[llvm] [RISCV] Improve performCONCAT_VECTORCombine stride matching (PR #68726)

[Craig Topper via llvm-commits]

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@@ -13821,6 +13822,58 @@ static SDValue performCONCAT_VECTORSCombine(SDNode *N, SelectionDAG &DAG,
     Align = std::min(Align, Ld->getAlign());
   }
 
+  // If the load ptrs can be decomposed into a common (Base + Index) with a
+  // common constant stride, then return the constant stride. This matcher
+  // enables some additional optimization since BaseIndexOffset is capable of
+  // decomposing the load ptrs to (add (add Base, Index), Stride) instead of
+  // (add LastPtr, Stride) or (add NextPtr, Stride) that matchForwardStrided and
+  // matchReverseStrided use respectively.
+  auto matchConstantStride = [&DAG, &N](ArrayRef<SDUse> Loads) {
+    // Initialize match constraints based on the first load. Initialize
+    // ConstStride by taking the difference between the offset of the first two
+    // loads.
+    if (Loads.size() < 2)
+      return SDValue();
+    BaseIndexOffset BaseLdBIO =
+        BaseIndexOffset::match(cast<LoadSDNode>(Loads[0]), DAG);
+    BaseIndexOffset LastLdBIO =
+        BaseIndexOffset::match(cast<LoadSDNode>(Loads[1]), DAG);
+    bool AllValidOffset =
+        BaseLdBIO.hasValidOffset() && LastLdBIO.hasValidOffset();
+    if (!AllValidOffset)
+      return SDValue();
+    bool BaseIndexMatch = BaseLdBIO.equalBaseIndex(LastLdBIO, DAG);
+    if (!BaseIndexMatch)
+      return SDValue();
+    int64_t ConstStride = LastLdBIO.getOffset() - BaseLdBIO.getOffset();
+
+    // Check that constraints hold for all subsequent loads and the ConstStride
+    // is the same.
+    for (auto Idx : enumerate(Loads.drop_front(2))) {
+      auto *Ld = cast<LoadSDNode>(Idx.value());
+      BaseIndexOffset BIO = BaseIndexOffset::match(Ld, DAG);
+      AllValidOffset &= BIO.hasValidOffset();
+      if (!AllValidOffset)
+        return SDValue();
+      BaseIndexMatch |= BaseLdBIO.equalBaseIndex(BIO, DAG);
+      // Add 3 to index because the first two loads have been processed before
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topperc wrote:

Is this comment stale? I don't see any `3`

https://github.com/llvm/llvm-project/pull/68726