[LLVMdev] Performance regression on ARM

Adam Nemet anemet at apple.com
Thu Oct 16 21:56:53 PDT 2014

> On Oct 16, 2014, at 3:04 AM, Renato Golin <renato.golin at linaro.org> wrote:
> On 16 October 2014 09:34, Hal Finkel <hfinkel at anl.gov> wrote:
>> Interesting. Looks like the problem is in (219545, 219569].
> Yes.

Chandler’s complex arithmetic changes are also in the range: r219557 in clang.  We saw it change the code in mandel-2 significantly.


>> and given the magnitude of the change, I think that the trip-count changes are more likely.
> Good point.
>> Of course, all of these things are bug fixes :( -- So how do we follow-up on this?
> Correctness before performance. Always.
> I'll create a bug on this pointing to the delta for someone to
> investigate. It doesn't have to be me, or the committer, the idea is
> that this is not high priority. At least, not for now.
> Once we have a way to track this more consistently, I think a good
> approach is to be pragmatic. So, something along the lines of working
> with the implementer, trying to understand the reason of the
> regression. It could be a bad implementation or just a target-specific
> reason for the regression. Depending on the importance of the
> regression and of the patch, I'd consider turning it off for ARM (for
> example, PR18996) and later investigating why.
> I'd only consider reverting the patch in extreme circumstances, for
> example, if we're close to a release AND the regression is big AND the
> patch is a new feature, etc. I believe that's what you were concerned
> about. :)
> cheers,
> --renato
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