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

[Florian Hahn 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);
+  if (!WalkBytes) {
     // Re-try with constant max backedge-taken count if using the symbolic one
     // failed.
     MaxBTC = SE.getConstantMaxBackedgeTakenCount(AR->getLoop());
     if (isa<SCEVCouldNotCompute>(MaxBTC))
       return false;
-    MaxBTC = SE.getNoopOrZeroExtend(
-        MaxBTC, WiderTy);
-    OffsetAtLastIter =
-        mulSCEVNoOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
-    if (!OffsetAtLastIter)
+    MaxBTC = SE.getNoopOrZeroExtend(MaxBTC, WiderTy);
+    WalkBytes = mulSCEVNoOverflow(MaxBTC, AbsStep, SE);
+    if (!WalkBytes)
       return false;
   }
 
-  const SCEV *OffsetEndBytes = addSCEVNoOverflow(
-      OffsetAtLastIter, SE.getNoopOrZeroExtend(EltSize, WiderTy), SE);
-  if (!OffsetEndBytes)
+  // Total length in bytes of the accessed range (from the first accessed
+  // byte through the end of the last access); WalkBytes + EltSize.
+  const SCEV *SpanBytes = addSCEVNoOverflow(
----------------
fhahn wrote:

nit: more in line with the comment above?
```suggestion
  const SCEV *AccessedBytes = addSCEVNoOverflow(
```

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