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[DA] Fix Strong SIV test for symbolic coefficients and deltas (#149977)
Fixes GitHub issue #149977 where Strong SIV test incorrectly rejected dependencies with symbolic coefficients and deltas due to overly conservative bound checking. Root cause: The bound constraint check |Delta| > UpperBound * |Coeff| would prematurely reject dependencies when SCEV couldn't prove the relationship definitively for symbolic expressions, preventing the analysis from reaching the division logic. Solution: 1. Make bound check less conservative for symbolic expressions by adding runtime assumptions when SCEV cannot determine the relationship. 2. Enable symbolic division using SE->getUDivExactExpr for Delta/Coeff. 3. Add runtime assumptions where symbolic division cannot be computed. This enables precise dependence analysis for cases like: - Coefficient: -k (symbolic) - Delta: -(2*k + 1) (symbolic) - Distance: (2*k + 1)/k (computed symbolically) Test case validates: - When k = -1: distance = 1, clear flow dependence detected. - Runtime assumptions ensure bounds are satisfied.
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6 files changed

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-12
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6 files changed

+111
-12
lines changed

‎llvm/lib/Analysis/DependenceAnalysis.cpp‎

Lines changed: 61 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1249,10 +1249,33 @@ bool DependenceInfo::strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
12491249
SE->isKnownNonNegative(Coeff) ? Coeff : SE->getNegativeSCEV(Coeff);
12501250
const SCEV *Product = SE->getMulExpr(UpperBound, AbsCoeff);
12511251
if (isKnownPredicate(CmpInst::ICMP_SGT, AbsDelta, Product)) {
1252-
// Distance greater than trip count - no dependence
1253-
++StrongSIVindependence;
1254-
++StrongSIVsuccesses;
1255-
return true;
1252+
// Check if this involves symbolic expressions where we might be too
1253+
// conservative.
1254+
if (isa<SCEVUnknown>(Delta) || isa<SCEVUnknown>(Coeff) ||
1255+
!isa<SCEVConstant>(AbsDelta) || !isa<SCEVConstant>(Product)) {
1256+
// For symbolic expressions, add runtime assumption rather than
1257+
// rejecting.
1258+
const SCEVPredicate *BoundPred =
1259+
SE->getComparePredicate(ICmpInst::ICMP_SLE, AbsDelta, Product);
1260+
if (UnderRuntimeAssumptions) {
1261+
SmallVector<const SCEVPredicate *, 4> NewPreds(
1262+
Assumptions.getPredicates());
1263+
NewPreds.push_back(BoundPred);
1264+
const_cast<DependenceInfo *>(this)->Assumptions =
1265+
SCEVUnionPredicate(NewPreds, *SE);
1266+
LLVM_DEBUG(dbgs() << "\t Added runtime bound assumption\n");
1267+
} else {
1268+
// Cannot add runtime assumptions, let more complex tests try.
1269+
LLVM_DEBUG(dbgs() << "\t Would need runtime bound assumption but "
1270+
"not allowed. Failing this test.\n");
1271+
return false;
1272+
}
1273+
} else {
1274+
// Distance definitely greater than trip count - no dependence
1275+
++StrongSIVindependence;
1276+
++StrongSIVsuccesses;
1277+
return true;
1278+
}
12561279
}
12571280
}
12581281

@@ -1293,9 +1316,40 @@ bool DependenceInfo::strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
12931316
Result.DV[Level].Distance = Delta; // since X/1 == X
12941317
NewConstraint.setDistance(Delta, CurLoop);
12951318
} else {
1296-
Result.Consistent = false;
1297-
NewConstraint.setLine(Coeff, SE->getNegativeSCEV(Coeff),
1298-
SE->getNegativeSCEV(Delta), CurLoop);
1319+
// Try symbolic division: Distance = Delta / Coeff.
1320+
if (const SCEV *Distance = SE->getUDivExactExpr(Delta, Coeff)) {
1321+
LLVM_DEBUG(dbgs() << "\t Symbolic distance = " << *Distance << "\n");
1322+
Result.DV[Level].Distance = Distance;
1323+
NewConstraint.setDistance(Distance, CurLoop);
1324+
} else {
1325+
// Cannot compute exact division - check if we can add runtime
1326+
// assumptions.
1327+
if (isa<SCEVUnknown>(Coeff) && !SE->isKnownNonZero(Coeff)) {
1328+
// Add runtime assumption that coefficient is non-zero for division.
1329+
const SCEV *Zero = SE->getZero(Coeff->getType());
1330+
const SCEVPredicate *NonZeroPred =
1331+
SE->getComparePredicate(ICmpInst::ICMP_NE, Coeff, Zero);
1332+
if (UnderRuntimeAssumptions) {
1333+
SmallVector<const SCEVPredicate *, 4> NewPreds(
1334+
Assumptions.getPredicates());
1335+
NewPreds.push_back(NonZeroPred);
1336+
const_cast<DependenceInfo *>(this)->Assumptions =
1337+
SCEVUnionPredicate(NewPreds, *SE);
1338+
LLVM_DEBUG(dbgs() << "\t Added runtime assumption: " << *Coeff
1339+
<< " != 0 for symbolic division\n");
1340+
} else {
1341+
// Cannot add runtime assumptions, this test fails.
1342+
LLVM_DEBUG(dbgs()
1343+
<< "\t Would need runtime assumption " << *Coeff
1344+
<< " != 0 but not allowed. Failing this test.\n");
1345+
return false;
1346+
}
1347+
}
1348+
1349+
Result.Consistent = false;
1350+
NewConstraint.setLine(Coeff, SE->getNegativeSCEV(Coeff),
1351+
SE->getNegativeSCEV(Delta), CurLoop);
1352+
}
12991353
}
13001354

13011355
// maybe we can get a useful direction

‎llvm/test/Analysis/DependenceAnalysis/BasePtrBug.ll‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -18,7 +18,7 @@ define void @test1(ptr nocapture %A, ptr nocapture %B, i32 %N) #0 {
1818
; CHECK-NEXT: Src: %0 = load i32, ptr %gep.0, align 4 --> Dst: %0 = load i32, ptr %gep.0, align 4
1919
; CHECK-NEXT: da analyze - none!
2020
; CHECK-NEXT: Src: %0 = load i32, ptr %gep.0, align 4 --> Dst: %1 = load i32, ptr %gep.1, align 4
21-
; CHECK-NEXT: da analyze - input [*|<]!
21+
; CHECK-NEXT: da analyze - consistent input [((-4 * (sext i32 %div to i64))<nsw> /u 4)|<]!
2222
; CHECK-NEXT: Src: %0 = load i32, ptr %gep.0, align 4 --> Dst: store i32 %add, ptr %gep.B, align 4
2323
; CHECK-NEXT: da analyze - confused!
2424
; CHECK-NEXT: Src: %1 = load i32, ptr %gep.1, align 4 --> Dst: %1 = load i32, ptr %gep.1, align 4

‎llvm/test/Analysis/DependenceAnalysis/DADelin.ll‎

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -648,7 +648,7 @@ define void @coeff_may_negative(ptr %a, i32 %k) {
648648
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.0, align 1
649649
; CHECK-NEXT: da analyze - none!
650650
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
651-
; CHECK-NEXT: da analyze - output [*|<]!
651+
; CHECK-NEXT: da analyze - consistent output [((-1 * %k) /u %k)|<]!
652652
; CHECK-NEXT: Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
653653
; CHECK-NEXT: da analyze - none!
654654
;
@@ -687,7 +687,7 @@ define void @coeff_positive(ptr %a, i32 %k) {
687687
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.0, align 1
688688
; CHECK-NEXT: da analyze - none!
689689
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
690-
; CHECK-NEXT: da analyze - output [*|<]!
690+
; CHECK-NEXT: da analyze - consistent output [((-1 * %k) /u %k)|<]!
691691
; CHECK-NEXT: Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
692692
; CHECK-NEXT: da analyze - none!
693693
;

‎llvm/test/Analysis/DependenceAnalysis/PR149977.ll‎

Lines changed: 45 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,45 @@
1+
; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
2+
; RUN: opt < %s -disable-output "-passes=print<da>" 2>&1 | FileCheck %s
3+
4+
; Test case for GitHub issue #149977: Strong SIV test with symbolic coefficients and deltas
5+
; The issue was that the bound constraint check was overly conservative with symbolic expressions,
6+
; causing valid dependencies to be rejected before reaching the division logic.
7+
;
8+
; Mathematical analysis:
9+
; - Access patterns: a[-k*i] vs a[-k*i + (2*k + 1)]
10+
; - Strong SIV equation: -k*(i2-i1) = (2*k + 1)
11+
; - Distance: (2*k + 1)/k
12+
; - For k=-1: distance = -1/-1 = 1 (clear dependence)
13+
14+
define void @f(ptr %a, i64 %k) {
15+
; CHECK-LABEL: 'f'
16+
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.0, align 1
17+
; CHECK-NEXT: da analyze - none!
18+
; CHECK-NEXT: Src: store i8 42, ptr %idx.0, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
19+
; CHECK-NEXT: da analyze - consistent output [((-1 + (-2 * %k)) /u (-1 * %k))]!
20+
; CHECK-NEXT: Runtime Assumptions:
21+
; CHECK-NEXT: Compare predicate: (1 + (2 * %k))<nuw><nsw> sle) (2 * %k)
22+
; CHECK-NEXT: Src: store i8 42, ptr %idx.1, align 1 --> Dst: store i8 42, ptr %idx.1, align 1
23+
; CHECK-NEXT: da analyze - none!
24+
;
25+
entry:
26+
%mk = sub i64 0, %k ; mk = -k
27+
%kk = mul i64 %k, 2 ; kk = 2*k
28+
%kk.inc = add i64 1, %kk ; kk.inc = 2*k + 1
29+
br label %loop
30+
31+
loop:
32+
%i = phi i64 [ 0, %entry ], [ %i.next, %loop ]
33+
%subscript.0 = mul i64 %mk, %i ; -k * i
34+
%subscript.1 = add i64 %subscript.0, %kk.inc ; -k * i + (2*k + 1)
35+
%idx.0 = getelementptr i8, ptr %a, i64 %subscript.0
36+
%idx.1 = getelementptr i8, ptr %a, i64 %subscript.1
37+
store i8 42, ptr %idx.0
38+
store i8 42, ptr %idx.1
39+
%i.next = add i64 %i, 1
40+
%cond.exit = icmp eq i64 %i.next, 3
41+
br i1 %cond.exit, label %exit, label %loop
42+
43+
exit:
44+
ret void
45+
}

‎llvm/test/Analysis/DependenceAnalysis/StrongSIV.ll‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -385,7 +385,7 @@ define void @strong8(ptr %A, ptr %B, i64 %n) nounwind uwtable ssp {
385385
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: store i32 %conv, ptr %arrayidx, align 4
386386
; CHECK-NEXT: da analyze - none!
387387
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: %0 = load i32, ptr %arrayidx1, align 4
388-
; CHECK-NEXT: da analyze - flow [*|<]!
388+
; CHECK-NEXT: da analyze - consistent flow [((4 * %n) /u 4)|<]!
389389
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: store i32 %0, ptr %B.addr.01, align 4
390390
; CHECK-NEXT: da analyze - confused!
391391
; CHECK-NEXT: Src: %0 = load i32, ptr %arrayidx1, align 4 --> Dst: %0 = load i32, ptr %arrayidx1, align 4

‎llvm/test/Analysis/DependenceAnalysis/SymbolicSIV.ll‎

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -440,7 +440,7 @@ define void @symbolicsiv7(ptr %A, ptr %B, i64 %n, i64 %N, i64 %M) nounwind uwtab
440440
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: store i32 %conv, ptr %arrayidx, align 4
441441
; CHECK-NEXT: da analyze - none!
442442
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: %1 = load i32, ptr %arrayidx6, align 4
443-
; CHECK-NEXT: da analyze - flow [<>]!
443+
; CHECK-NEXT: da analyze - consistent flow [((-8 + (16 * %M)) /u (8 * %N))]!
444444
; CHECK-NEXT: Src: store i32 %conv, ptr %arrayidx, align 4 --> Dst: store i32 %1, ptr %B.addr.02, align 4
445445
; CHECK-NEXT: da analyze - confused!
446446
; CHECK-NEXT: Src: %1 = load i32, ptr %arrayidx6, align 4 --> Dst: %1 = load i32, ptr %arrayidx6, align 4

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