[llvm] [LAA] Avoid unprofitable unit-stride versioning (PR #225331)

[Ramkumar Ramachandra via llvm-commits]

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@@ -3248,39 +3247,27 @@ void LoopAccessInfo::collectStridedAccess(Value *MemAccess) {
   // of various possible stride specializations, considering the alternatives
   // of using gather/scatters (if available).
 
-  const SCEV *MaxBTC = PSE->getSymbolicMaxBackedgeTakenCount();
-
-  // Match the types so we can compare the stride and the MaxBTC.
-  // The Stride can be positive/negative, so we sign extend Stride;
-  // The backedgeTakenCount is non-negative, so we zero extend MaxBTC.
-  const DataLayout &DL = TheLoop->getHeader()->getDataLayout();
-  uint64_t StrideTypeSizeBits = DL.getTypeSizeInBits(StrideExpr->getType());
-  uint64_t BETypeSizeBits = DL.getTypeSizeInBits(MaxBTC->getType());
-  const SCEV *CastedStride = StrideExpr;
-  const SCEV *CastedBECount = MaxBTC;
   ScalarEvolution *SE = PSE->getSE();
-  if (BETypeSizeBits >= StrideTypeSizeBits)
-    CastedStride = SE->getNoopOrSignExtend(StrideExpr, MaxBTC->getType());
-  else
-    CastedBECount = SE->getZeroExtendExpr(MaxBTC, StrideExpr->getType());
-  const SCEV *StrideMinusBETaken = SE->getMinusSCEV(CastedStride, CastedBECount);
-  // Since TripCount == BackEdgeTakenCount + 1, checking:
-  // "Stride >= TripCount" is equivalent to checking:
-  // Stride - MaxBTC> 0
-  if (SE->isKnownPositive(StrideMinusBETaken)) {
-    LLVM_DEBUG(
-        dbgs() << "LAA: Stride>=TripCount; No point in versioning as the "
-                  "Stride==1 predicate will imply that the loop executes "
-                  "at most once.\n");
+  const SCEV *MaxBTC = PSE->getSymbolicMaxBackedgeTakenCount();
+  if (!LoopGuards)
+    LoopGuards.emplace(ScalarEvolution::LoopGuards::collect(TheLoop, *SE));
+  MaxBTC = SE->applyLoopGuards(MaxBTC, *LoopGuards);
+
+  // Evaluate the guarded trip count under the unit-stride predicate instead of
+  // comparing the stride and trip count, which may use different integer
+  // extensions. Keep the predicate local: we have not decided to version the
+  // access yet.
+  const SCEV *One = SE->getOne(StrideBase->getType());
+  const SCEVPredicate *StrideIsOne = SE->getEqualPredicate(StrideBase, One);
+  if (SE->isLoopEntryGuardedByCond(TheLoop, ICmpInst::ICMP_NE, StrideBase,
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artagnon wrote:

I think we also need SE->isAvailableAtEntry(StrideBase) to avoid crashing in isLoopEntryGuardedByCond

https://github.com/llvm/llvm-project/pull/225331