[DA] Fix zero coeff bug in Strong SIV test with runtime assumptions #155037
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@llvm/pr-subscribers-llvm-analysis Author: Sebastian Pop (sebpop) ChangesFix GitHub issue #149991 where Strong SIV test incorrectly concludes 'none!' for symbolic coefficients that could be zero, leading to 0/0 undef behavior. The issue occurs in subscripts like {base,+,coeff} where coeff is symbolic:
The Strong SIV test's Delta=0 case assumed 0/X=0 where X is the coefficient, but when X could be zero, we have 0/0 which is undefined. The analysis needs to be conservative when the coefficient might be zero. When coefficient is SCEVUnknown and cannot be proven non-zero at compile time, use SCEV range analysis to attempt proving coefficient > 0. If this fails, add a runtime assumption 'coeff > 0' to the dependence result. This allows precise analysis when possible (none! under assumption coeff > 0) while maintaining correctness by exposing the required assumption. Test cases:
Full diff: https://github.com/llvm/llvm-project/pull/155037.diff 3 Files Affected:
diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index f33e04e804e3d..50e0fb7b3bcd3 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -1282,7 +1282,28 @@ bool DependenceInfo::strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
Result.DV[Level].Direction &= Dependence::DVEntry::EQ;
++StrongSIVsuccesses;
} else if (Delta->isZero()) {
- // since 0/X == 0
+ // Check if coefficient could be zero. If so, 0/0 is undefined and we
+ // cannot conclude that only same-iteration dependencies exist.
+ // When coeff=0, all iterations access the same location.
+ if (isa<SCEVUnknown>(Coeff) && !SE->isKnownNonZero(Coeff)) {
+ // Use SCEV range analysis to prove coefficient > 0 in loop context.
+ const SCEV *Zero = SE->getZero(Coeff->getType());
+
+ // Ask SCEV's range analysis if it can prove Coeff > Zero
+ if (SE->isKnownPredicate(ICmpInst::ICMP_SGT, Coeff, Zero)) {
+ LLVM_DEBUG(
+ dbgs()
+ << "\t Coefficient proven positive by SCEV range analysis\n");
+ } else {
+ // Cannot prove at compile time, add runtime assumption
+ const SCEVPredicate *Pred =
+ SE->getComparePredicate(ICmpInst::ICMP_SGT, Coeff, Zero);
+ const_cast<DependenceInfo *>(this)->Assumptions.push_back(Pred);
+ LLVM_DEBUG(dbgs() << "\t Added runtime assumption: " << *Coeff
+ << " > 0\n");
+ }
+ }
+ // since 0/X == 0 (where X is known non-zero)
Result.DV[Level].Distance = Delta;
NewConstraint.setDistance(Delta, CurLoop);
Result.DV[Level].Direction &= Dependence::DVEntry::EQ;
@@ -3574,8 +3595,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
if (!isLoadOrStore(Src) || !isLoadOrStore(Dst)) {
// can only analyze simple loads and stores, i.e., no calls, invokes, etc.
LLVM_DEBUG(dbgs() << "can only handle simple loads and stores\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
}
const MemoryLocation &DstLoc = MemoryLocation::get(Dst);
@@ -3586,8 +3607,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
case AliasResult::PartialAlias:
// cannot analyse objects if we don't understand their aliasing.
LLVM_DEBUG(dbgs() << "can't analyze may or partial alias\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
case AliasResult::NoAlias:
// If the objects noalias, they are distinct, accesses are independent.
LLVM_DEBUG(dbgs() << "no alias\n");
@@ -3601,8 +3622,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
// The dependence test gets confused if the size of the memory accesses
// differ.
LLVM_DEBUG(dbgs() << "can't analyze must alias with different sizes\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
}
Value *SrcPtr = getLoadStorePointerOperand(Src);
@@ -3621,8 +3642,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
// We check this upfront so we don't crash in cases where getMinusSCEV()
// returns a SCEVCouldNotCompute.
LLVM_DEBUG(dbgs() << "can't analyze SCEV with different pointer base\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
}
// Even if the base pointers are the same, they may not be loop-invariant. It
@@ -3634,8 +3655,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
if (!isLoopInvariant(SrcBase, SrcLoop) ||
!isLoopInvariant(DstBase, DstLoop)) {
LLVM_DEBUG(dbgs() << "The base pointer is not loop invariant.\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
}
uint64_t EltSize = SrcLoc.Size.toRaw();
@@ -3646,8 +3667,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
if (!SE->isKnownMultipleOf(SrcEv, EltSize, Assume) ||
!SE->isKnownMultipleOf(DstEv, EltSize, Assume)) {
LLVM_DEBUG(dbgs() << "can't analyze SCEV with different offsets\n");
- return std::make_unique<Dependence>(Src, Dst,
- SCEVUnionPredicate(Assume, *SE));
+ return std::make_unique<Dependence>(
+ Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE));
}
if (!Assume.empty()) {
@@ -3670,7 +3691,7 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
LLVM_DEBUG(dbgs() << " common nesting levels = " << CommonLevels << "\n");
LLVM_DEBUG(dbgs() << " maximum nesting levels = " << MaxLevels << "\n");
- FullDependence Result(Src, Dst, SCEVUnionPredicate(Assume, *SE),
+ FullDependence Result(Src, Dst, SCEVUnionPredicate(this->Assumptions, *SE),
PossiblyLoopIndependent, CommonLevels);
++TotalArrayPairs;
diff --git a/llvm/test/Analysis/DependenceAnalysis/DADelin.ll b/llvm/test/Analysis/DependenceAnalysis/DADelin.ll
index 8f94a455d3724..c7b9865329214 100644
--- a/llvm/test/Analysis/DependenceAnalysis/DADelin.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/DADelin.ll
@@ -649,8 +649,13 @@ define void @coeff_may_negative(ptr %a, i32 %k) {
; CHECK-NEXT: da analyze - none!
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
; CHECK-NEXT: da analyze - output [*|<]!
+; CHECK-NEXT: Runtime Assumptions:
+; CHECK-NEXT: Compare predicate: %k sgt) 0
; CHECK-NEXT: Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
; CHECK-NEXT: da analyze - none!
+; CHECK-NEXT: Runtime Assumptions:
+; CHECK-NEXT: Compare predicate: %k sgt) 0
+; CHECK-NEXT: Compare predicate: %k sgt) 0
;
entry:
br label %loop
@@ -688,8 +693,13 @@ define void @coeff_positive(ptr %a, i32 %k) {
; CHECK-NEXT: da analyze - none!
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
; CHECK-NEXT: da analyze - output [*|<]!
+; CHECK-NEXT: Runtime Assumptions:
+; CHECK-NEXT: Compare predicate: %k sgt) 0
; CHECK-NEXT: Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
; CHECK-NEXT: da analyze - none!
+; CHECK-NEXT: Runtime Assumptions:
+; CHECK-NEXT: Compare predicate: %k sgt) 0
+; CHECK-NEXT: Compare predicate: %k sgt) 0
;
entry:
br label %loop
diff --git a/llvm/test/Analysis/DependenceAnalysis/zero-coefficient.ll b/llvm/test/Analysis/DependenceAnalysis/zero-coefficient.ll
new file mode 100644
index 0000000000000..d6ad924da722b
--- /dev/null
+++ b/llvm/test/Analysis/DependenceAnalysis/zero-coefficient.ll
@@ -0,0 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -disable-output "-passes=print<da>" 2>&1 | FileCheck %s
+
+; Test case for GitHub issue #149991: Strong SIV test with symbolic coefficient
+; that could be zero. Fixed using runtime assumptions: assume coefficient > 0.
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+
+define void @test_zero_coefficient(ptr %a, i64 %k) {
+; CHECK-LABEL: 'test_zero_coefficient'
+; CHECK-NEXT: Src: store i8 42, ptr %idx, align 1 --> Dst: store i8 42, ptr %idx, align 1
+; CHECK-NEXT: da analyze - none!
+; CHECK-NEXT: Runtime Assumptions:
+; CHECK-NEXT: Compare predicate: %k sgt) 0
+;
+entry:
+ br label %loop
+
+loop:
+ %i = phi i64 [ 0, %entry ], [ %i.next, %loop ]
+ %subscript = mul i64 %i, %k ; When %k=0, all iterations access %a[0]
+ %idx = getelementptr i8, ptr %a, i64 %subscript
+ store i8 42, ptr %idx
+ %i.next = add i64 %i, 1
+ %cond.exit = icmp eq i64 %i.next, 100
+ br i1 %cond.exit, label %exit, label %loop
+
+exit:
+ ret void
+}
|
…ce tests Previously, predicates were collected using a local `Assume` vector. This patch: 1. Removes local `Assume` vector, uses class member `Assumptions` instead. 2. Adds a `getNonRedundantAssumptions()` helper for deduplication. 3. Extends predicate collection to all dependence tests.
…tions (llvm#149977) Fix GitHub issue llvm#149991 where Strong SIV test incorrectly concludes 'none' for symbolic coefficients that could be zero, leading to 0/0 undefined behavior. The issue occurs in subscripts like {base,+,coeff} where coeff is symbolic: - When coeff != 0: different iterations access different locations - When coeff = 0: all iterations access the same location (many dependencies) The Strong SIV test's Delta=0 case assumed 0/X=0 where X is the coefficient, but when X could be zero, we have 0/0 which is undefined. The analysis needs to be conservative when the coefficient might be zero. Solution: When coefficient is SCEVUnknown and cannot be proven non-zero at compile time, use SCEV range analysis to attempt proving coefficient > 0. If this fails, add a runtime assumption 'coeff > 0' to the dependence result. This allows precise analysis when possible (none under assumption coeff > 0) while maintaining correctness by exposing the required assumption. Test cases: - zero-coefficient.ll: New test for the reported bug - DADelin.ll: Updated to expect runtime assumptions for symbolic coefficients
Meinersbur
reviewed
Aug 25, 2025
| const SCEV *Zero = SE->getZero(Coeff->getType()); | ||
|
|
||
| // Ask SCEV's range analysis if it can prove Coeff > Zero | ||
| if (SE->isKnownPredicate(ICmpInst::ICMP_SGT, Coeff, Zero)) { |
There was a problem hiding this comment.
Isn't this (or should be) deduction already redudant with SE->isKnownNonZero?
Comment on lines
+3588
to
+3595
| auto I = UniqueAssumptions.begin(); | ||
| while (I != UniqueAssumptions.end()) { | ||
| if (P->implies(*I, *SE)) { | ||
| I = UniqueAssumptions.erase(I); | ||
| } else { | ||
| ++I; | ||
| } | ||
| } |
There was a problem hiding this comment.
Suggested change
| auto I = UniqueAssumptions.begin(); | |
| while (I != UniqueAssumptions.end()) { | |
| if (P->implies(*I, *SE)) { | |
| I = UniqueAssumptions.erase(I); | |
| } else { | |
| ++I; | |
| } | |
| } | |
| erase_if(UniqueAssumptions, [](SCEVPredicate *) {...}) |
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Fix GitHub issue #149991 where Strong SIV test incorrectly concludes 'none!' for symbolic coefficients that could be zero, leading to 0/0 undef behavior.
The issue occurs in subscripts like {base,+,coeff} where coeff is symbolic:
The Strong SIV test's Delta=0 case assumed 0/X=0 where X is the coefficient, but when X could be zero, we have 0/0 which is undefined. The analysis needs to be conservative when the coefficient might be zero.
When coefficient is SCEVUnknown and cannot be proven non-zero at compile time, use SCEV range analysis to attempt proving coefficient > 0. If this fails, add a runtime assumption 'coeff > 0' to the dependence result.
This allows precise analysis when possible (none! under assumption coeff > 0) while maintaining correctness by exposing the required assumption.
Test cases: