[Ffmpeg-cvslog] r6729 - trunk/configure

[Guillaume POIRIER]

Hi,

On 10/18/06, M?ns Rullg?rd <mru at inprovide.com> wrote:
>
> Guillaume POIRIER said:
> > Hi,
> >
> > On 10/18/06, M?ns Rullg?rd <mru at inprovide.com> wrote:
> >>
> >> gpoirier said:
> >> > Author: gpoirier
> >> > Date: Wed Oct 18 14:33:42 2006
> >> > New Revision: 6729
> >> >
> >> > Modified:
> >> >    trunk/configure
> >> >
> >> > Log:
> >> > rename --tune to --cpu and make --cpu pass the apropriate -march=xx option to
> >> the
> >> > compiler.
> >> > Note that previous implementation (--tune) wasn't consistent with regards to
> >> > setting -march/-mcpu/-mtune whereas current --cpu now is
> >>
> >> This is wrong.  GCC is inconsistent between architectures about the names of
> >> those
> >> flags.  This change is likely to break builds on all but x86 machines.
> >
> > Let me see. As far as I can see, x86 is the only arch for which -march
> > is documented as setting -mpcu or -mtune too.
> >
> > On other arches, it's not documented as such (but maybe the behaviour
> > is the same, who knows?)
> >
> > M?ns, if you haven't already, please read the patch and tell me if it
> > does what it's supposed to:
> >   --cpu sets the base intruction emitting scheme (-march);
> >   --tune (the one that is to come when I write it, not the old --tune)
> > sets the scheduling heuristic and other things that just make it run
> > better on the selected target. This is supposed to be controlled by
> > -mcpu or -mtune depending on GCC version (prior 3.4 uses -mcpu, the
> > newer uses -mtune).
> >
> > So as far as I can see, unless you give me further details in what is
> > wrong with the way --cpu is implemented, my implementation seems
> > correct to me...
>
> What you say is true for x86, but not most other arches.  On some -mcpu
> does more or less what -march does on x86, and -march doesn't exist (error).
> Some other arches have both -mcpu and -march, but with different meanings
> than x86.  The same goes for -mtune.  Some arches have it, others don't, and
> where it does exist, it means different things.  The GCC info manual has
> all the details.

Ok, thanks for the enlightenment. I guess I'm off to a lot of headache
since there are not only plenty of arches, but also plenty of
different gcc versions..

Guillaume
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