[PATCH] D25682: [LoopUnroll] Keep the loop test only on the first iteration of max-or-zero loops

[John Brawn via llvm-commits]

john.brawn created this revision.
john.brawn added reviewers: sanjoy, christof, haicheng, mzolotukhin.
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When we have a loop with a known upper bound on the number of iterations, and furthermore know that either the number of iterations will be either exactly that upper bound or zero, then we can fully unroll up to that upper bound keeping only the first loop test to check for the zero iteration case.

Most of the work here is in plumbing this 'max-or-zero' information from the part of scalar evolution where it's detected through to loop unrolling. I've also gone for the safe default of 'false' everywhere but howManyLessThans which could probably be improved.


Repository:
  rL LLVM

https://reviews.llvm.org/D25682

Files:
  include/llvm/Analysis/ScalarEvolution.h
  include/llvm/Transforms/Utils/UnrollLoop.h
  lib/Analysis/ScalarEvolution.cpp
  lib/Transforms/Scalar/LoopUnrollPass.cpp
  lib/Transforms/Utils/LoopUnroll.cpp
  test/Analysis/ScalarEvolution/trip-count13.ll
  test/Analysis/ScalarEvolution/trip-count14.ll
  test/Transforms/LoopUnroll/full-unroll-keep-first-exit.ll

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