[llvm] [SCCP] Relax two-instruction range checks (PR #158495)
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Sun Sep 14 19:31:15 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-function-specialization
Author: Yingwei Zheng (dtcxzyw)
<details>
<summary>Changes</summary>
If we know x in R1, the range check `x in R2` can be relaxed into `x in Union(R2, Inverse(R1))`.
Fixes regressions introduced by https://github.com/llvm/llvm-project/pull/156497.
Compile-time impact: https://llvm-compile-time-tracker.com/compare.php?from=ead4f3e271fdf6918aef2ede3a7134811147d276&to=fcd06a82a8e04120aa24cd521953740c9ca5d3d4&stat=instructions%3Au
---
Full diff: https://github.com/llvm/llvm-project/pull/158495.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+50)
- (added) llvm/test/Transforms/SCCP/relax-range-checks.ll (+92)
``````````diff
diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index 84485176ad4ff..e239cad257141 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -19,8 +19,10 @@
#include "llvm/Analysis/ValueLattice.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstVisitor.h"
+#include "llvm/IR/Instructions.h"
#include "llvm/IR/NoFolder.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Casting.h"
@@ -284,6 +286,54 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
return Sub;
}
+ // Relax range checks.
+ if (auto *ICmp = dyn_cast<ICmpInst>(&Inst)) {
+ Value *X;
+ auto MatchTwoInstructionExactRangeCheck =
+ [&]() -> std::optional<ConstantRange> {
+ const APInt *RHSC;
+ if (!match(ICmp->getOperand(1), m_APInt(RHSC)))
+ return std::nullopt;
+
+ Value *LHS = ICmp->getOperand(0);
+ ICmpInst::Predicate Pred = ICmp->getPredicate();
+ const APInt *Offset;
+ if (match(LHS, m_OneUse(m_AddLike(m_Value(X), m_APInt(Offset)))))
+ return ConstantRange::makeExactICmpRegion(Pred, *RHSC).sub(*Offset);
+ // Match icmp eq/ne X & NegPow2, C
+ if (ICmp->isEquality()) {
+ const APInt *Mask;
+ if (match(LHS, m_OneUse(m_And(m_Value(X), m_NegatedPower2(Mask)))) &&
+ RHSC->countr_zero() >= Mask->countr_zero()) {
+ ConstantRange CR(*RHSC, *RHSC - *Mask);
+ return Pred == ICmpInst::ICMP_EQ ? CR : CR.inverse();
+ }
+ }
+ return std::nullopt;
+ };
+
+ if (auto CR = MatchTwoInstructionExactRangeCheck()) {
+ ConstantRange LRange = GetRange(X);
+ if (LRange.isFullSet())
+ return nullptr;
+ auto NewCR = CR->exactUnionWith(LRange.inverse());
+ if (!NewCR)
+ return nullptr;
+ // Avoid transforming cases which do not relax the range.
+ if (NewCR.value() == *CR)
+ return nullptr;
+ ICmpInst::Predicate Pred;
+ APInt RHS;
+ if (NewCR->getEquivalentICmp(Pred, RHS)) {
+ IRBuilder<NoFolder> Builder(&Inst);
+ Value *NewICmp =
+ Builder.CreateICmp(Pred, X, ConstantInt::get(X->getType(), RHS));
+ InsertedValues.insert(NewICmp);
+ return NewICmp;
+ }
+ }
+ }
+
return nullptr;
}
diff --git a/llvm/test/Transforms/SCCP/relax-range-checks.ll b/llvm/test/Transforms/SCCP/relax-range-checks.ll
new file mode 100644
index 0000000000000..90722f350aa9e
--- /dev/null
+++ b/llvm/test/Transforms/SCCP/relax-range-checks.ll
@@ -0,0 +1,92 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=sccp -S | FileCheck %s
+
+define i1 @relax_range_check(i8 range(i8 0, 5) %x) {
+; CHECK-LABEL: define i1 @relax_range_check(
+; CHECK-SAME: i8 range(i8 0, 5) [[X:%.*]]) {
+; CHECK-NEXT: [[ADD:%.*]] = add nsw i8 [[X]], -3
+; CHECK-NEXT: [[RET:%.*]] = icmp uge i8 [[X]], 3
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %add = add i8 %x, -3
+ %ret = icmp ult i8 %add, 2
+ ret i1 %ret
+}
+
+define i1 @relax_range_check_highbits_check(i8 range(i8 2, 0) %x) {
+; CHECK-LABEL: define i1 @relax_range_check_highbits_check(
+; CHECK-SAME: i8 range(i8 2, 0) [[X:%.*]]) {
+; CHECK-NEXT: [[AND:%.*]] = and i8 [[X]], -2
+; CHECK-NEXT: [[RET:%.*]] = icmp ult i8 [[X]], 4
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %and = and i8 %x, -2
+ %ret = icmp eq i8 %and, 2
+ ret i1 %ret
+}
+
+; Negative tests.
+
+define i1 @relax_range_check_one_instruction(i8 range(i8 0, 5) %x) {
+; CHECK-LABEL: define i1 @relax_range_check_one_instruction(
+; CHECK-SAME: i8 range(i8 0, 5) [[X:%.*]]) {
+; CHECK-NEXT: [[RET:%.*]] = icmp ult i8 [[X]], 2
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %ret = icmp ult i8 %x, 2
+ ret i1 %ret
+}
+
+define i1 @relax_range_check_not_profitable(i8 range(i8 0, 6) %x) {
+; CHECK-LABEL: define i1 @relax_range_check_not_profitable(
+; CHECK-SAME: i8 range(i8 0, 6) [[X:%.*]]) {
+; CHECK-NEXT: [[ADD:%.*]] = add nsw i8 [[X]], -3
+; CHECK-NEXT: [[RET:%.*]] = icmp ult i8 [[ADD]], 2
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %add = add i8 %x, -3
+ %ret = icmp ult i8 %add, 2
+ ret i1 %ret
+}
+
+define i1 @relax_range_check_unknown_range(i64 %x) {
+; CHECK-LABEL: define i1 @relax_range_check_unknown_range(
+; CHECK-SAME: i64 [[X:%.*]]) {
+; CHECK-NEXT: [[AND:%.*]] = and i64 [[X]], -67108864
+; CHECK-NEXT: [[TMP1:%.*]] = icmp eq i64 [[AND]], 0
+; CHECK-NEXT: ret i1 [[TMP1]]
+;
+ %and = and i64 %x, -67108864
+ %test = icmp eq i64 %and, 0
+ ret i1 %test
+}
+
+define i1 @relax_range_check_highbits_check_multiuse(i8 range(i8 2, 0) %x) {
+; CHECK-LABEL: define i1 @relax_range_check_highbits_check_multiuse(
+; CHECK-SAME: i8 range(i8 2, 0) [[X:%.*]]) {
+; CHECK-NEXT: [[AND:%.*]] = and i8 [[X]], -2
+; CHECK-NEXT: call void @use(i8 [[AND]])
+; CHECK-NEXT: [[RET:%.*]] = icmp eq i8 [[AND]], 2
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %and = and i8 %x, -2
+ call void @use(i8 %and)
+ %ret = icmp eq i8 %and, 2
+ ret i1 %ret
+}
+
+define i1 @relax_range_check_multiuse(i8 range(i8 0, 5) %x) {
+; CHECK-LABEL: define i1 @relax_range_check_multiuse(
+; CHECK-SAME: i8 range(i8 0, 5) [[X:%.*]]) {
+; CHECK-NEXT: [[ADD:%.*]] = add nsw i8 [[X]], -3
+; CHECK-NEXT: call void @use(i8 [[ADD]])
+; CHECK-NEXT: [[RET:%.*]] = icmp ult i8 [[ADD]], 2
+; CHECK-NEXT: ret i1 [[RET]]
+;
+ %add = add i8 %x, -3
+ call void @use(i8 %add)
+ %ret = icmp ult i8 %add, 2
+ ret i1 %ret
+}
+
+declare void @use(i8)
``````````
</details>
https://github.com/llvm/llvm-project/pull/158495
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