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Re: [graphite] Weekly phone call notes
On Thu, Apr 30, 2009 at 12:06 AM, Tobias Grosser
<grosser@fim.uni-passau.de> wrote:
> On Wed, 2009-04-29 at 23:57 +0200, Richard Guenther wrote:
>> On Wed, Apr 29, 2009 at 11:34 PM, Tobias Grosser
>> <grosser@fim.uni-passau.de> wrote:
>> > Hi gcc developers, hi graphities
>> >
>> > here are some notes from our weekly phone call. Unfortunately I missed
>> > to send out the notes from the last two phone calls, but I hope to get
>> > them out more regulary. Believe in me! ;-)
>> >
>> > http://gcc.gnu.org/wiki/Graphite_Phone_Call/2009_04_29
>> >
>> > Attendees: Li, Jan, Konrad, Sebastian, Tobias, David, Christophe
>> >
>> > ? ? ?* Reductions: Diego OK with going out of SSA.
>>
>> You will loose all points-to information. ?I think going out of SSA is
>> a very bad idea.
>
> It makes live a lot easier and it is just for reductions. ;-) But you
> are right we should get SSA back.
I don't see how SSA form makes anything more complicated ;)
> The point was more if it is the right moment to try to regenerate SSA as
> the polyhedral model is actually not single assignment. Or actually the
> other way around. More single assignment than SSA.
Certainly different single assignment. But for parameters it should be a
non-problem, induction variables are implicit anyway and yes - reductions
may be interesting - but the only thing you need to do is move the
reduction variable "out of SSA" (or rather, by the fact you recognize
a reduction you have done enough work already - code generation
simply needs to create a PHI node for the reduction).
> We postponed this complication or silently hoped that a later pass might
> clean up our mess. Actually Sebastian already looked at the output and
> there seems to be a later gcc pass, that cleans it up and reintroduces
> SSA. But we have to investigate this further or at least document it
> correctly. The final goal has to be to get an optimized loop, that is
> still in SSA. The way we take is not completely clear yet.
Well, the challenge is to retain the per SSA name information across
Graphite. At some point we need to stop re-computing points-to
information because we cannot do so with retaining IPA results.
Richard.