[FFmpeg-devel] [RFC] Disable gcc auto-vectorisation

Måns Rullgård mans
Wed Jul 15 11:33:38 CEST 2009

M?ns Rullg?rd <mans at mansr.com> writes:

> Jason Garrett-Glaser <darkshikari at gmail.com> writes:
>> 2009/7/13 M?ns Rullg?rd <mans at mansr.com>:
>>> Since gcc 4.3, the auto-vectoriser (-ftree-vectorize) has been enabled
>>> at -O3 optimisation level. ?This vectoriser is, mildly put, useless.
>>> When it doesn't produce outright broken code, it is a gamble whether
>>> the result is any faster whatsoever, and we have seen cases where it
>>> introduced a significant slowdown.
>>> My proposal is to disable this misfeature by adding
>>> -fno-tree-vectorize to the compiler flags when using gcc.
>>> Does anyone have a good reason for not doing this?
>> The only reason I can imagine to not do this is that it makes GCC look
>> less bad on default ffmpeg builds, giving us less to make fun of.
> Uoti claimed on IRC he saw a slowdown when adding -fno-tree-vectorize
> on x86.  I'd like some more details, and hard figures, on this.
> Meanwhile, I suggest adding this flag for all non-x86 gcc builds.  I
> have not seen any significant performance benefit, only random
> fluctuations of 1% or less, when testing this on PPC and ARM/NEON.  I
> don't know of any other architectures where gcc even attempts to
> support SIMD instructions.

If nobody complains, I'll do this today.

M?ns Rullg?rd
mans at mansr.com

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