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

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402236

src/video/SDL_blit_A.c

@@ -1566,6 +1566,7 @@ static void BlitRGBtoRGBPixelAlphaMMX3DNOW(SDL_BlitInfo *info)
 	int dstskip = info->d_skip >> 2;
 	SDL_PixelFormat* sf = info->src;
 	Uint32 amask = sf->Amask;
+	Uint32 ashift = sf->Ashift;
 
 	__asm__ (
 	/* make mm6 all zeros. */
@@ -1587,7 +1588,7 @@ static void BlitRGBtoRGBPixelAlphaMMX3DNOW(SDL_BlitInfo *info)
 	/* get alpha channel shift */
 	"movd      %1, %%mm5\n\t" /* Ashift -> mm5 */
 
-	  : /* nothing */ : "m" (sf->Amask), "m" (sf->Ashift) );
+	  : /* nothing */ : "m" (amask), "m" (ashift) );
 
 	while(height--) {