IRA copy heuristics
Jeff Law
law@redhat.com
Fri Sep 5 03:51:00 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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