[PATCH] D11764: [DAGCombiner] Attempt to mask vectors before zero extension instead of after.

[Simon Pilgrim]

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RKSimon added reviewers: chandlerc, uweigand, andreadb, spatel.
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For cases where we TRUNCATE and then ZERO_EXTEND to a larger size (often from vector legalization), see if we can mask the source data and then ZERO_EXTEND (instead of after a ANY_EXTEND). This can help avoid having to generate a larger mask, and possibly applying it to several sub-vectors.

(zext (truncate x)) -> (zext (and(x, m))

This is the first of a number of minor patches to help improve the conversion of byte masks to clear mask shuffles.

Ulrich - this includes some test changes in SystemZ, please can you check them? A quick search inducates they make sense but I want to make sure.

Repository:
  rL LLVM

http://reviews.llvm.org/D11764

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/SystemZ/insert-05.ll
  test/CodeGen/SystemZ/int-conv-04.ll
  test/CodeGen/SystemZ/int-conv-08.ll
  test/CodeGen/X86/avx2-conversions.ll
  test/CodeGen/X86/vec_cast2.ll
  test/CodeGen/X86/vector-zext.ll

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