[Ffmpeg-devel] [PATH] test if cpu supports CMOV

[Michael Niedermayer]

Hi

On Fri, Oct 13, 2006 at 05:35:13PM +0800, Zuxy Meng wrote:
> Hi,
> 
> 2006/10/13, Guillaume Poirier <gpoirier at mplayerhq.hu>:
> >Hi,
> >The attached patch (hopefully) allows to test for the presence of CMOV
> >instruction, introduced in _real_ i686 CPUs.
> >It's needed for latest and greatest CABAC code, as some cpus that
> >claim to be i686 do not support cmov (such as VIA C3).
> >
> >This patch creates #define HAVE_CMOV 1 for config.h and
> >TARGET_CMOV=yes for config.mak.
> >I don't know if the name is too wise, maybe it would be better to have
> > HAVE_X86_CMOV to specify that were are talking about x86 in
> >particular (other CPU support conditional move too, sadly, PPC doesn't).
> >

this will break runtime cpudetect (yeah i know my crap code already
broke it but thats not supposed to stay that way in svn)

solution:
1. clean configure so that
--cpu  selects the minimum cpu required (-mcpu/-march)
--tune selects the target cpu for which to tune but doent cause possibly
       unsupported instructions to be generated (-mtune)
2. set HAVE_CMOV depening on --cpu


> 
> Why not detect it at run-time in cputest.c, just like what we've done
> for 3DNow!, SSE, etc.?

rejected, considering the small gain and huge increase in complexity 
(=duplicate half of h264.c in the object file and thats just what cabac
would cause, there are a few more cmovs around IIRC)
of course you can do the work benchmark it and submit a patch if you
dissagree, ill apply it if its clean, doesnt unreasonable increase
object or source size and theres no speed loss relative to the current
code

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In the past you could go to a library and read, borrow or copy any book
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