[PATCH] Add a Scalarize pass

[Pekka Jääskeläinen]

  Hi,

  It doesn't have to be a "domain specific language" for one to want to use vector datatypes/intrinsics.
  One point of view is that if the programmer has used them, he/she has done a target-specific optimization, assuming it is profitable to use those -- and it might be so for the original target at hand. Or the vector datatypes might be natural for the algorithm/problem at hand but not map optimally to the platform at hand.

  To port the performance of that code to a machine with different SIMD or other fine grained parallel hardware (VLIW/superscalar), it might be profitable to first undo the user's vector code to map the code better to the parallel resources of the target at hand. In that case, the idea is to scalarize the explicit vector intrinsics and then revectorize with the target specific properties.

  In the current set of LLVM passes, I think this could be done selectively from the vectorizers: if they see that the programmer's vectorization decisions render the code less vectorizable as a whole (e.g. if the loop vectorizer cannot horizontally vectorize the loops just because of it, or if the local vectorizers could find better pairings from scalar code), they can try to scalarize the code first and then vectorize more efficiently for the target at hand.

  I agree this should not be on by default. I see it most beneficial when done selectively based on both the code and the target at hand. But it should not be left as a "backend pass" either because it might help the vectorizers.

  BR,
  Pekka

http://llvm-reviews.chandlerc.com/D2112