Re: Some thoughts and quetsions about the data flow infrastracture

[Ian Lance Taylor <iant at google dot com>]

"Vladimir N. Makarov" <vmakarov@redhat.com> writes:

> I know some work is being done on incremental df analysis.  It could
> decrease time for rescanning RTL between passes.  Let us hope on this.

My understanding is that on dataflow-branch the DF information is now
fully incremental.


I don't really grasp where you are headed with this criticism.
Criticism can be a good thing.  But I think it would help me, at
least, if you were more clear about what you see as the alternative.
Moving all the RTL passes into tree-ssa might be a good idea.  But it
would be orders of magnitude more work than the dataflow work has
required, so it is not comparable.

We could contrain the dataflow work to be a 100% improvement on all
platforms before it can be committed.  But that would be extremely
difficult, because they would continually be catching up to problems
introduced in mainline.  For example, I slowed them down by a few days
as they fit my recent lower-subreg.c patch into the DF framework.  So
I think that imposing such a requirement would be unwise.  I believe
it would be tantamount to saying that we can never make a major
infrastructure change.  For tree-ssa (admittedly a much bigger change)
we accepted slowdowns when it was committed because we believed that
the end result would be better.

I think that you should channel your concern into looking at the code
on dataflow-branch and seeing how it can be improved, or determining
that it is hopeless.  I don't think this is a discussion which can be
usefully carried on in the abstract.

Ian