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

[Craig Topper via llvm-commits]

================
@@ -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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topperc wrote:

Could simplify this loop to

```
bool IsUndef[2] = { true, true};
for (unsigned i = 0; i < Mask.size(); i++)
  IsUndef[i % 2] &= (Mask[i] == -1);
```

but maybe the array is too subtle?

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