[llvm] [InstrCombin] fold icmp with add/sub instructions having the same operands (PR #143241)
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Sat Jun 7 00:41:40 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Acthinks Yang (Acthinks)
<details>
<summary>Changes</summary>
Closes#<!-- -->143211
---
Full diff: https://github.com/llvm/llvm-project/pull/143241.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+41)
- (added) llvm/test/Transforms/InstCombine/icmp-subadd.ll (+114)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index c112fae351817..1054d091fd226 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -7728,6 +7728,47 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
}
}
+ // In case of a comparison with add/sub instructions having the same operands,
+ // check whether cmp operands have same signed no wrap. If so, just compare
+ // the sub's second operand and zero.
+ // For example:
+ // %tmp1 = sub nsw i8 %x, %y
+ // %tmp2 = add nsw i8 %x, %y
+ // %cmp = icmp sgt i8 %tmp1, %tmp2
+ // transform this into:
+ // %cmp = icmp slt i32 %y, 0
+ // This handles similar cases to transform.
+ {
+ Value *A, *B;
+ auto *I0 = dyn_cast<OverflowingBinaryOperator>(Op0);
+ auto *I1 = dyn_cast<OverflowingBinaryOperator>(Op1);
+ bool UnsignedCmp = ICmpInst::isUnsigned(Pred);
+ bool SignedCmp = ICmpInst::isSigned(Pred);
+ bool EqualityCmp = ICmpInst::isEquality(Pred);
+
+ if (I0 && I1) {
+ bool I0NUW = I0->hasNoUnsignedWrap();
+ bool I1NUW = I1->hasNoUnsignedWrap();
+ bool I0NSW = I0->hasNoSignedWrap();
+ bool I1NSW = I1->hasNoSignedWrap();
+ bool Swaped = false;
+ if ((UnsignedCmp && I0NUW && I1NUW) || (SignedCmp && I0NSW && I1NSW) ||
+ (EqualityCmp && I0NUW && I0NSW && I1NUW && I1NSW)) {
+ if (I0->getOpcode() == Instruction::Add &&
+ I1->getOpcode() == Instruction::Sub) {
+ std::swap(I0, I1);
+ Swaped = true;
+ }
+ if (match(I0, m_Sub(m_Value(A), m_Value(B))) &&
+ (match(I1, m_Add(m_Specific(A), m_Specific(B))) ||
+ match(I1, m_Add(m_Specific(B), m_Specific(A))))) {
+ return new ICmpInst(Swaped ? Pred : I.getSwappedPredicate(), B,
+ ConstantInt::get(Op0->getType(), 0));
+ }
+ }
+ }
+ }
+
// Try to optimize equality comparisons against alloca-based pointers.
if (Op0->getType()->isPointerTy() && I.isEquality()) {
assert(Op1->getType()->isPointerTy() &&
diff --git a/llvm/test/Transforms/InstCombine/icmp-subadd.ll b/llvm/test/Transforms/InstCombine/icmp-subadd.ll
new file mode 100644
index 0000000000000..46e5ebe57a1b0
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/icmp-subadd.ll
@@ -0,0 +1,114 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i1 @test-same-operands-sub-add-nsw-icmp-sgt(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-same-operands-sub-add-nsw-icmp-sgt(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp slt i8 [[B]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nsw i8 %a, %b
+ %add = add nsw i8 %a, %b
+ %cmp = icmp sgt i8 %sub, %add
+ ret i1 %cmp
+}
+
+define i1 @test-same-operands-sub-add-nsw-icmp-slt(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-same-operands-sub-add-nsw-icmp-slt(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i8 [[B]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nsw i8 %a, %b
+ %add = add nsw i8 %a, %b
+ %cmp = icmp slt i8 %sub, %add
+ ret i1 %cmp
+}
+
+define i1 @test-same-operands-sub-add-nsw-icmp-sle(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-same-operands-sub-add-nsw-icmp-sle(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i8 [[B]], -1
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nsw i8 %a, %b
+ %add = add nsw i8 %a, %b
+ %cmp = icmp sle i8 %sub, %add
+ ret i1 %cmp
+}
+
+define i1 @test-same-operands-sub-add-nsw-nuw-icmp-eq(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-same-operands-sub-add-nsw-nuw-icmp-eq(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[B]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nuw nsw i8 %a, %b
+ %add = add nuw nsw i8 %a, %b
+ %cmp = icmp eq i8 %sub, %add
+ ret i1 %cmp
+}
+
+; Check not folded
+define i1 @test-same-operands-sub-add-nsw-icmp-eq(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-same-operands-sub-add-nsw-icmp-eq(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i8 [[A]], [[B]]
+; CHECK-NEXT: [[ADD:%.*]] = add nsw i8 [[A]], [[B]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i8 [[SUB]], [[ADD]]
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nsw i8 %a, %b
+ %add = add nsw i8 %a, %b
+ %cmp = icmp eq i8 %sub, %add
+ ret i1 %cmp
+}
+
+define i1 @test-add-sub-nsw-icmp-sgt(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-add-sub-nsw-icmp-sgt(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i8 [[B]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nsw i8 %a, %b
+ %add = add nsw i8 %a, %b
+ %cmp = icmp sgt i8 %add, %sub
+ ret i1 %cmp
+}
+
+define i1 @test-add-sub-nuw-icmp-uge(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-add-sub-nuw-icmp-uge(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: ret i1 true
+;
+ %sub = sub nuw i8 %a, %b
+ %add = add nuw i8 %a, %b
+ %cmp = icmp uge i8 %add, %sub
+ ret i1 %cmp
+}
+
+; Check not folded
+define i1 @test-add-sub-nuw-icmp-sge(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-add-sub-nuw-icmp-sge(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: [[SUB:%.*]] = sub nuw i8 [[A]], [[B]]
+; CHECK-NEXT: [[ADD:%.*]] = add nuw i8 [[A]], [[B]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sge i8 [[ADD]], [[SUB]]
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %sub = sub nuw i8 %a, %b
+ %add = add nuw i8 %a, %b
+ %cmp = icmp sge i8 %add, %sub
+ ret i1 %cmp
+}
+
+define i1 @test-add-swap-sub-nuw-icmp-uge(i8 %a, i8 %b) {
+; CHECK-LABEL: define i1 @test-add-swap-sub-nuw-icmp-uge(
+; CHECK-SAME: i8 [[A:%.*]], i8 [[B:%.*]]) {
+; CHECK-NEXT: ret i1 true
+;
+ %sub = sub nuw i8 %a, %b
+ %add = add nuw i8 %b, %a
+ %cmp = icmp uge i8 %add, %sub
+ ret i1 %cmp
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/143241
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