[llvm] [SLP]Add runtime stride support for strided loads. (PR #81517)

[Alexey Bataev via llvm-commits]

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@@ -3890,6 +3891,122 @@ static bool isReverseOrder(ArrayRef<unsigned> Order) {
   });
 }
 
+/// Checks if the provided list of pointers \p Pointers represents the strided
+/// pointers for type ElemTy. If they are not, std::nullopt is returned.
+/// Otherwise, if \p Inst is not specified, just initialized optional value is
+/// returned to show that the pointers represent strided pointers. If \p Inst
+/// specified, the runtime stride is materialized before the given \p Inst.
+/// \returns std::nullopt if the pointers are not pointers with the runtime
+/// stride, nullptr or actual stride value, otherwise.
+static std::optional<Value *>
+calculateRtStride(ArrayRef<Value *> PointerOps, Type *ElemTy,
+                  const DataLayout &DL, ScalarEvolution &SE,
+                  SmallVectorImpl<unsigned> &SortedIndices,
+                  Instruction *Inst = nullptr) {
+  SmallVector<const SCEV *> SCEVs;
+  const SCEV *PtrSCEVA = nullptr;
+  const SCEV *PtrSCEVB = nullptr;
+  for (Value *Ptr : PointerOps) {
+    const SCEV *PtrSCEV = SE.getSCEV(Ptr);
+    if (!PtrSCEV)
+      return std::nullopt;
+    SCEVs.push_back(PtrSCEV);
+    if (!PtrSCEVA && !PtrSCEVB) {
+      PtrSCEVA = PtrSCEVB = PtrSCEV;
+      continue;
+    }
+    const SCEV *Diff = SE.getMinusSCEV(PtrSCEV, PtrSCEVA);
+    if (!Diff || isa<SCEVCouldNotCompute>(Diff))
+      return std::nullopt;
+    if (Diff->isNonConstantNegative()) {
+      PtrSCEVA = PtrSCEV;
+      continue;
+    }
+    const SCEV *Diff1 = SE.getMinusSCEV(PtrSCEVB, PtrSCEV);
+    if (!Diff1 || isa<SCEVCouldNotCompute>(Diff1))
+      return std::nullopt;
+    if (Diff1->isNonConstantNegative()) {
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alexey-bataev wrote:

I need to check that it is also not a constant

https://github.com/llvm/llvm-project/pull/81517