[llvm] [InstCombine] Fold comparisons of llvm.usub.sat result with its LHS (PR #214108)
Haram Jeong via llvm-commits
llvm-commits at lists.llvm.org
Wed Aug 5 22:17:33 PDT 2026
https://github.com/haramj updated https://github.com/llvm/llvm-project/pull/214108
>From 4f129d5bf7af0bca1be84885d40e83e46dbb0f9b Mon Sep 17 00:00:00 2001
From: haramjeong <04harams77 at gmail.com>
Date: Thu, 6 Aug 2026 13:54:12 +0900
Subject: [PATCH 1/2] [InstCombine] Add tests for usub.sat comparisons with its
LHS
---
.../Transforms/InstCombine/icmp-usub-sat.ll | 87 +++++++++++++++++++
1 file changed, 87 insertions(+)
diff --git a/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll b/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
index 2cd07b17af580..1c0233aba2229 100644
--- a/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
@@ -425,6 +425,92 @@ define <2 x i1> @icmp_eq_vector_multiuse_negative_equal(<2 x i8> %arg) {
ret <2 x i1> %cmp
}
+define i1 @icmp_eq_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[SAT]], [[X]]
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp eq i64 %sat, %x
+ ret i1 %cmp
+}
+
+define i1 @icmp_eq_lhs_nonzero_constant_commuted(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant_commuted
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], [[SAT]]
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp eq i64 %x, %sat
+ ret i1 %cmp
+}
+
+define i1 @icmp_ult_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ult_lhs_nonzero_constant
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
+; CHECK-NEXT: [[CMP:%.*]] = icmp ult i64 [[SAT]], [[X]]
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp ult i64 %sat, %x
+ ret i1 %cmp
+}
+
+define i1 @icmp_eq_lhs_nonzero_constant_multiuse(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant_multiuse
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[SAT]], [[X]]
+; CHECK-NEXT: call void @use.i64(i64 [[SAT]])
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp eq i64 %sat, %x
+ call void @use.i64(i64 %sat)
+ ret i1 %cmp
+}
+
+define i1 @icmp_ult_lhs_nonzero_constant_multiuse(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ult_lhs_nonzero_constant_multiuse
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
+; CHECK-NEXT: [[CMP:%.*]] = icmp ult i64 [[SAT]], [[X]]
+; CHECK-NEXT: call void @use.i64(i64 [[SAT]])
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp ult i64 %sat, %x
+ call void @use.i64(i64 %sat)
+ ret i1 %cmp
+}
+
+define i1 @icmp_eq_lhs_zero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_lhs_zero_constant
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: ret i1 true
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 0)
+ %cmp = icmp eq i64 %sat, %x
+ ret i1 %cmp
+}
+
+define <2 x i1> @icmp_ult_lhs_nonzero_constant_vector(<2 x i8> %x) {
+; CHECK-LABEL: define <2 x i1> @icmp_ult_lhs_nonzero_constant_vector
+; CHECK-SAME: (<2 x i8> [[X:%.*]]) {
+; CHECK-NEXT: [[SAT:%.*]] = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> [[X]], <2 x i8> splat (i8 10))
+; CHECK-NEXT: [[CMP:%.*]] = icmp ult <2 x i8> [[SAT]], [[X]]
+; CHECK-NEXT: ret <2 x i1> [[CMP]]
+;
+ %sat = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> %x, <2 x i8> <i8 10, i8 10>)
+ %cmp = icmp ult <2 x i8> %sat, %x
+ ret <2 x i1> %cmp
+}
+
declare i8 @llvm.usub.sat.i8(i8, i8)
declare i16 @llvm.usub.sat.i16(i16, i16)
declare i32 @llvm.usub.sat.i32(i32, i32)
@@ -437,3 +523,4 @@ declare <2 x i8> @llvm.usub.sat.v2i8(<2 x i8>, <2 x i8>)
declare void @use.i8(i8)
declare void @use.v2i8(<2 x i8>)
+declare void @use.i64(i64)
>From 534e5cfb97b19c0ac05da2d17ddb45e6077d51cb Mon Sep 17 00:00:00 2001
From: haramjeong <04harams77 at gmail.com>
Date: Thu, 6 Aug 2026 14:16:51 +0900
Subject: [PATCH 2/2] [InstCombine] Fold usub.sat comparisons with its LHS
---
.../InstCombine/InstCombineCompares.cpp | 33 +++++++-
.../Transforms/InstCombine/icmp-usub-sat.ll | 83 +++++++++++++------
2 files changed, 91 insertions(+), 25 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 70d584740f5b9..f5a275331a073 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3952,7 +3952,35 @@ Instruction *InstCombinerImpl::foldICmpEqIntrinsicWithConstant(
return nullptr;
}
-/// Fold an icmp with LLVM intrinsics
+/// Fold comparisons of llvm.usub.sat with its LHS.
+static Instruction *foldICmpUSubSatWithLHS(ICmpInst &Cmp,
+ const SimplifyQuery &Q) {
+ ICmpInst::Predicate Pred = Cmp.getPredicate();
+ Value *Op0 = Cmp.getOperand(0);
+ Value *Op1 = Cmp.getOperand(1);
+ Value *X, *Y;
+
+ // For Y != 0:
+ // usub.sat(X, Y) == X --> X == 0
+ // usub.sat(X, Y) != X --> X != 0
+ // usub.sat(X, Y) < X --> X != 0
+ // X > usub.sat(X, Y) --> X != 0
+ if (!match(&Cmp,
+ m_c_ICmp(m_Intrinsic<Intrinsic::usub_sat>(m_Value(X), m_Value(Y)),
+ m_Deferred(X))))
+ return nullptr;
+
+ bool IsSupportedPredicate = CmpInst::isEquality(Pred) ||
+ (Pred == ICmpInst::ICMP_ULT && Op1 == X) ||
+ (Pred == ICmpInst::ICMP_UGT && Op0 == X);
+ if (!IsSupportedPredicate || !isKnownNonZero(Y, Q))
+ return nullptr;
+
+ ICmpInst::Predicate NewPred =
+ CmpInst::isEquality(Pred) ? Pred : ICmpInst::ICMP_NE;
+ return new ICmpInst(NewPred, X, Constant::getNullValue(X->getType()));
+}
+
static Instruction *
foldICmpIntrinsicWithIntrinsic(ICmpInst &Cmp,
InstCombiner::BuilderTy &Builder) {
@@ -7853,6 +7881,9 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
if (Instruction *Res = canonicalizeICmpPredicate(I))
return Res;
+ if (Instruction *Res = foldICmpUSubSatWithLHS(I, Q))
+ return Res;
+
if (Instruction *Res = foldICmpWithConstant(I))
return Res;
diff --git a/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll b/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
index 1c0233aba2229..f612b192b43cb 100644
--- a/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-usub-sat.ll
@@ -425,11 +425,10 @@ define <2 x i1> @icmp_eq_vector_multiuse_negative_equal(<2 x i8> %arg) {
ret <2 x i1> %cmp
}
-define i1 @icmp_eq_lhs_nonzero_constant(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant
+define i1 @icmp_eq_usub_sat_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_usub_sat_lhs_nonzero_constant
; CHECK-SAME: (i64 [[X:%.*]]) {
-; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[SAT]], [[X]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], 0
; CHECK-NEXT: ret i1 [[CMP]]
;
%sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
@@ -437,11 +436,10 @@ define i1 @icmp_eq_lhs_nonzero_constant(i64 %x) {
ret i1 %cmp
}
-define i1 @icmp_eq_lhs_nonzero_constant_commuted(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant_commuted
+define i1 @icmp_eq_usub_sat_lhs_nonzero_constant_commuted(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_usub_sat_lhs_nonzero_constant_commuted
; CHECK-SAME: (i64 [[X:%.*]]) {
-; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], [[SAT]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], 0
; CHECK-NEXT: ret i1 [[CMP]]
;
%sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
@@ -449,11 +447,10 @@ define i1 @icmp_eq_lhs_nonzero_constant_commuted(i64 %x) {
ret i1 %cmp
}
-define i1 @icmp_ult_lhs_nonzero_constant(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_ult_lhs_nonzero_constant
+define i1 @icmp_ult_usub_sat_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ult_usub_sat_lhs_nonzero_constant
; CHECK-SAME: (i64 [[X:%.*]]) {
-; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
-; CHECK-NEXT: [[CMP:%.*]] = icmp ult i64 [[SAT]], [[X]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[X]], 0
; CHECK-NEXT: ret i1 [[CMP]]
;
%sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
@@ -461,11 +458,33 @@ define i1 @icmp_ult_lhs_nonzero_constant(i64 %x) {
ret i1 %cmp
}
-define i1 @icmp_eq_lhs_nonzero_constant_multiuse(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_eq_lhs_nonzero_constant_multiuse
+define i1 @icmp_ne_usub_sat_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ne_usub_sat_lhs_nonzero_constant
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[X]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp ne i64 %sat, %x
+ ret i1 %cmp
+}
+
+define i1 @icmp_ugt_usub_sat_lhs_nonzero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ugt_usub_sat_lhs_nonzero_constant
+; CHECK-SAME: (i64 [[X:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[X]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 10)
+ %cmp = icmp ugt i64 %x, %sat
+ ret i1 %cmp
+}
+
+define i1 @icmp_eq_usub_sat_lhs_nonzero_constant_multiuse(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_usub_sat_lhs_nonzero_constant_multiuse
; CHECK-SAME: (i64 [[X:%.*]]) {
; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[SAT]], [[X]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], 0
; CHECK-NEXT: call void @use.i64(i64 [[SAT]])
; CHECK-NEXT: ret i1 [[CMP]]
;
@@ -475,11 +494,11 @@ define i1 @icmp_eq_lhs_nonzero_constant_multiuse(i64 %x) {
ret i1 %cmp
}
-define i1 @icmp_ult_lhs_nonzero_constant_multiuse(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_ult_lhs_nonzero_constant_multiuse
+define i1 @icmp_ult_usub_sat_lhs_nonzero_constant_multiuse(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_ult_usub_sat_lhs_nonzero_constant_multiuse
; CHECK-SAME: (i64 [[X:%.*]]) {
; CHECK-NEXT: [[SAT:%.*]] = call i64 @llvm.usub.sat.i64(i64 [[X]], i64 10)
-; CHECK-NEXT: [[CMP:%.*]] = icmp ult i64 [[SAT]], [[X]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[X]], 0
; CHECK-NEXT: call void @use.i64(i64 [[SAT]])
; CHECK-NEXT: ret i1 [[CMP]]
;
@@ -489,8 +508,8 @@ define i1 @icmp_ult_lhs_nonzero_constant_multiuse(i64 %x) {
ret i1 %cmp
}
-define i1 @icmp_eq_lhs_zero_constant(i64 %x) {
-; CHECK-LABEL: define i1 @icmp_eq_lhs_zero_constant
+define i1 @icmp_eq_usub_sat_lhs_zero_constant(i64 %x) {
+; CHECK-LABEL: define i1 @icmp_eq_usub_sat_lhs_zero_constant
; CHECK-SAME: (i64 [[X:%.*]]) {
; CHECK-NEXT: ret i1 true
;
@@ -499,11 +518,10 @@ define i1 @icmp_eq_lhs_zero_constant(i64 %x) {
ret i1 %cmp
}
-define <2 x i1> @icmp_ult_lhs_nonzero_constant_vector(<2 x i8> %x) {
-; CHECK-LABEL: define <2 x i1> @icmp_ult_lhs_nonzero_constant_vector
+define <2 x i1> @icmp_ult_usub_sat_lhs_nonzero_constant_vector(<2 x i8> %x) {
+; CHECK-LABEL: define <2 x i1> @icmp_ult_usub_sat_lhs_nonzero_constant_vector
; CHECK-SAME: (<2 x i8> [[X:%.*]]) {
-; CHECK-NEXT: [[SAT:%.*]] = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> [[X]], <2 x i8> splat (i8 10))
-; CHECK-NEXT: [[CMP:%.*]] = icmp ult <2 x i8> [[SAT]], [[X]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne <2 x i8> [[X]], zeroinitializer
; CHECK-NEXT: ret <2 x i1> [[CMP]]
;
%sat = call <2 x i8> @llvm.usub.sat.v2i8(<2 x i8> %x, <2 x i8> <i8 10, i8 10>)
@@ -511,6 +529,21 @@ define <2 x i1> @icmp_ult_lhs_nonzero_constant_vector(<2 x i8> %x) {
ret <2 x i1> %cmp
}
+define i1 @icmp_eq_usub_sat_lhs_known_nonzero_rhs(i64 %x, i64 %y) {
+; CHECK-LABEL: define i1 @icmp_eq_usub_sat_lhs_known_nonzero_rhs
+; CHECK-SAME: (i64 [[X:%.*]], i64 [[Y:%.*]]) {
+; CHECK-NEXT: [[COND:%.*]] = icmp ne i64 [[Y]], 0
+; CHECK-NEXT: call void @llvm.assume(i1 [[COND]])
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[X]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %cond = icmp ne i64 %y, 0
+ call void @llvm.assume(i1 %cond)
+ %sat = call i64 @llvm.usub.sat.i64(i64 %x, i64 %y)
+ %cmp = icmp eq i64 %sat, %x
+ ret i1 %cmp
+}
+
declare i8 @llvm.usub.sat.i8(i8, i8)
declare i16 @llvm.usub.sat.i16(i16, i16)
declare i32 @llvm.usub.sat.i32(i32, i32)
@@ -521,6 +554,8 @@ declare <2 x i32> @llvm.usub.sat.v2i32(<2 x i32>, <2 x i32>)
declare <2 x i16> @llvm.usub.sat.v2i16(<2 x i16>, <2 x i16>)
declare <2 x i8> @llvm.usub.sat.v2i8(<2 x i8>, <2 x i8>)
+declare void @llvm.assume(i1)
+
declare void @use.i8(i8)
declare void @use.v2i8(<2 x i8>)
declare void @use.i64(i64)
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