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

[Sebastian Pop via llvm-commits]

================
@@ -1368,7 +1369,35 @@ 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)) {
----------------
sebpop wrote:

I rewrote this code like this to avoid enumerating all possible expressions that Coeff can take at this location:
```
    if (isa<SCEVConstant>(Coeff) || SE->isKnownNonZero(Coeff)) {
      LLVM_DEBUG(
          dbgs() << "\t    Coefficient proven non-zero by SCEV analysis\n");
    } else {
      // Cannot prove at compile time, would need runtime assumption.
[...]
    }
```

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