From: Gabriel Gambetta SDL-1.2
authorRyan C. Gordon <icculus@icculus.org>
Tue, 21 Nov 2006 23:17:00 +0000
branchSDL-1.2
changeset 3899081aecdb0911
parent 3898 899d07087694
child 3900 ce3a2bd11305
From: Gabriel Gambetta
To: SDL Mailing List <sdl@libsdl.org>
Date: Fri, 17 Nov 2006 00:39:29 -0200
Subject: [SDL] Alpha blending bug - fixed?

It turns out mm5 never got the correct value. This failed :

[...]
"movd %1, %%mm5\n\t"
: : "m" (amask), "m" (sf->Ashift) );


mm5 got 0xFF000018 instead of 0x00000018. However I did this :

Uint32 ashift = sf->Ashift;
[...]
"movd %1, %%mm5\n\t"
: : "m" (amask), "m" (ashift) );

and everything worked fine.

The only thing I could find out was that ashift is 32-bit aligned but
sf->Ashift isn't:

printf("ashift %8X [%d]\n", &ashift, (int)(&ashift) % 4);
printf("sf->Ashift %8X [%d]\n", &sf->Ashift, (int)(&sf->Ashift) % 4);

ashift at BFF0E0C4 [0]
sf->Ashift at 08AFB9CD [1]

Could this be the problem? If it is, the fix is extremely trivial.
Should I submit a patch?

--Gabriel
src/video/SDL_blit_A.c
     1.1 --- a/src/video/SDL_blit_A.c	Tue Nov 21 20:49:35 2006 +0000
     1.2 +++ b/src/video/SDL_blit_A.c	Tue Nov 21 23:17:00 2006 +0000
     1.3 @@ -1566,6 +1566,7 @@
     1.4  	int dstskip = info->d_skip >> 2;
     1.5  	SDL_PixelFormat* sf = info->src;
     1.6  	Uint32 amask = sf->Amask;
     1.7 +	Uint32 ashift = sf->Ashift;
     1.8  
     1.9  	__asm__ (
    1.10  	/* make mm6 all zeros. */
    1.11 @@ -1587,7 +1588,7 @@
    1.12  	/* get alpha channel shift */
    1.13  	"movd      %1, %%mm5\n\t" /* Ashift -> mm5 */
    1.14  
    1.15 -	  : /* nothing */ : "m" (sf->Amask), "m" (sf->Ashift) );
    1.16 +	  : /* nothing */ : "m" (amask), "m" (ashift) );
    1.17  
    1.18  	while(height--) {
    1.19