[FFmpeg-devel] FASTDIV macro

Måns Rullgård mans
Sun Nov 9 14:35:15 CET 2008

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?

>> 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.

What about lavc/mathops.h?

M?ns Rullg?rd
mans at mansr.com

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