[llvm] [LAA] Be more careful when evaluating AddRecs at symbolic max BTC. (PR #128061)

[Florian Hahn via llvm-commits]

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@@ -206,11 +206,31 @@ std::pair<const SCEV *, const SCEV *> llvm::getStartAndEndForAccess(
   const SCEV *ScStart;
   const SCEV *ScEnd;
 
+  auto &DL = Lp->getHeader()->getDataLayout();
+  Type *IdxTy = DL.getIndexType(PtrExpr->getType());
+  const SCEV *EltSizeSCEV = SE->getStoreSizeOfExpr(IdxTy, AccessTy);
   if (SE->isLoopInvariant(PtrExpr, Lp)) {
     ScStart = ScEnd = PtrExpr;
   } else if (auto *AR = dyn_cast<SCEVAddRecExpr>(PtrExpr)) {
     ScStart = AR->getStart();
-    ScEnd = AR->evaluateAtIteration(MaxBECount, *SE);
+    if (!isa<SCEVCouldNotCompute>(BTC))
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fhahn wrote:

Thanks for digging into this! One clarification is that we evaluate at BTC = UNSIGNED_MAX. So `{0, +, 1}` won't wrap, but adding `1` will (`getStartAndEndForAccess` will compute the first address after the last access).

When we have strides larger than 1, the last accessed address will be something like `%start + stride * UNSIGNED_MAX`, which should wrap to something like `%start - %stride`. I am not entirely sure if there may be other wrapping issues with how `evaluateAtIteration` internally computes the result, but the original end point computed for `runtime_checks_with_symbolic_max_btc_neg_1` should illustrates that: start == end due to adding %stride to the result of evaluateAtIteration.

https://github.com/llvm/llvm-project/pull/128061