[llvm] [LAA][NFC] Refactor deref no-wrap check; expose broken reverse-loop bounds (PR #211960)

[Aleksandr Popov via llvm-commits]

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@@ -277,45 +281,51 @@ static bool evaluatePtrAddRecAtMaxBTCWillNotWrap(
     LoopGuards.emplace(ScalarEvolution::LoopGuards::collect(AR->getLoop(), SE));
   MaxBTC = SE.applyLoopGuards(MaxBTC, *LoopGuards);
 
-  const SCEV *OffsetAtLastIter =
-      mulSCEVNoOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
-  if (!OffsetAtLastIter) {
+  const SCEV *AbsStep = SE.getAbsExpr(Step, /*IsNSW=*/false);
+  // Total distance (in bytes) walked between the first and the last
+  // accessed pointer; MaxBTC * |Step|.
+  const SCEV *WalkBytes = mulSCEVNoOverflow(MaxBTC, AbsStep, SE);
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aleks-tmb wrote:

Agreed, though `OffsetAtLastIter` wouldn't be quite accurate for a negative-step loop. Renamed to `DistToLastIter` - hopefully that fits both directions.

https://github.com/llvm/llvm-project/pull/211960