[llvm] [LoopVectorize] Perform loop versioning for some early exit loops (PR #120603)

[David Sherwood via llvm-commits]

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@@ -1609,6 +1609,43 @@ bool LoopVectorizationLegality::canVectorizeLoopNestCFG(
   return Result;
 }
 
+bool LoopVectorizationLegality::analyzePotentiallyFaultingLoads(
+    SmallVectorImpl<LoadInst *> *Loads) {
+  LLVM_DEBUG(dbgs() << "LV: Looking for potentially faulting loads in loop "
+                       "with uncountable early exit:\n");
+  for (LoadInst *LI : *Loads) {
+    LLVM_DEBUG(dbgs() << "LV:   Load: " << *LI << '\n');
+    Value *Ptr = LI->getPointerOperand();
+    if (!Ptr)
+      return false;
+    const SCEV *PtrExpr = PSE.getSCEV(Ptr);
+    const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(PtrExpr);
+    // TODO: Deal with loop invariant pointers.
+    if (!AR || AR->getLoop() != TheLoop || !AR->isAffine())
+      return false;
+    auto Step = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*PSE.getSE()));
+    if (!Step)
+      return false;
+    const SCEV *Start = AR->getStart();
+
+    // Make sure the step is positive and matches the object size in memory.
+    // TODO: Extend this to cover more cases.
+    auto &DL = LI->getDataLayout();
+    APInt EltSize(DL.getIndexTypeSizeInBits(Ptr->getType()),
+                  DL.getTypeStoreSize(LI->getType()).getFixedValue());
+
+    // Also discard element sizes that are not a power of 2, since the loop
+    // vectorizer can only perform loop versioning with pointer alignment
+    // checks for vector loads that are power-of-2 in size.
+    if (EltSize != Step->getAPInt() || !EltSize.isPowerOf2())
+      return false;
+
+    LLVM_DEBUG(dbgs() << "LV: SCEV for Load Ptr: " << *Start << '\n');
+    PotentiallyFaultingLoads.push_back({LI, Start});
----------------
david-arm wrote:

I've changed the list to now only require `Start` and the load type.

https://github.com/llvm/llvm-project/pull/120603