[FFmpeg-devel] FASTDIV macro

[Michael Niedermayer]

On Sun, Nov 09, 2008 at 01:35:15PM +0000, M?ns Rullg?rd wrote:
> Michael Niedermayer <michaelni at gmx.at> writes:
> 
> > On Sat, Nov 08, 2008 at 09:49:24PM +0000, M?ns Rullg?rd wrote:
> >> libavutil/internal.h defines a macro, FASTDIV(), for fast 32/16-bit
> >> division my means of multiplying by a table value.  If the
> >> architecture is not ARM or x86, which have asm versions, this macro is
> >> defined as a normal division if CONFIG_FASTDIV is not set.  The odd
> >> thing is, nothing ever sets CONFIG_FASTDIV.  Something is clearly not
> >> right here.
> >> 
> >> I see these alternatives to fix it:
> >> 
> >> 1. Always use the table multiplication.
> >> 2. Enable CONFIG_FASTDIV by default.
> >> 3. Disable CONFIG_FASTDIV by default, adding configure option to
> >>    enable.
> >> 4. Always use plain division if no asm available.
> >> 
> >> Opinions?
> >
> > the variant that is faster should be used unless CONFIG_SMALL is enabled
> > in which case the table should be avoided except when the speed difference
> > is enourmious.
> 
> How do we know which is faster?

benchmark


> 
> >> There is also a question of where this macro belongs.  It uses a table
> >> defined in lavc, and its only use outside lavc is in ff_sqrt(), which
> >> is only used in lavc.  Would it make sense to move these to
> >> lavc/mathops.h, where other similar macros are defined?
> >> Alternatively, mathops.h could be moved to lavu.  I'd like it to be
> >> consistent.  The same goes for a number of other macros of this type.
> >
> > i think that at least the table should be moved to lavu
> 
> What would be a good place?  mathematics.c has some other tables (for
> ff_sqrt() and av_log2()), so maybe that would be a good place.

ok


> 
> What about lavc/mathops.h?

i dont know, but at least the per file FRAC_BITS #define system should be
replaced by something else first.

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

The greatest way to live with honor in this world is to be what we pretend
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