[MPlayer-dev-eng] Re: [SURVEY] change default CPU target when compiled with runtime CPU detection

[Guillaume POIRIER]

Hi,

On 11/12/05, Guillaume POIRIER <poirierg at gmail.com> wrote:
> Hi,
> I've committed a patch to configure script to support VIA/Cyrix C2
> CPU. C2 is really an El Cheapo cpu as it doesn't feature any buzz
> words like SSE or 3dnow! (shame!).
> Anyway, in order to support this little baby, we'd need to set default
> "mcpu" from i686 to i586.
>
> Here's the corresponding code in configure script (note the i686 is
> _*probably*_ the most common CPU):
>
>   else # if test "$_runtime_cpudetection" = no
>     # i686 is probably the most common CPU - optimize for it
>     _mcpu="$cpuopt=i686"
>     # at least i486 required, for bswap instruction
>     _march="-march=i486"

According to GCC's man page, changing mcpu to i586 just changes the
scheduling of instructions, when it's -march that tells what
instructions are allowed, so the proper solution is not there.

The guy who reported the problem mentioned that changing this like:

        #if __CPU__ >= 686 && !defined(RUNTIME_CPUDETECT)

to #if 0 was fixing the problem for him. Now I'm wondering how _CPU_
is set. I it looking at a variable set by the configure script (which
I doubt very much)? What to do?

Guillaume
--
Reading doesn't hurt, really!
  -- Dominik 'Rathann' Mierzejewski