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Re: patch pings
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On 04/15/11 10:26, Bernd Schmidt wrote:
> On 04/15/2011 04:18 PM, Jeff Law wrote:
>
>> http://gcc.gnu.org/ml/gcc-patches/2011-03/msg02247.html
>
> I don't know. I sympathize with the goal, but I'm not too happy about
> the structure of this patch. Doesn't this do the scan once for every
> reload in an insn? It seems to me like the loop (or rather, a function
> call to it) should at least be placed at the top of choose_reload_regs
> rather than in allocate_reload_reg.
Perhaps. As you noted in the original discussion we could do something
like save distance to next reuse or something like that in the reload
chain, the use that information to help drive selection of the reload
register.
I poked at it a little and it just didn't seem to be worth the
additional effort.
In reality while we potentially scan for each reload, the cost in
practice is very very small.
>
> In my tests (i686-linux, compiling my standard set of testcases) the
> patch appears to have, in general, very little effect on code quality.
> It might on average be slightly better, but I've also seen several cases
> where we do worse. If we go to so much effort to do scanning of
> subsequent insns it ought to be possible to do better.
That is consistent with my tests. Remember the main purpose was to
improve the predictability, the minor improvement in code generation was
a pleasant side effect. And yes, it sometimes regressed as well, my
overall impression was it was a minor improvement.
I'm not going to push hard for these changes -- I'm comfortable keeping
them on my branch -- I've already spent more time on them than I can
reasonably justify :-)
Jeff
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