[FFmpeg-devel] [RFC] clobbers for XMM registers

[Michael Niedermayer]

On Fri, Oct 01, 2010 at 12:42:33AM +0100, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Thu, Sep 30, 2010 at 04:02:00PM -0300, Ramiro Polla wrote:
> >> 2010/9/30 M?ns Rullg?rd <mans at mansr.com>:
> >> > Ramiro Polla <ramiro.polla at gmail.com> writes:
> >> >> 2010/9/30 M?ns Rullg?rd <mans at mansr.com>:
> >> >>> Ramiro Polla <ramiro.polla at gmail.com> writes:
> >> >>>> What about
> >> >>>> #if HAVE_XMM_CLOBBERS
> >> >>>> # ? ?define XMM_CLOBBERS(a, ...) __VA_ARGS__
> >> >>>> #else
> >> >>>> # ? ?define XMM_CLOBBERS(a, ...) a
> >> >>>> #endif
> >> >>>>
> >> >>>> to be used as in lavc/x86/fft_sse.c:
> >> >>>> ? ? ? ? :"+r"(j), "+r"(k)
> >> >>>> ? ? ? ? :"r"(output+n4), "r"(output+n4*3),
> >> >>>> ? ? ? ? ?"m"(*m1m1m1m1)
> >> >>>> ? ? ? ? XMM_CLOBBERS(, : "%xmm0", "%xmm1", "%xmm7")
> >> >>>> ? ? );
> >> >>>
> >> >>> That falls over if any other clobbers are needed.
> >> >>
> >> >> If any other clobbers are needed they could be written before the macro.
> >> >
> >> > That won't work.
> >> 
> >> : "eax" XMM_CLOBBERS(,, "%xmm0", "%xmm1", "%xmm7")
> >> 
> >> But I agree having "cc" as dummy is simpler.
> >
> > it may be simpler but your variant is more correct
> 
> If "cc" is a no-op neither variant is more correct.  Mine is a hell of
> a lot prettier though.

unless "cc" is documented to be a no-op on x86 we should not assume it is

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.
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