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Re: RFC: IRA patch to reduce lifetimes


On Mon, May 21, 2012 at 9:33 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Wed, Apr 11, 2012 at 7:35 AM, Bernd Schmidt <bernds@codesourcery.com> wrote:
>> On 12/23/2011 05:31 PM, Vladimir Makarov wrote:
>>> On 12/21/2011 09:09 AM, Bernd Schmidt wrote:
>>>> This patch was an experiment to see if we can get the same improvement
>>>> with modifications to IRA, making it more tolerant to over-aggressive
>>>> scheduling. THe idea is that if an instruction sets a register A, and
>>>> all its inputs are live and unmodified for the lifetime of A, then
>>>> moving the instruction downwards towards its first use is going to be
>>>> beneficial from a register pressure point of view.
>>>>
>>>> That alone, however, turns out to be too aggressive, performance drops
>>>> presumably because we undo too many scheduling decisions. So, the patch
>>>> detects such situations, and splits the pseudo; a new pseudo is
>>>> introduced in the original setting instruction, and a copy is added
>>>> before the first use. If the new pseudo does not get a hard register, it
>>>> is removed again and instead the setting instruction is moved to the
>>>> point of the copy.
>>>>
>>>> This gets up to 6.5% on 456.hmmer on the mips target I was working on;
>>>> an embedded benchmark suite also seems to have a (small) geomean
>>>> improvement. On x86_64, I've tested spec2k, where specint is unchanged
>>>> and specfp has a tiny performance regression. All these tests were done
>>>> with a gcc-4.6 based tree.
>>>>
>>>> Thoughts? Currently the patch feels somewhat bolted on to the side of
>>>> IRA, maybe there's a nicer way to achieve this?
>>>>
>>> I think that is an excellent idea. ?I used analogous approach for
>>> splitting pseudo in IRA on loop bounds even if it gets hard register
>>> inside and outside loops. ?The copies are removed if the live ranges
>>> were not spilled in reload.
>>>
>>> I have no problem with this patch. ?It is just a small change in IRA.
>>
>> Sounds like you're happier with the patch than I am, so who am I to argue.
>>
>> Here's an updated version against current trunk, with some cc0 bugfixes
>> that I've since discovered to be necessary. Bootstrapped and tested (but
>> not benchmarked again) on i686-linux. Ok?
>>
>>
>> Bernd
>
> This caused:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53411
>

This also caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53495


-- 
H.J.


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