Fixed bug #33

Mike Frysinger wrote:
> with libsdl-1.2.9, some games (like bomberclone) started
> segfaulting in Gentoo
[...snip...]
> the last change in the last hunk:
[...snip...]
> if i change the statement to read:
> (table[which].blit_features & GetBlitFeatures()) == GetBlitFeatures()
> bomberclone no longer segfaults on my box

Alex Volkov wrote:
> The test "(table[which].blit_features & GetBlitFeatures()) ==
> table[which].blit_features)" is correct, and the previous
> "(table[which].cpu_mmx == SDL_HasMMX())" was actually broken.

I think there is potentially a slightly different cause of the above problem.
During the introduction of the Altivec code, the blit_table struct field
'alpha' got changed from a straightforward enum to a bitmask, which makes
perfect sense by itself. However, now the table driven blitter selection code
in SDL_CalculateBlitN() can choose the wrong blitters when searching for a
NO_ALPHA blitter because of the following code:
    int a_need = 0;
    ...
    (a_need & table[which].alpha) == a_need &&
When searching through the normal_blit_2[] table, a SET_ALPHA blitter (like
Blit_RGB565_ARGB8888) can now be selected instead of a NO_ALPHA one, causing
alpha channel bits to appear in a non-alpha destination surface. I suppose this
could theoretically be an indirect cause of the segfault mentioned above.

I *think* this can be fixed by changing to
    int a_need = NO_ALPHA;

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401548

src/video/SDL_blit_N.c

@@ -2433,7 +2433,7 @@ SDL_loblit SDL_CalculateBlitN(SDL_Surface *surface, int blit_index)
 		}
 	} else {
 		/* Now the meat, choose the blitter we want */
-		int a_need = 0;
+		int a_need = NO_ALPHA;
 		if(dstfmt->Amask)
 		    a_need = srcfmt->Amask ? COPY_ALPHA : SET_ALPHA;
 		table = normal_blit[srcfmt->BytesPerPixel-1];