[PATCH] [AArch64] Improve the target-specific cost model to better handle signed division by a power of 2.

[Chad Rosier]

Hi grosbach, t.p.northover, Jiangning, jmolloy,

This patch improve the target-specific cost model to better handle signed division by a power of 2. The immediate result is that this enables the SLP vectorizer to do a better job.

The ADD/CMP/SELECT/SRA is the expansion for the scalar version, which was added in r213758.  I assume this is the cost we should return. The generated vector code still uses the target-independent series of SRA/SRL/ADD/SRA, however.  I may need to clarify the comments, so suggestions are welcome.

We could also investigate using the ADD/CMP/SELECT/SRA combination for vectors, but that would require additional performance evaluation (and I think it's an orthogonal issue).

This should address PR20714.

 Chad

http://reviews.llvm.org/D5469

Files:
  lib/Target/AArch64/AArch64TargetTransformInfo.cpp
  test/Transforms/SLPVectorizer/AArch64/sdiv-pow2.ll
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