[PATCH] [x86] Add a ZERO_EXTEND_VECTOR_INREG DAG node and use it when widening vector types to be legal and a ZERO_EXTEND node is encountered.

[Chandler Carruth]

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Comment at: lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp:750
@@ +749,3 @@
+  return DAG.getNode(ISD::BITCAST, DL, VT,
+                     DAG.getVectorShuffle(SrcVT, DL, Zero, Src, ShuffleMask));
+}
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Matt Arsenault wrote:
> Benjamin Kramer wrote:
> > Can we get away with creating illegal shuffles here or should we ask the backend if the mask is legal first?
> I'm pretty sure this needs to be checked. I fixed a similar issue in http://reviews.llvm.org/D4320
I believe we can get away with this, but it's a bit dicey I agree.

First, to Matt's point -- we can *definitely* create illegal types here, i don't know whot D4320 is talking about. If you look at SelectionDAGISel::CodeGenAndEmitDAG, you'll see that we re-type-legalize after the vector operation legalization. There are also specific comments around the vector operation legalization that it is expected to at times produce illegal types.

What we can't do here is produce an illegal *vector* type which could then result in an illegal vector operation, which could then result in scalarization with an illegal scalar type. We won't get a chance to fix the scalarized version unless we produce it in this pass of the legalizer.

So the risk in the current approach is that I don't know if the legalizer correctly recurses on the newly created VECTOR_SHUFFLE node to legalize that operation. If it doesn't then we would need to recurse on it within VectorLegalizer. I'll check that.

http://reviews.llvm.org/D4405