[llvm-dev] llvm (the middle-end) is getting slower, December edition

Mikhail Zolotukhin via llvm-dev
2016-12-19T23:55:06Z

> On Dec 17, 2016, at 7:48 PM, Davide Italiano via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
>
>
>
> On Dec 17, 2016 7:41 PM, "Sean Silva" <chisophugis at
gmail.com <mailto:chisophugis at gmail.com>> wrote:
>
>
> On Sat, Dec 17, 2016 at 6:32 PM, Davide Italiano via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> On Sat, Dec 17, 2016 at 1:35 PM, Davide Italiano <davide at freebsd.org
<mailto:davide at freebsd.org>> wrote:
> [...]
>
> > I don't have an infrastructure to measure the runtime performance
> > benefits/regression of clang, but I have for `game7`.
> > I wasn't able to notice any fundamental speedup (at least, not
> > something that justifies a 2x build-time).
> >
>
> Just to provide numbers (using Sean's Mathematica template, thanks),
> here's a plot of the CDF of the frames in a particular level/scene.
> The curves pretty much match, and the picture in the middle shows a
> relative difference of 0.4% between Jun and Dec (which could be very
> possibly be in the noise).
>
> With 5-10 runs per binary you should be able to reliably measure down to
0.5% difference on game7 (50 usec difference per frame). With just one run per
binary like you have here I would not trust a 0.4% difference.
>
> Yeah, I agree. This was mostly a sanity check to understand if there was a
significant improvement at runtime. I ran the same test 7 times in the last our
but the difference is always in the noise, FWIW.
FWIW, opt has an option '-run-twice', which might be helpful if compile
time of a particular unit is too small (it would be nice to change it to
-run-n-times though to make it more flexible).

Michael
>
> -- Sean Silva
>
> On the same scene, the difference between -O3 and -flto is 12%, FWIW.
> So it seems that at least for this particular case all the compile
> time didn't buy any runtime improvement.
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
>
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