[llvm] [VPlan] Use SCEV to prove non-aliasing for stores at different offsets. (PR #170347)

[Ramkumar Ramachandra via llvm-commits]

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@@ -139,14 +139,54 @@ bool VPlanTransforms::tryToConvertVPInstructionsToVPRecipes(
   return true;
 }
 
+// Return true if \p A and \p B are known to not alias for all VFs in the plan,
+// checked via the distance between the accesses
+static bool isNoAliasViaDistance(VPReplicateRecipe *A, VPReplicateRecipe *B,
+                                 ScalarEvolution &SE, const Loop &L,
+                                 VPTypeAnalysis &TypeInfo) {
+  if (A->getOpcode() != Instruction::Store ||
+      B->getOpcode() != Instruction::Store)
+    return false;
+
+  VPValue *AddrA = A->getOperand(1);
+  const SCEV *SCEVA = vputils::getSCEVExprForVPValue(AddrA, SE, &L);
+  VPValue *AddrB = B->getOperand(1);
+  const SCEV *SCEVB = vputils::getSCEVExprForVPValue(AddrB, SE, &L);
+  if (isa<SCEVCouldNotCompute>(SCEVA) || isa<SCEVCouldNotCompute>(SCEVB))
+    return false;
+
+  const SCEV *Distance = SE.getMinusSCEV(SCEVA, SCEVB);
+  auto *ConstDist = dyn_cast<SCEVConstant>(Distance);
+  if (!ConstDist)
+    return false;
+
+  const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
+  Type *TyA = TypeInfo.inferScalarType(A->getOperand(0));
+  TypeSize SizeA = DL.getTypeStoreSize(TyA);
+  Type *TyB = TypeInfo.inferScalarType(B->getOperand(0));
+  TypeSize SizeB = DL.getTypeStoreSize(TyB);
+
+  // Use the maximum store size to ensure no overlap from either direction.
+  uint64_t MaxStoreSize =
+      std::max(SizeA.getFixedValue(), SizeB.getFixedValue());
+  const APInt &DistValue = ConstDist->getAPInt();
+  auto VFs = B->getParent()->getPlan()->vectorFactors();
+  ElementCount MaxVF = *max_element(VFs, ElementCount::isKnownLT);
+  return DistValue.abs().uge(
+      MaxVF.multiplyCoefficientBy(MaxStoreSize).getFixedValue());
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artagnon wrote:

Will getFixedValue not assert here, as MaxVF could be a scalable? Also, is it possible to pass BestVF or hasn't that been decided yet?

https://github.com/llvm/llvm-project/pull/170347