[PATCH] [CodeGen] Combine shuffle from concat+bitcast scalar to avoid the smaller vector type.

[Ahmed Bougacha]

Hi spatel, qcolombet,

Code that deals with small vectors but worries about performance (is hardware-aware) often passes the vectors around (on some targets as scalars, thanks to type coercion), but immediately shuffles them into a native-width vector, to do proper operations that we can't reasonably do a horrible job at.  Promoting small vector types leads to some really nasty stuff, and people actively try to avoid that now ;)

Anyway, this means a pretty common pattern is this:

      (vector_shuffle (v8i8 concat_vectors (v2i8 bitcast (i16)), undef..), M)

which we'll usually end up scalarizing.  Instead, we can turn it into:

      (vector_shuffle (v8i8 bitcast (v4i16 scalar_to_vector (i16))), M)

which lets us deal with native-width types all the time (this should be the foremost canonicalization goal whenever dealing with vectors IMHO, but I digress).

http://reviews.llvm.org/D8884

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/AArch64/concat_vectors-combines.ll

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