IRA copy heuristics

Jeff Law law@redhat.com
Fri Sep 5 03:19:00 GMT 2008


Peter Bergner wrote:
> On Thu, 2008-09-04 at 20:28 -0400, David Edelsohn wrote:
>   
>> On Thu, Sep 4, 2008 at 7:39 PM, Vladimir Makarov <vmakarov@redhat.com> wrote:
>>     
>>> Meanwhile I am going to submit your second patch with an added
>>> comment.  The patch permits gcc to generate the same quality code as
>>> before your first patch.
>>>       
>> Why?
>>
>> As Richard said before:
>>
>> "... it changes
>> the heuristics _without any explanation of why this is necessary_.
>> IMO, that's unacceptable for our shiny, new (and generally very nice)
>> register allocator.  And I think it's unacceptable even if it happens
>> to fix a performance regression."
>>     
>
> I have to agree with Richard and David here.  I find it troubling that
> allocation order affects performance by anything other than a small
> amount due to heuristic noise.  It might be in the end there is a 
> valid reason on why Richard's patch has a positive benefit, but until
> we know why, I'd rather wait.
>
> Peter
>   
Agreed.  Let's at least have a reasonable explanation of why this is 
affecting performance so much.  In theory, this kind of change ought to 
be in the noise.

Jeff



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