[SDL] Alpha blending bug - fixed?

[Frank Mehnert]

Hi,

the original code takes the _address_ of the sf->Ashift variable and passes
this address to the assembler, because the 'm' constraint is used! Therefore
the movd instruction will not only load the content of sf->Ashift into
the mm5 register but also the following bits of the SDL_Pixelformat
structure (the lower three bytes of Rmask). The original propose of using the
local ashift variable (which is 32 bits wide) as input for the assembler
statement was correct.

To force gcc to load sf->Amask into a register you could also use

-         : /* nothing */ : "m" (sf->Amask), "m" (sf->Ashift) );
+         : /* nothing */ : "r" (sf->Amask), "r" ((Uint32) sf->Ashift) );

Note the "r" constraint.

Kind regards,

Frank

On Tuesday 21 November 2006 22:51, Gabriel Gambetta wrote:
> Thanks for listening :)
>
> The patch you sent doesn't even compile -
>
> ./src/video/SDL_blit_A.c: In function 'BlitRGBtoRGBPixelAlphaMMX3DNOW':
> ./src/video/SDL_blit_A.c:1571: error: memory input 1 is not directly
> addressable
>
> In any case I guess GCC should be smart enough to make this work as
> expected, whether it's an alignment issue or a data size issue. GCC bug
> maybe?
>
> 	--Gabriel
>
> El mar, 21-11-2006 a las 16:30 -0500, Ryan C. Gordon escribió:
> > > The only thing I could find out was that ashift is 32-bit aligned but
> > > sf->Ashift isn't:
> >
> > I don't think it's an alignment issue, since GCC should be smart enough
> > to get it into a register even if unaligned...however, sf->AShift is 8
> > bits, and ashift is 32, which is probably why you get different
> > results...it probably landed in (say) %al instead of %eax, leaving junk
> > in the rest of the register's bits.
> >
> > Try the attached patch and see if it works, too.
>
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-- 
## Dept. of Computer Science, Dresden University of Technology, Germany ##
## http://os.inf.tu-dresden.de/~fm3                                     ##
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