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

[Michael Maitland via llvm-commits]

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@@ -13862,13 +13915,21 @@ static SDValue performCONCAT_VECTORSCombine(SDNode *N, SelectionDAG &DAG,
   SDValue Stride = matchForwardStrided(Ptrs);
   if (!Stride) {
     Stride = matchReverseStrided(Ptrs);
-    Reversed = true;
-    // TODO: At this point, we've successfully matched a generalized gather
-    // load.  Maybe we should emit that, and then move the specialized
-    // matchers above and below into a DAG combine?
-    if (!Stride)
-      return SDValue();
+    if (Stride) {
+      Reversed = true;
+      Stride = DAG.getNegative(Stride, DL, Stride->getValueType(0));
+    } else {
+      Stride = matchConstantStride(N->ops());
+      if (Stride) {
+        Reversed = cast<ConstantSDNode>(Stride)->getSExtValue() < 0;
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michaelmaitland wrote:

There is a use of Reversed below.

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