[FFmpeg-devel] [RFC] Runtime-inited versus Hardcoded tables.

[Reimar Döffinger]

Hello,
On Thu, Jan 31, 2008 at 02:14:53PM +0100, Zdenek Kabelac wrote:
> But there is always chance you will want to install the ffmpeg into
> some limited sized eeprom/CD/distro and you want to save some memory
> for other things - so while you could have a lot of dynamic memory -
> you are limited by the ROM size....

The tables in .bss solve that problem already though.

> It's probably not so easy to say which tables should be created for
> the codec initialization - should it take less then 0.1ms on 400MHz
> CPU :) though for me I would even support the idea to have the tables
> created always when codec is opened/closed.
> But it might be nice to have the build with 'static table' or
> dynamicaly computed tables - I think there is already zillion of
> defines thus this one extra wouldn't be so problematic - routines for
> table generation could be held in one .c file - and added only when
> such build is requested.

I assume that you mean having the tables in the codec context. As said,
I think that sometimes that might be preferable, but you seem to miss
the biggest drawbacks: An application that decodes lots of streams will
have these tables lots of times in memory, and maybe more importantly it
means one additional pointer dereference for each access to the table,
possibly even causing register spill in an innermost loop.
And if the table is used in asm code, you might also need different code.

Greetings,
Reimar Doeffinger