[llvm] [DA] Fix zero coeff bug in Strong SIV test with runtime assumptions (#149991) (PR #155037)

[Michael Kruse via llvm-commits]

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@@ -1282,7 +1282,40 @@ 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_NE, Coeff, Zero)) {
+        LLVM_DEBUG(
+            dbgs()
+            << "\t    Coefficient proven non-zero by SCEV range analysis\n");
+      } else {
+        // Cannot prove at compile time, would need runtime assumption.
+        if (UnderRuntimeAssumptions) {
+          const SCEVPredicate *Pred =
+              SE->getComparePredicate(ICmpInst::ICMP_NE, Coeff, Zero);
+          SmallVector<const SCEVPredicate *, 4> NewPreds(
+              Assumptions.getPredicates());
+          NewPreds.push_back(Pred);
+          const_cast<DependenceInfo *>(this)->Assumptions =
+              SCEVUnionPredicate(NewPreds, *SE);
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Meinersbur wrote:

Consider refactoring this. There already is `SCEVUnionPredicate::add` but it is private, mayb add a method that reurns the union.

https://github.com/llvm/llvm-project/pull/155037