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Re: [PING] Move costs, allocno costs, and per-register costs


Vladimir Makarov <vmakarov@redhat.com> writes:
> Richard Sandiford wrote:
>> Hi Vlad,
>>
>> Thanks for the message.
>>
>> Vladimir Makarov <vmakarov@redhat.com> writes:
>>   
>>> Richard Sandiford wrote:
>>>     
>>>> http://gcc.gnu.org/ml/gcc-patches/2008-11/msg00325.html
>>>> Fixes a problem with the costs of union classes.  Needed to fix
>>>> MIPS dspr2-MULT{,U}.c regressions.
>>>>
>>>>   
>>>>       
>>>   Sorry, Richard.  I missed that email.  I had some problems with my 
>>> email (an still messages for some time is  lost).
>>>
>>>   You did a big job comparing  so many targets.  I know about the 
>>> problem.  Recently  I tried to modify IRA (Chaitin-Briggs algorithm) to 
>>> permit allocnos to be of union of cover classes.  It could solve the 
>>> problem you are trying to solve by using smaller cover classes.  It also 
>>> could solve one IRA drawback in comparison with the old RA.  
>>> Memory-memory movements (like arr[i] = arr [j]) are implemented usually 
>>> as  two insns using a temporary  register.  The  temporary register 
>>> could be  of two cover classes (e.g. general purpose or float regs).  
>>>     
>>
>> Yes, this would solve the problem in the case I'm seeing, in that it
>> would be natural to treat GR_REGS and ACC_REGS as separate cover classes.
>> But would you get rid of the "preferred class" code at the same time?
>> If not, I think the underlying problem would still be there.
>>
>>   
> Yes, it addressed that problem if cover classes are uniform.   But as I 
> wrote there are a lot of other serious problems with this approach of 
> using several cover classes for allocnos.   Therefore I am not going to 
> submit this code (but may be continue work on this).

OK.  Hope it goes well, if you do decide to keep working on it.
It sounds like it could be useful if you can iron out the wrinkles.

>> To put it another way: having a single cover class is a fairly important
>> characteristic of the new RA.  On the other hand, I don't think the
>> problem I'm fixing _is_ a fundamental characteristic.  It seems to me
>> that IRA is simply internally inconsistent in its handling of certain
>> costs.  The inconsistency isn't fundamental to the algorithm or the
>> implementation: it seems to be more unintentional than anything.
>>
>> Specifically, one part of IRA says that if A and B have cost X,
>> A union B also has cost X.  But this deduction (which is borrowed
>> from the old RA, correctly IMO) isn't transmitted to the code that
>> sets ALLOCNO_COVER_CLASS_COST to the cost of A union B.
>
> I am agree with this.  Actually the code was taken from regclass.c but I 
> changed it so many times that I lost the count of this.  Your fresh look 
> found the problem.
>
> Anyway I tested the patch on some benchmarks on x86/x86_64.  It looks ok 
> (I found no difference in the results).
>
> You can commit the patch to the trunk.  It looks ok to me.

Thanks, applied.

Richard


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