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

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

On Sat, 2010-03-06 at 12:44 +0000, José Fonseca wrote: 
> On Mon, 2010-03-01 at 09:03 -0800, Michel Dänzer wrote:
> > 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.
> 
> It's not only implementation dependent and slow -- it is also buggy!
> 
> gcc-4.4.3 is doing something very fishy to single bit fields. 
> 
> See the attached code. ff ff ff ff is expected, but ff ff ff 01 is
> printed with gcc-4.4.3. Even without any optimization. gcc-4.3.4 works
> fine.
> 
> Am I missing something or is this effectively a bug?

No idea, I just try to stay away from bitfields as much as possible.


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