[FFmpeg-devel] [PATCH] configure: enable runtime cpu detect by default.

Michael Niedermayer michaelni at gmx.at
Sun Sep 23 15:49:28 CEST 2012

On Sun, Sep 23, 2012 at 10:32:33AM +0800, Xidorn Quan wrote:
> On Sun, Sep 23, 2012 at 9:57 AM, Michael Niedermayer <michaelni at gmx.at>wrote:
> > On Fri, Sep 21, 2012 at 06:15:39PM +0200, Michael Niedermayer wrote:
> > > with the exception of libpostproc runtime_cpudetect = no is not
> > > supported thus having no as default really is quite odd, it results in
> > > a libpostproc where HAVE_MMX* have very different meaning from the
> > > rest of ffmpeg and it breaks any x86 cpu that doesnt support mmx2
> > > because mmx2 is hardcoded as a result
> > >
> > > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> >
> > if there are no objections i will apply this tomorrow, it fixes
> > libpostproc on pre MMX2 cpus
> >
> Most people should prefer to compile for their own cpu which is
> hardly changed. Only those who distribute binary packages would like
> to enable runtime cpu detection.

no strong disagreement here in principle but

> So I think it is better to disable
> it by default, and notice the distributors to enable it themselves.

The problem is currently ffmpeg does not support "no cpu detection
at runtime", only libpostproc supports this mode
so when you disable runtime detect (as its default now) it really
disables it just for 2% of the code, 98% still does runtime_cpudetect

The second problem is that the build system has no support to detect
the cpu features of your cpu at all so when runtime_cpudetect is
disabled it will not compile libpostproc with a feature set matching
your cpu but it will compile with a random feature set. And this
breaks on pre MMX2 cpus as described previously.
or to say it differently a ./configure && make will not create a
working libpostproc on pre MMX2 cpus, this patch fixes this

Ill apply it later today or tomorrow unless i hear objections


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Good people do not need laws to tell them to act responsibly, while bad
people will find a way around the laws. -- Plato
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