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Re: [patch] tree-ssa-phiopt.c: Update a comment about the pass
On Wed, 2005-04-20 at 14:18 -0400, Kazu Hirata wrote:
> Hi Jeff,
>
> > Whoops. Not PHI-OPT, but the redundant phi removal pass. From
> > a quick looksie, it appears that pass was removed by Diego. I
> > haven't looked, but I hope Diego put in a copy propagation pass
> > in its place since the whole purpose of the redundant phi removal
> > pass was to kill these kinds of degenerate phis.
>
> Diego's copy prop does not remove degenerate PHI nodes, but it makes
> them unused. DCE does the actual removal.
And it appears that we do not have cprop & DCE scheduled immediately
after phi-opt, which means this stuff is lying around a long longer
than is necessary (and a lot longer than it used to be since we used
to have redundant phi removal scheduled soon after phi-opt).
I'll note this would be another excellent case where having an API
into the cprop code which allowed a pass which uncovers const/copy
propagation opportunity to propagate the values to their uses would
be helpful and probably significantly lower overhead than having
to schedule cprop & dce passes.
Jeff