[llvm] [RISCV] Improve lowering of spread(2) shuffles (PR #118658)

[Philip Reames via llvm-commits]

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@@ -5331,17 +5331,36 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
     // Extract the halves of the vectors.
     MVT HalfVT = VT.getHalfNumVectorElementsVT();
 
+    // Recognize if one half is actually undef; the matching above will
+    // otherwise reuse the even stream for the undef one.  This improves
+    // spread(2) shuffles.
+    bool EvenIsUndef = true, OddIsUndef = true;
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preames wrote:

Seems reasonable to me, updating.

https://github.com/llvm/llvm-project/pull/118658