[PATCH] D50248: [GuardWidening] Insert fictive guards to loop preheader to increase chances of widening

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.
mkazantsev added reviewers: fedor.sergeev, apilipenko, sanjoy, reames.

Sometimes we have something like:

  preheader:
    guard(%c1)[];
    %c2 =<some cond>;
    br i1 %loop
  
  loop:
    ...
    guard(%c2)
    ...

Despite `%c2` being loop invariant, we cannot widen the 2nd guard into
the 1st one in case if `%c2` is not available at the point of 1st guard. To
solve that situation, we can insert the fictive guard with `true` condition
like this:

  preheader:
    guard(%c1)[];
    %c2 =<some cond>;
    guard(true)[];
    br i1 %loop
  
  loop:
    ...
    guard(%c2)
    ...

And then widen the condition of guard in loop into this fictive guard. In case
if it didn't help, we can just safely delete the fictive guard we've just inserted.


https://reviews.llvm.org/D50248

Files:
  lib/Transforms/Scalar/GuardWidening.cpp
  test/Transforms/GuardWidening/fictive-guards.ll

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