Re: Iterating over RTL in Graphite

[Arnaldo <arnaldo dot cruz at upr dot edu>]

On Sat, Feb 18, 2012 at 6:46 PM, Arnaldo <arnaldo.cruz@upr.edu> wrote:
> On Fri, Feb 17, 2012 at 10:15 PM, Tobias Grosser <tobias@grosser.es> wrote:
>> On 02/17/2012 08:34 PM, David Malcolm wrote:
>>>
>>> On Thu, 2012-02-16 at 19:17 -0400, Arnaldo wrote:
>>>>
>>>> Hello everyone,
>>>>
>>>> I'm working on an extension to the Graphite pass of GCC 4.4.0. ?My
>>>> intention is to associate costs to RTL instructions by adding them as
>>>> RTX attributes to a machine description file, and to read them back
>>>> during the Graphite pass by iterating through each basic block.
>>>>
>>>> Is the RTL available during this optimization pass? I'm not sure this
>>>> is the case as I get a segfault when trying to iterate over the RTL
>>>> with the code below ("internal compiler error: Segmentation fault"). I
>>>> don't need the fully resolved RTL, just to be able to read the
>>>> attribute given an RTL instruction.
>>>>
>>>> I've tried debugging the compiler with gdb but it can't find the
>>>> debugging symbols even though they're there. ?I'll keep trying to get
>>>> gdb to work but any leads on reading these attributes from within
>>>> Graphite is greatly appreciated.
>>>
>>> I don't know about GCC 4.4, but a while back I wrote a script using my
>>> GCC Python plugin to draw a "subway map" of GCC 4.6's passes:
>>> http://gcc.gnu.org/ml/gcc/2011-07/msg00157.html
>>> which you can see here:
>>> http://gcc-python-plugin.readthedocs.org/en/latest/tables-of-passes.html
>>>
>>> If I reading things correctly, the graphite passes happen whilst the
>>> code is still in gimple form: the blocks are converted to RTL form in
>>> the "expand" pass, which happens about 20 or so passes later.
>>>
>>> Caveat: I'm not familiar with the insides of the graphite, and am
>>> relatively new to gcc's insides, so I could be wrong; also the script
>>> relies on the pass flags, and they're not necessarily correct either...
>>
>>
>> Yes, graphite works on GIMPLE. I believe I have never seen RTL when working
>> on graphite, so I doubt it is easily available. (Maybe it is, but it is
>> definitely not used within graphite).
>>
>> Cheers
>> Tobi
>>
>
> Thanks David and Tobias, that helped.
>
> I started looking into the expand pass (cfgexpand.c), there's an
> interesting function there:
> static basic_block expand_gimple_basic_block (basic_block bb)
>
> When I get a change I'll make it 'public' and see if I can call it
> from graphite.c
>
> -Arnaldo

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.

-Arnaldo