Re: [Mesa3d-dev] Mesa (master): util: Code generate functions to pack and unpack a single pixel.

[Michel D. <mi...@da...>]

On Fri, 2010-02-26 at 08:47 -0800, Jose Fonseca wrote: 
> Module: Mesa
> Branch: master
> Commit: 9beb302212a2afac408016cbd7b93c8b859e4910
> URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=9beb302212a2afac408016cbd7b93c8b859e4910
> 
> Author: José Fonseca <jfo...@vm...>
> Date:   Fri Feb 26 16:45:22 2010 +0000
> 
> util: Code generate functions to pack and unpack a single pixel.
> 
> Should work correctly for all pixel formats except SRGB formats.
> 
> Generated code made much simpler by defining the pixel format as
> a C structure. For example this is the generated structure for
> PIPE_FORMAT_B6UG5SR5S_NORM:
> 
> union util_format_b6ug5sr5s_norm {
>    uint16_t value;
>    struct {
>       int r:5;
>       int g:5;
>       unsigned b:6;
>    } chan;
> };

José, are you aware that the memory layout of bitfields is mostly
implementation dependent? IME this makes them mostly unusable for
modelling hardware in a portable manner.

> Not used everywhere yet because it seems compiled code is slower than
> bitshift arithmetic by some misterious reason. So we should generate
> bitshift arithmetic at least for the simple UNORM pixel formats.

Due to above I'd recommend always using bitshift arithmetic rather than
bitfields anyway. :)


-- 
Earthling Michel Dänzer           |                http://www.vmware.com
Libre software enthusiast         |          Debian, X and DRI developer