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Re: IRA copy heuristics
Vladimir Makarov <vmakarov@redhat.com> writes:
> Richard Sandiford wrote:
>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>
>>> On Tue, Sep 2, 2008 at 8:37 AM, Vladimir Makarov <vmakarov@redhat.com> wrote:
>>>
>>>>> If using DF seems like the Right Thing, we could simply apply both
>>>>> patches, which would give a similar same allocno order to the one
>>>>> we have now. But it seemed better to look a bit deeper first...
>>>>>
>>>>>
>>>> Richard, please apply the both patches. As I wrote above there is no
>>>> SPECFP regression anymore with the patches. They also solves some
>>>> testsuite regressions concerning EH.
>>>>
>>>>
>>> Hi Richard,
>>>
>>> Could you please apply your use DF patch? It fixes EH regressions
>>> as well as 434.zeusmp in SPEC CPU 2006?
>>>
>>
>> As I said yesterday, I'm reluctant to apply the first patch,
>> because without further analysis, there's a danger it's just
>> papering over a deeper problem.
>>
>> It's interesting that it fixes EH regressions for you too though.
>> That was what the patch was originally meant to do, but I thought
>> I'd only seem the regressions I was fixing on MIPS, not on x86_64.
>> Which target did you see them on?
>>
>>
> Richard, please apply the both patches because I know that they will not
> introduce performance regression. I'll check what happens to SPEC2000
> without the second patch (allocno ordering) later. If it is ok we could
> remove the second patch.
Sorry Vlad, my reply to HJ crossed with your message.
I'm really against applying the ALLOCNO_COMPARE patch. Unlike the DF
patch, It isn't needed to fix a correctness regression. And 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.
Experience suggests that if we apply this patch now, no-one will
ever look at the deeper problem, and we'll be lumped with magic goo
that no-one really understands. After all, this issue is about three
months old now. (And that certainly isn't meant to be a criticism.
We're all busy people. But it's because we're busy people that I'm
so reluctant to apply the patch.)
But as I said to HJ, I'm happy to apply the DF patch in isolation,
as long as we accept that the benefit of fixing a correctness
regression outweighs the potential performance regression.
Richard