[PATCH] D75348: [DAGCombiner] recognize shuffle (shuffle X, Mask0), Mask --> splat X

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

Sounds good to me modulo legality check.



================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:19264
+/// Combine shuffle of shuffle of the form:
+/// shuf (shuf0 V, undef, MaskOp0), undef, Mask --> splat V
+static SDValue formSplatFromShuffles(ShuffleVectorSDNode *Shuf,
----------------
I'd suggest `OuterShuf` and `InnerShuf`


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:19277
+  assert(NumElts == MaskOp0.size() && "Mask length mismatch");
+  SmallVector<int, 32> NewMask(NumElts, -1);
+  int SplatIndex = -1;
----------------
`CombinedMask`? Not sure.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:19282-19286
+    if (MaskElt == -1 || MaskOp0[MaskElt] == -1)
+      continue;
+
+    // Peek through the shuffle masks to get the underlying source element.
+    int MaskOp0Elt = MaskOp0[MaskElt];
----------------
Might it be more obvious to do
```
    int MaskElt = Mask[i];
    if (MaskElt == -1)
      continue;
    // Peek through the shuffle masks to get the underlying source element.
    int MaskOp0Elt = MaskOp0[MaskElt];
    if (MaskOp0Elt == -1)
      continue;

```
?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D75348/new/

https://reviews.llvm.org/D75348