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The pass was not doing anything incorrect, but
it missed some opportunities to optimize. On
a copy heavy example I observed that, in the
output of the pass a phi of the following shape
remained:
%a =w phi @A %c, @B %a
Originally the phi for %a was:
%a =w phi @A %b, @B %a
Since %b was discovered a copy of %c, %a should
have been eliminated and replaced with %c.
I think the problem is that knowledge on the
first argument of the phi %a changes as the
algorithm progresses, a more detailed walk-
through follows.
In the first round of the algoritm, %a is
discovered to be a copy of its first argument
%b.
phi(%b, %a) -> %b
In the second round, %a is computed as the phi
of %c (since the first argument changed) and %b
(the result of the first iteration), in our
lattice, the glb of those two is bottom.
phi(%c, %b) -> %a (Bottom)
Finally, there is a third round in which we
compute %a as the phi of %a and %c, which again,
gives bottom.
phi(%c, %a) -> %a (Bottom)
The bug is not tied to a phi of a copy, for
example, if the first argument is speculated
to be a copy of 0 and then, this knowledge is
retracted; we will compute in sequence:
phi(0, %a) -> 0
phi(%b, 0) -> %a (Bottom)
phi(%b, %a) -> %a (Bottom)
The way I fixed this is by ignoring arguments
of the phi that were discovered to be copies of
the phi node itself. This will make the last
rounds above do the correct thing.
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