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

[Florian Hahn via llvm-commits]

================
@@ -139,14 +140,77 @@ bool VPlanTransforms::tryToConvertVPInstructionsToVPRecipes(
   return true;
 }
 
-// Check if a memory operation doesn't alias with memory operations in blocks
-// between FirstBB and LastBB using scoped noalias metadata.
-// For load hoisting, we only check writes in one direction.
-// For store sinking, we check both reads and writes bidirectionally.
+/// Helper for extra no-alias checks via known-safe recipe and SCEV.
+class SinkStoreInfo {
+  const SmallPtrSetImpl<VPRecipeBase *> &ExcludeRecipes;
+  VPReplicateRecipe &GroupLeader;
+  ScalarEvolution &SE;
+  const Loop &L;
+  VPTypeAnalysis &TypeInfo;
+
+  // 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
+  bool isNoAliasViaDistance(VPReplicateRecipe *A, VPReplicateRecipe *B) const {
+    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 APInt *Distance;
+    if (!match(SE.getMinusSCEV(SCEVA, SCEVB), m_scev_APInt(Distance)))
+      return false;
+
+    const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
+    Type *TyA = TypeInfo.inferScalarType(A->getOperand(0));
+    uint64_t SizeA = DL.getTypeStoreSize(TyA);
+    Type *TyB = TypeInfo.inferScalarType(B->getOperand(0));
+    uint64_t SizeB = DL.getTypeStoreSize(TyB);
+    // Use the maximum store size to ensure no overlap from either direction.
+    // Currently only handles fixed sizes, as it is only used for
+    // replicating VPReplicateRecipes.
+    uint64_t MaxStoreSize = std::max(SizeA, SizeB);
+
+    auto VFs = B->getParent()->getPlan()->vectorFactors();
+    ElementCount MaxVF = *max_element(VFs, ElementCount::isKnownLT);
+    return Distance->abs().uge(
+        MaxVF.multiplyCoefficientBy(MaxStoreSize).getFixedValue());
+  }
+
+public:
+  SinkStoreInfo(const SmallPtrSetImpl<VPRecipeBase *> &ExcludeRecipes,
+                VPReplicateRecipe &GroupLeader, ScalarEvolution &SE,
+                const Loop &L, VPTypeAnalysis &TypeInfo)
+      : ExcludeRecipes(ExcludeRecipes), GroupLeader(GroupLeader), SE(SE), L(L),
+        TypeInfo(TypeInfo) {}
+
+  /// Return true if \p R should be skipped during alias checking, either
+  /// because it's in the exclude set or because no-alias can be proven via
+  /// SCEV.
+  bool shouldSkip(VPRecipeBase &R) const {
+    if (ExcludeRecipes.contains(&R))
+      return true;
+    if (auto *Store = dyn_cast<VPReplicateRecipe>(&R))
+      return isNoAliasViaDistance(Store, &GroupLeader);
+    return false;
----------------
fhahn wrote:

Done thanks

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