[PATCH] Add support to recognize non SIMD kind of parallelism in SLPVectorizer

[Karthik Bhat]

Hi Arnold,
Thanks for the pointers. Updated the cost model for SK_Alternate Shuffles.
The cost model is as follows-
1) In BasicTTI created a function getAltShuffleOverhead. 
As you mentioned the conservative cost of shuffle is the cost of extracting the element from an index from vector + cost of inserting the element into the result vector for all elements in result vector.
Since we will be alternatively picking elements from the 2 vectors which are of same type(i.e. element 0 of 1st vector, element 1 of 2nd vector, element 3 of 1st vector and so on..) the functions just runs a loop to calculate the cost of extracting elements from each index and adds it up to the cost of inserting the element at that index.

We have overidden the getShuffleCost for X86 and ARM. Created 2 tables NEONAltShuffleTable and X86AltShuffleTable with more accurate cost of shuffle. The cost here represents the number of instructions required to generate the shuffled vector.

Please if you could let me know your inputs on this.

For <8 x i16> as well we should be generating the correct mask as the logic to create mask is generic. The logic to create mask is we take the vector length(8 in this case) and run a loop from 0 to length alternatively selecting loopindex(i) and lengthofVector+i. So in this case as we will be generating sequence (0,8+1,2,8+3,4,8+5,6,8+7) i.e. <0,9, 2, 11,4,13, 6, 15>.

I tried to test it on a sample test but it exits before entering buildTree in vectorizeStoreChain as VF>ChainLen. I will try to come up with a working test to check <8 x i16> and <16 x i8> but i feel we should be generating correct mask in both these cases as per our logic.

Thanks once again for your time and inputs.
Regards
Karthik Bhat

http://reviews.llvm.org/D4015

Files:
  include/llvm/Analysis/TargetTransformInfo.h
  lib/CodeGen/BasicTargetTransformInfo.cpp
  lib/Target/ARM/ARMTargetTransformInfo.cpp
  lib/Target/X86/X86TargetTransformInfo.cpp
  lib/Transforms/Vectorize/SLPVectorizer.cpp
  test/Transforms/SLPVectorizer/X86/addsub.ll
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