[FFmpeg-devel] [PATCH 2/3] x86: asm for sign/zero_extend()

[Michael Niedermayer]

On Sun, Feb 21, 2010 at 02:37:17AM +0000, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Sun, Feb 21, 2010 at 12:35:58AM +0000, Mans Rullgard wrote:
> >> ---
> >>  libavcodec/x86/mathops.h |   18 ++++++++++++++++++
> >>  1 files changed, 18 insertions(+), 0 deletions(-)
> >> 
> >> diff --git a/libavcodec/x86/mathops.h b/libavcodec/x86/mathops.h
> >> index 010cfb7..0c17f35 100644
> >> --- a/libavcodec/x86/mathops.h
> >> +++ b/libavcodec/x86/mathops.h
> >> @@ -97,4 +97,22 @@ static inline uint32_t NEG_USR32(uint32_t a, int8_t s){
> >>      return a;
> >>  }
> >>  
> >> +#define sign_extend sign_extend
> >> +static inline int sign_extend(int val, unsigned bits)
> >> +{
> >> +    __asm__ ("shll %1, %0 \n\t"
> >> +             "sarl %1, %0 \n\t"
> >> +             : "+&r" (val) : "ic" ((uint8_t)-bits));
> >> +    return val;
> >> +}
> >> +
> >
> >> +#define zero_extend zero_extend
> >> +static inline unsigned zero_extend(unsigned val, unsigned bits)
> >> +{
> >> +    __asm__ ("shll %1, %0 \n\t"
> >> +             "shrl %1, %0 \n\t"
> >> +             : "+&r" (val) : "ic" ((uint8_t)-bits));
> >> +    return val;
> >
> > if bits is a constant (which i guess it is quite often)
> > then this is quite inefficient.
> > val & 0x00007FFF
> > for example is more efficient in that case.
> > also its not certain 2 shifts are faster than =-1, >>, &
> > on all x86 cpus
> 
> Tell that to whoever wrote the asm for the NEG_*SR32 functions.
> Oh, wait... that was you...

true, my code is not optimal, ill look into this but that might
or might not be anytime soon.
still please dont copy my mistakes

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Observe your enemies, for they first find out your faults. -- Antisthenes
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