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

[Boyao Wang via llvm-commits]

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@@ -3248,42 +3249,33 @@ 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");
+  if (!SE->isAvailableAtLoopEntry(StrideExpr, TheLoop))
+    return;
+
+  const SCEV *MaxBTC = PSE->getSymbolicMaxBackedgeTakenCount();
+  if (!LoopGuards)
+    LoopGuards.emplace(ScalarEvolution::LoopGuards::collect(TheLoop, *SE));
+  MaxBTC = SE->applyLoopGuards(MaxBTC, *LoopGuards);
----------------
BoyaoWang430 wrote:

Without applying the loop guards here, trip_count_subtract_offset in llvm/test/Transforms/LoopVectorize/unit-stride-versioning.ll would fail to prove the backedge count is zero when N == 1. Because howFarToZero() uses loop guards to compute the constant maximum, but doesn't apply those guards to the symbolic maximum.

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