[llvm] [LAA] Fix off-by-EltSize in negative-step deref bounds check (PR #211964)

[Florian Hahn via llvm-commits]

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@@ -264,15 +274,15 @@ static bool evaluatePtrAddRecAtMaxBTCWillNotWrap(
   Step = SE.getNoopOrSignExtend(Step, WiderTy);
   MaxBTC = SE.getNoopOrZeroExtend(MaxBTC, WiderTy);
 
-  // For the computations below, make sure they don't unsigned wrap.
-  // FIXME: for a negative step the lowest accessed address is not
-  // AR->getStart() but AR->evaluateAtIteration(MaxBTC, SE); the check below
-  // therefore compares StartPtr against the highest accessed address instead
-  // of the lowest.
-  if (!SE.isKnownPredicate(CmpInst::ICMP_UGE, AR->getStart(), StartPtr))
+  const SCEV *LowestAddr = IsKnownNonNegative
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fhahn wrote:

can we use SCEVUse instead of `const SCEV*` here?
```suggestion
 SCEVUse LowestAddr = IsKnownNonNegative
```

https://github.com/llvm/llvm-project/pull/211964