
}

{
Value *X;
const APInt *C;
Expand Down
1 change: 1 addition & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineInternal.h
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Expand Up @@ -725,6 +725,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
Instruction *foldICmpBinOp(ICmpInst &Cmp, const SimplifyQuery &SQ);
Instruction *foldICmpWithMinMax(Instruction &I, MinMaxIntrinsic *MinMax,
Value *Z, CmpPredicate Pred);
Instruction *foldICmpWithClamp(ICmpInst &Cmp, Value *X, MinMaxIntrinsic *Min);
Instruction *foldICmpEquality(ICmpInst &Cmp);
Instruction *foldIRemByPowerOfTwoToBitTest(ICmpInst &I);
Instruction *foldSignBitTest(ICmpInst &I);
Expand Down
295 changes: 295 additions & 0 deletions llvm/test/Transforms/InstCombine/icmp-clamp.ll
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@@ -0,0 +1,295 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

declare void @use(i32)

define i1 @test_i32_eq(i32 %x) {
; CHECK-LABEL: define i1 @test_i32_eq(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[X]], 95
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[TMP1]], 256
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

define i1 @test_i32_ne(i32 %x) {
; CHECK-LABEL: define i1 @test_i32_ne(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[X]], -161
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[TMP1]], -256
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp ne i32 %v2, %x
ret i1 %cmp
}

define i1 @test_i32_eq_no_add(i32 %x) {
; CHECK-LABEL: define i1 @test_i32_eq_no_add(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[X]], 161
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 0)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

define i1 @test_i32_ne_no_add(i32 %x) {
; CHECK-LABEL: define i1 @test_i32_ne_no_add(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ugt i32 [[X]], 160
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 0)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp ne i32 %v2, %x
ret i1 %cmp
}

define i1 @test_unsigned_eq(i32 %x) {
; CHECK-LABEL: define i1 @test_unsigned_eq(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[X]], -10
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[TMP1]], 91
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.umax.i32(i32 %x, i32 10)
%v2 = tail call i32 @llvm.umin.i32(i32 %v1, i32 100)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

define i1 @test_unsigned_ne(i32 %x) {
; CHECK-LABEL: define i1 @test_unsigned_ne(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[X]], -101
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[TMP1]], -91
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.umax.i32(i32 %x, i32 10)
%v2 = tail call i32 @llvm.umin.i32(i32 %v1, i32 100)
%cmp = icmp ne i32 %v2, %x
ret i1 %cmp
}


; Different bit widths
define i1 @test_i8_eq(i8 %x) {
; CHECK-LABEL: define i1 @test_i8_eq(
; CHECK-SAME: i8 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i8 [[X]], 50
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i8 [[TMP1]], 101
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i8 @llvm.smax.i8(i8 %x, i8 -50)
%v2 = tail call i8 @llvm.smin.i8(i8 %v1, i8 50)
%cmp = icmp eq i8 %v2, %x
ret i1 %cmp
}

define i1 @test_i16_eq(i16 %x) {
; CHECK-LABEL: define i1 @test_i16_eq(
; CHECK-SAME: i16 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i16 [[X]], 1000
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i16 [[TMP1]], 2001
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i16 @llvm.smax.i16(i16 %x, i16 -1000)
%v2 = tail call i16 @llvm.smin.i16(i16 %v1, i16 1000)
%cmp = icmp eq i16 %v2, %x
ret i1 %cmp
}

define i1 @test_i64_eq(i64 %x) {
; CHECK-LABEL: define i1 @test_i64_eq(
; CHECK-SAME: i64 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[X]], 1
; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i64 [[TMP1]], -1
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i64 @llvm.smax.i64(i64 %x, i64 -1)
%v2 = tail call i64 @llvm.smin.i64(i64 %v1, i64 9223372036854775806)
%cmp = icmp eq i64 %v2, %x
ret i1 %cmp
}

; Negative tests - wrong predicate
define i1 @test_wrong_pred_slt(i32 %x) {
; CHECK-LABEL: define i1 @test_wrong_pred_slt(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[X]], 160
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp slt i32 %v2, %x
ret i1 %cmp
}


; Negative tests - not a clamp pattern
define i1 @test_not_clamp_pattern(i32 %x, i32 %y) {
; CHECK-LABEL: define i1 @test_not_clamp_pattern(
; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
; CHECK-NEXT: [[V1:%.*]] = tail call i32 @llvm.smax.i32(i32 [[Y]], i32 -95)
; CHECK-NEXT: [[V2:%.*]] = tail call i32 @llvm.smin.i32(i32 [[V1]], i32 160)
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[V2]], [[X]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %y, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Negative tests - Lo >= Hi
define i1 @test_invalid_range(i32 %x) {
; CHECK-LABEL: define i1 @test_invalid_range(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 50
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 100)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 50)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Negative tests - Lo is minimum signed value
define i1 @test_lo_min_signed(i32 %x) {
; CHECK-LABEL: define i1 @test_lo_min_signed(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp slt i32 [[X]], 161
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -2147483648)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Negative tests - Hi is maximum signed value
define i1 @test_hi_max_signed(i32 %x) {
; CHECK-LABEL: define i1 @test_hi_max_signed(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[X]], -96
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 2147483647)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Negative tests - Hi is maximum unsigned value
define i1 @test_hi_max_unsigned(i32 %x) {
; CHECK-LABEL: define i1 @test_hi_max_unsigned(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CMP:%.*]] = icmp ugt i32 [[X]], 9
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.umax.i32(i32 %x, i32 10)
%v2 = tail call i32 @llvm.umin.i32(i32 %v1, i32 4294967295)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Multi-use tests - multiple uses of max
define i1 @test_multi_use_max(i32 %x) {
; CHECK-LABEL: define i1 @test_multi_use_max(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[V1:%.*]] = tail call i32 @llvm.smax.i32(i32 [[X]], i32 -95)
; CHECK-NEXT: call void @use(i32 [[V1]])
; CHECK-NEXT: [[V2:%.*]] = tail call i32 @llvm.smin.i32(i32 [[V1]], i32 160)
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[V2]], [[X]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
call void @use(i32 %v1)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Multi-use tests - multiple uses of min
define i1 @test_multi_use_min(i32 %x) {
; CHECK-LABEL: define i1 @test_multi_use_min(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[V1:%.*]] = tail call i32 @llvm.smax.i32(i32 [[X]], i32 -95)
; CHECK-NEXT: [[V2:%.*]] = tail call i32 @llvm.smin.i32(i32 [[V1]], i32 160)
; CHECK-NEXT: call void @use(i32 [[V2]])
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[V2]], [[X]]
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
call void @use(i32 %v2)
%cmp = icmp eq i32 %v2, %x
ret i1 %cmp
}

; Commuted tests
define i1 @test_commuted_eq(i32 %x) {
; CHECK-LABEL: define i1 @test_commuted_eq(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[X]], 95
; CHECK-NEXT: [[CMP:%.*]] = icmp ult i32 [[TMP1]], 256
; CHECK-NEXT: ret i1 [[CMP]]
;
%v1 = tail call i32 @llvm.smax.i32(i32 %x, i32 -95)
%v2 = tail call i32 @llvm.smin.i32(i32 %v1, i32 160)
%cmp = icmp eq i32 %x, %v2
ret i1 %cmp
}


; Vector tests - splat constants
define <2 x i1> @test_vec_splat_eq(<2 x i32> %x) {
; CHECK-LABEL: define <2 x i1> @test_vec_splat_eq(
; CHECK-SAME: <2 x i32> [[X:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = add <2 x i32> [[X]], splat (i32 50)
; CHECK-NEXT: [[CMP:%.*]] = icmp ult <2 x i32> [[TMP1]], splat (i32 101)
; CHECK-NEXT: ret <2 x i1> [[CMP]]
;
%v1 = tail call <2 x i32> @llvm.smax.v2i32(<2 x i32> %x, <2 x i32> <i32 -50, i32 -50>)
%v2 = tail call <2 x i32> @llvm.smin.v2i32(<2 x i32> %v1, <2 x i32> <i32 50, i32 50>)
%cmp = icmp eq <2 x i32> %v2, %x
ret <2 x i1> %cmp
}

; Vector tests - poison elements
define <2 x i1> @test_vec_poison_eq(<2 x i32> %x) {
; CHECK-LABEL: define <2 x i1> @test_vec_poison_eq(
; CHECK-SAME: <2 x i32> [[X:%.*]]) {
; CHECK-NEXT: [[V1:%.*]] = tail call <2 x i32> @llvm.smax.v2i32(<2 x i32> [[X]], <2 x i32> <i32 -50, i32 poison>)
; CHECK-NEXT: [[V2:%.*]] = tail call <2 x i32> @llvm.smin.v2i32(<2 x i32> [[V1]], <2 x i32> <i32 50, i32 poison>)
; CHECK-NEXT: [[CMP:%.*]] = icmp eq <2 x i32> [[V2]], [[X]]
; CHECK-NEXT: ret <2 x i1> [[CMP]]
;
%v1 = tail call <2 x i32> @llvm.smax.v2i32(<2 x i32> %x, <2 x i32> <i32 -50, i32 poison>)
%v2 = tail call <2 x i32> @llvm.smin.v2i32(<2 x i32> %v1, <2 x i32> <i32 50, i32 poison>)
%cmp = icmp eq <2 x i32> %v2, %x
ret <2 x i1> %cmp
}

; Vector tests - non-splat
define <2 x i1> @test_vec_non_splat_eq(<2 x i32> %x) {
; CHECK-LABEL: define <2 x i1> @test_vec_non_splat_eq(
; CHECK-SAME: <2 x i32> [[X:%.*]]) {
; CHECK-NEXT: [[V1:%.*]] = tail call <2 x i32> @llvm.smax.v2i32(<2 x i32> [[X]], <2 x i32> <i32 -50, i32 -30>)
; CHECK-NEXT: [[V2:%.*]] = tail call <2 x i32> @llvm.smin.v2i32(<2 x i32> [[V1]], <2 x i32> <i32 50, i32 70>)
; CHECK-NEXT: [[CMP:%.*]] = icmp eq <2 x i32> [[V2]], [[X]]
; CHECK-NEXT: ret <2 x i1> [[CMP]]
;
%v1 = tail call <2 x i32> @llvm.smax.v2i32(<2 x i32> %x, <2 x i32> <i32 -50, i32 -30>)
%v2 = tail call <2 x i32> @llvm.smin.v2i32(<2 x i32> %v1, <2 x i32> <i32 50, i32 70>)
%cmp = icmp eq <2 x i32> %v2, %x
ret <2 x i1> %cmp
}