[PATCH] Make SLP vectorizer consider the cost that vectorized instruction cannot use memory operand as destination on X86

[Wei Mi]

Hi nadav, aschwaighofer,

This is the patch to fix the performance problem reported in https://llvm.org/bugs/show_bug.cgi?id=23510.

Many X86 scalar instructions support using memory operand as destination but most vector instructions do not support it. In SLP cost evaluation, 

scalar version:
  t1 = load [mem];
  t1 = shift 5, t1          
  store t1, [mem]
  ...
  t4 = load [mem4];
  t4 = shift 5, t4          
  store t4, [mem4]

slp vectorized version:
  v1 = vload [mem];
  v1 = vshift 5, v1
  store v1, [mem]

SLP cost model thinks there will be 12 - 3 = 9 insns savings. But scalar version can be converted to the following form on x86 while vectorized instruction cannot:

[mem1] = shift 5, [mem1]
[mem2] = shift 5, [mem2]
[mem3] = shift 5, [mem3]
[mem4] = shift 5, [mem4]

We add the extra cost VL * 2 to the SLP cost evaluation to handle such case (VL is the vector length).

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D10352

Files:
  include/llvm/Analysis/TargetTransformInfo.h
  include/llvm/Analysis/TargetTransformInfoImpl.h
  lib/Analysis/TargetTransformInfo.cpp
  lib/Target/X86/X86TargetTransformInfo.cpp
  lib/Target/X86/X86TargetTransformInfo.h
  lib/Transforms/Vectorize/SLPVectorizer.cpp
  test/Transforms/SLPVectorizer/X86/pr23510.ll

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