[FFmpeg-devel] [PATCH] Further optimization of base64 decode using AV_WB32.

[Reimar Döffinger]

On Sat, Jan 21, 2012 at 06:13:19PM +0100, Reimar Döffinger wrote:
> On Sat, Jan 21, 2012 at 05:56:32PM +0100, Michael Niedermayer wrote:
> > On Sat, Jan 21, 2012 at 05:52:27PM +0100, Reimar Döffinger wrote:
> > > This is somewhat questionable.
> > > The biggest issue is that av_bswap32 is not replaced
> > > with our asm version on gcc 4.5 or newer.
> > > This causes gcc to generate horrible code that is slower
> > > than the unoptimized variant.
> > > Old:                                  248852 decicycles
> > > New with gcc's attempt at av_bswap32: 256576 decicycles
> > > New with our bswap32:                 200260 decicycles
> > [...]
> > > diff --git a/libavutil/x86/bswap.h b/libavutil/x86/bswap.h
> > > index 52ffb4d..aa39d97 100644
> > > --- a/libavutil/x86/bswap.h
> > > +++ b/libavutil/x86/bswap.h
> > > @@ -37,7 +37,7 @@ static av_always_inline av_const unsigned av_bswap16(unsigned x)
> > >  }
> > >  #endif /* !AV_GCC_VERSION_AT_LEAST(4,1) */
> > >  
> > > -#if !AV_GCC_VERSION_AT_LEAST(4,5)
> > > +#if 1 || !AV_GCC_VERSION_AT_LEAST(4,5)
> > >  #define av_bswap32 av_bswap32
> > >  static av_always_inline av_const uint32_t av_bswap32(uint32_t x)
> > >  {
> > 
> > also make sure -cpu/arch/tune is set so gcc is allowed to use bswap
> > (its 486+) so not possible for gcc to use on strict x86
> 
> It is a x86_64 build, so I'd hope that gcc will not try to "optimize"
> of 486 on that...

gcc version is actually 4.6.2 and it fails to use the bswap instruction
regardless whether I use no extra options, -march=native, -m32, -m32
-march=native.
In all cases the code without our inline bswap is significantly slower
(ca. 20%).
I have no idea where the claim that gcc would recognize the bswap comes
from (hm, I haven't tested if the << 8 confuses it though, will now).