[llvm] [SLPVectorizer] Widen constant strided loads. (PR #162324)

[Alexey Bataev via llvm-commits]

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@@ -6942,32 +6941,99 @@ bool BoUpSLP::analyzeConstantStrideCandidate(
     const SmallVectorImpl<unsigned> &SortedIndices, const int64_t Diff,
     Value *Ptr0, Value *PtrN, StridedPtrInfo &SPtrInfo) const {
   const size_t Sz = PointerOps.size();
-  if (!isStridedLoad(PointerOps, ScalarTy, Alignment, Diff, Sz))
+  SmallVector<int64_t> SortedOffsetsFromBase(Sz);
+  // Go through `PointerOps` in sorted order and record offsets from `Ptr0`.
+  for (unsigned I : seq<unsigned>(Sz)) {
+    Value *Ptr =
+        SortedIndices.empty() ? PointerOps[I] : PointerOps[SortedIndices[I]];
+    SortedOffsetsFromBase[I] =
+        *getPointersDiff(ScalarTy, Ptr0, ScalarTy, Ptr, *DL, *SE);
+  }
+
+  // The code below checks that `SortedOffsetsFromBase` looks as follows:
+  // ```
+  // [
+  //   (e_{0, 0}, e_{0, 1}, ..., e_{0, GroupSize - 1}), // first group
+  //   (e_{1, 0}, e_{1, 1}, ..., e_{1, GroupSize - 1}), // secon group
+  //   ...
+  //   (e_{NumGroups - 1, 0}, e_{NumGroups - 1, 1}, ..., e_{NumGroups - 1,
+  //   GroupSize - 1}), // last group
+  // ]
+  // ```
+  // The distance between consecutive elements within each group should all be
+  // the same `StrideWithinGroup`. The distance between the first elements of
+  // consecutive groups should all be the same `StrideBetweenGroups`.
+
+  int64_t StrideWithinGroup =
+      SortedOffsetsFromBase[1] - SortedOffsetsFromBase[0];
+  // Determine size of the first group. Later we will check that all other
+  // groups have the same size.
+  auto isEndOfGroupIndex = [=, &SortedOffsetsFromBase](unsigned Idx) {
+    return SortedOffsetsFromBase[Idx] - SortedOffsetsFromBase[Idx - 1] !=
+           StrideWithinGroup;
+  };
+  unsigned GroupSize = *llvm::find_if(seq<unsigned>(1, Sz), isEndOfGroupIndex);
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alexey-bataev wrote:

Are you sure it always finds the required element?

https://github.com/llvm/llvm-project/pull/162324