[llvm] b6231f5 - [DA] Add overflow check in ExactSIV (#157086)

[via llvm-commits]

Author: Ryotaro Kasuga
Date: 2025-09-19T20:08:42+09:00
New Revision: b6231f5197820026adc452707e920847364f2e6f

URL: https://github.com/llvm/llvm-project/commit/b6231f5197820026adc452707e920847364f2e6f
DIFF: https://github.com/llvm/llvm-project/commit/b6231f5197820026adc452707e920847364f2e6f.diff

LOG: [DA] Add overflow check in ExactSIV (#157086)

This patch adds an overflow check to the `exactSIVtest` function to fix
the issue demonstrated in the test case added in #157085. This patch
only fixes one of the routines. To fully resolve the test case, the
other functions need to be addressed as well.

Added: 
    

Modified: 
    llvm/lib/Analysis/DependenceAnalysis.cpp
    llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index 0f77a1410e83b..6e576e866b310 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -1170,6 +1170,15 @@ const SCEVConstant *DependenceInfo::collectConstantUpperBound(const Loop *L,
   return nullptr;
 }
 
+/// Returns \p A - \p B if it guaranteed not to signed wrap. Otherwise returns
+/// nullptr. \p A and \p B must have the same integer type.
+static const SCEV *minusSCEVNoSignedOverflow(const SCEV *A, const SCEV *B,
+                                             ScalarEvolution &SE) {
+  if (SE.willNotOverflow(Instruction::Sub, /*Signed=*/true, A, B))
+    return SE.getMinusSCEV(A, B);
+  return nullptr;
+}
+
 // testZIV -
 // When we have a pair of subscripts of the form [c1] and [c2],
 // where c1 and c2 are both loop invariant, we attack it using
@@ -1626,7 +1635,9 @@ bool DependenceInfo::exactSIVtest(const SCEV *SrcCoeff, const SCEV *DstCoeff,
   assert(0 < Level && Level <= CommonLevels && "Level out of range");
   Level--;
   Result.Consistent = false;
-  const SCEV *Delta = SE->getMinusSCEV(DstConst, SrcConst);
+  const SCEV *Delta = minusSCEVNoSignedOverflow(DstConst, SrcConst, *SE);
+  if (!Delta)
+    return false;
   LLVM_DEBUG(dbgs() << "\t    Delta = " << *Delta << "\n");
   NewConstraint.setLine(SrcCoeff, SE->getNegativeSCEV(DstCoeff), Delta,
                         CurLoop);
@@ -1716,6 +1727,7 @@ bool DependenceInfo::exactSIVtest(const SCEV *SrcCoeff, const SCEV *DstCoeff,
   // explore directions
   unsigned NewDirection = Dependence::DVEntry::NONE;
   APInt LowerDistance, UpperDistance;
+  // TODO: Overflow check may be needed.
   if (TA.sgt(TB)) {
     LowerDistance = (TY - TX) + (TA - TB) * TL;
     UpperDistance = (TY - TX) + (TA - TB) * TU;

diff  --git a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
index 2a809c32d7d21..e8e7cb11bb23e 100644
--- a/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/ExactSIV.ll
@@ -841,7 +841,7 @@ define void @exact14(ptr %A) {
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 0, ptr %idx.0, align 1 --> Dst: store i8 0, ptr %idx.0, align 1
 ; CHECK-SIV-ONLY-NEXT:    da analyze - none!
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 0, ptr %idx.0, align 1 --> Dst: store i8 1, ptr %idx.1, align 1
-; CHECK-SIV-ONLY-NEXT:    da analyze - none!
+; CHECK-SIV-ONLY-NEXT:    da analyze - output [*|<]!
 ; CHECK-SIV-ONLY-NEXT:  Src: store i8 1, ptr %idx.1, align 1 --> Dst: store i8 1, ptr %idx.1, align 1
 ; CHECK-SIV-ONLY-NEXT:    da analyze - none!
 ;