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Re: Iterating over RTL in Graphite


On Mon, Mar 5, 2012 at 9:52 AM, Arnaldo <arnaldo.cruz@upr.edu> wrote:
> On Mon, Mar 5, 2012 at 10:00 AM, Michael Matz <matz@suse.de> wrote:
>> Hi,
>>
>> On Mon, 5 Mar 2012, Arnaldo wrote:
>>
>>> I couldn't get cfgexpand.c:basic_block expand_gimple_basic_block
>>> (basic_block bb) to work by calling it directly because there is some
>>> preprocessing in gimple_expand_cfg() that has to be done first. ?But
>>> calling gimple_expand_cfg() modifies the CFG and asserts will fail later
>>> on during compilation.
>>>
>>> I think the only way to solve this would be to somehow duplicate the
>>> current cfun structure when entering the part of Graphite I'm extending,
>>> then calling push_cfun(), gimple_expand_cfg(), extracting the BBs with
>>> the RTL and calling pop_cfun() before continuing.
>>
>> Really, you're barking up the wrong tree. ?graphite doesn't work on the
>> RTL IL, it'll work only on gimple. ?expanding is what we call the process
>> of transforming gimple to RTL, and that process destroys gimple. ?Hence
>> you can't do that when still at the gimple side of things as there are
>> still passes to run that run in gimple.
>>
>> Whatever you want to do with graphite, you have to do it at the gimple
>> level.
>>
>>
>> Ciao,
>> Michael.
>
> Richard, Michael,
>
> I have to find a way to generate the RTL because I have profiled an
> instruction set and I need access to these costs during my extension
> to the Graphite pass. ?I planed to add these costs as attributes to
> the RTX patterns in the machine description file and read the back
> from Graphite. ?Gimple seems to be too high-level to associate its
> statements to machine costs.
>
> I know this is not the way GCC was designed but the optimization I'm
> working on needs access to the profile. ?Maybe there's a better way of
> doing this? ?What I'm attempting to do now is to duplicate the current
> cfun so that I can expand and read the RTL attributes and then discard
> this cfun before continuing with the compilation.

You can look in the SSA loop optimizations (tree-ssa-loop-ivopts.c?)
for some code that basically compiles little snippets from GIMPLE to
RTL, computes RTX costs, and then tries to map those back to GIMPLE.
It's very ugly and limited accuracy.

- David


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