[FFmpeg-devel] [PATCH 0/2] Fix strict aliasing violations in AAC and AC3 parsers

Benoit Fouet benoit.fouet
Mon Dec 15 13:59:50 CET 2008


On 12/15/2008 10:35 AM, M?ns Rullg?rd wrote:
> M?ns Rullg?rd <mans at mansr.com> writes:
>
>> "Jason Garrett-Glaser" <darkshikari at gmail.com> writes:
>>
>>>> am i interperting this correctly, that the code is hugely faster with
>>>> -fno-strict-aliasing
>>>> ?
>>>> and iam of course in favor of the faster variant for h264.c
>>> Yes, but it makes absolutely no sense why it would be faster--the
>>> oprofile says even the asm functions are faster which of course makes
>>> no sense at all.
>>>
>>> Others in #ffmpeg-devel have reported small speed increases or
>>> decreases, but nothing like what I posted.
>> Changes in decoding time after adding -fno-strict-aliasing on ARM:
>>
>> H.264   -2.4%
>> MPEG4   +0.14%
>> MPEG2   -2.3%
>> Vorbis  +0.08%
>> Flac    +3.6%
>> MP3     +2.6%
>> AC3     +0.83%
>> DTS     +0.81%
>
> BTW, that was with codesourcery gcc 4.2.1.
>
>> I will test with Jason's sample on Core2 and see if I get anything
>> resembling his results.
>
> Tested on Core2, and I get no such numbers:
>
> vanilla              203.7867 s
> -fno-strict-aliasing 204.2257 s
>
> This was with gcc 4.2.4.
>

same with gcc 4.3.2 on a Core2:
vanilla              245.095s
-fno-strict-aliasing 247.687s

Ben





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