[PATCH] D14296: [LV][LAA] Add a layer over SCEV to apply run-time checked knowledge on SCEV expressions

[silviu.baranga@arm.com via llvm-commits]

sbaranga created this revision.
sbaranga added reviewers: mzolotukhin, anemet.
sbaranga added a subscriber: llvm-commits.
Herald added a subscriber: sanjoy.

This change creates a layer over ScalarEvolution for LAA and LV, and centralizes the
usage of SCEV predicates. The SCEVPredicatedLayer takes the statically deduced knowledge
by ScalarEvolution and applies the knowledge from the SCEV predicates. The end goal is
that both LAA and LV should use this interface everywhere.

This also solves a problem involving the result of SCEV expression rewritting when
the predicate changes. Suppose we have the expression (sext {a,+,b}) and two predicates
  P1: {a,+,b} has nsw
  P2: b = 1.

Applying P1 and then P2 gives us {a,+,1}, while applying P2 and the P1 gives us
sext({a,+,1}) (the AddRec expression was changed by P2 so P1 no longer applies).
The SCEVPredicatedLayer maintains the order of transformations by feeding back
the results of previous transformations into new transformations, and therefore
avoiding this issue.

The SCEVPredicatedLayer maintains a cache to remember the results of previous
SCEV rewritting results. This also has the benefit of reducing the overall number
of expression rewrites.

http://reviews.llvm.org/D14296

Files:
  include/llvm/Analysis/LoopAccessAnalysis.h
  include/llvm/Analysis/ScalarEvolution.h
  include/llvm/Transforms/Utils/LoopUtils.h
  lib/Analysis/LoopAccessAnalysis.cpp
  lib/Transforms/Utils/LoopUtils.cpp
  lib/Transforms/Vectorize/LoopVectorize.cpp

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