From Bug #36:

There are a couple of issues with the selection of Altivec alpha-blitting
routines in CalculateAlphaBlit() in src/video/SDL_Blit_A.c.

1) There's no check for the presence of Altivec when checking if the
Blit32to565PixelAlphaAltivec() routine can be selected.

2) Altivec cannot be used in video memory, and there's no check if the
destination surface is a hardware surface. (Alpha-blitting to a hardware
surface with GPU support is a bad idea, but somebody's bound to do it anyway.)

Patch to fix these attached.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401243

src/video/SDL_blit_A.c

@@ -2145,7 +2145,8 @@ SDL_loblit SDL_CalculateAlphaBlit(SDL_Surface *surface, int blit_index)
 		return BlitNto1SurfaceAlphaKey;
 	    else
 #ifdef USE_ALTIVEC_BLITTERS
-        if (sf->BytesPerPixel == 4 && df->BytesPerPixel == 4 && SDL_HasAltiVec())
+	if (sf->BytesPerPixel == 4 && df->BytesPerPixel == 4 &&
+	    !(surface->map->dst->flags & SDL_HWSURFACE) && SDL_HasAltiVec())
             return Blit32to32SurfaceAlphaKeyAltivec;
         else
 #endif
@@ -2192,7 +2193,7 @@ SDL_loblit SDL_CalculateAlphaBlit(SDL_Surface *surface, int blit_index)
 		else
 #endif
 #ifdef USE_ALTIVEC_BLITTERS
-        if(SDL_HasAltiVec())
+	if(!(surface->map->dst->flags & SDL_HWSURFACE) && SDL_HasAltiVec())
             return BlitRGBtoRGBSurfaceAlphaAltivec;
         else
 #endif
@@ -2200,7 +2201,8 @@ SDL_loblit SDL_CalculateAlphaBlit(SDL_Surface *surface, int blit_index)
 		}
 		else
 #ifdef USE_ALTIVEC_BLITTERS
-        if((sf->BytesPerPixel == 4) && SDL_HasAltiVec())
+        if((sf->BytesPerPixel == 4) &&
+	   !(surface->map->dst->flags & SDL_HWSURFACE) && SDL_HasAltiVec())
             return Blit32to32SurfaceAlphaAltivec;
         else
 #endif
@@ -2219,9 +2221,9 @@ SDL_loblit SDL_CalculateAlphaBlit(SDL_Surface *surface, int blit_index)
 
 	case 2:
 #ifdef USE_ALTIVEC_BLITTERS
-        if(sf->BytesPerPixel == 4 && 
+	if(sf->BytesPerPixel == 4 && !(surface->map->dst->flags & SDL_HWSURFACE) &&
            df->Gmask == 0x7e0 &&
-           df->Bmask == 0x1f)
+	   df->Bmask == 0x1f && SDL_HasAltiVec())
             return Blit32to565PixelAlphaAltivec;
         else
 #endif
@@ -2252,14 +2254,15 @@ SDL_loblit SDL_CalculateAlphaBlit(SDL_Surface *surface, int blit_index)
 		else
 #endif
 #ifdef USE_ALTIVEC_BLITTERS
-        if(SDL_HasAltiVec())
+	if(!(surface->map->dst->flags & SDL_HWSURFACE) && SDL_HasAltiVec())
             return BlitRGBtoRGBPixelAlphaAltivec;
         else
 #endif
 		    return BlitRGBtoRGBPixelAlpha;
 	    }
 #ifdef USE_ALTIVEC_BLITTERS
-        if (sf->Amask && sf->BytesPerPixel == 4 && SDL_HasAltiVec())
+        if (sf->Amask && sf->BytesPerPixel == 4 &&
+	    !(surface->map->dst->flags & SDL_HWSURFACE) && SDL_HasAltiVec())
             return Blit32to32PixelAlphaAltivec;
         else
 #endif