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Re: RFC: IRA patch to reduce lifetimes


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

Attachment: ira-moveable.diff
Description: Text document


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