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Bug 648

Summary: GCC optimizations break BlitRGBtoRGBSurfaceAlpha128MMX
Product: SDL Reporter: Neil T. <neil1357>
Component: videoAssignee: Ryan C. Gordon <icculus>
Status: RESOLVED FIXED QA Contact: Sam Lantinga <slouken>
Severity: normal    
Priority: P2 CC: sezeroz
Version: HG 1.2Keywords: target-1.2.14
Hardware: x86   
OS: Linux   
Attachments: png from previous comment
patch from above comment
example problem showing the problem with the blitter

Description Neil T. 2008-11-28 02:17:26 UTC 
BlitRGBtoRGBSurfaceAlpha128MMX passes the "load" value (SDL-1.2/src/video/SDL_blit_A.c:240 rev 4144) as a Uint8 to the asm block, so GCC seems to be assuming that the upper doubleword doesn't need to be initialized as it is reinitialized later as 0x00010101. This results in a mask like 0x0000200000fefefeULL instead of 0x00fefefe00fefefeULL.
Causes issues with surface alpha=128 blitting: http://www.boswars.org/dev/screenbug.png
Compiled with default options from ./configure (-g -O2)
GCC version: Debian 4.3.2-1.
Comment 1 Neil T. 2008-11-28 02:45:15 UTC 
Getting an error from Bugzilla when trying to add an attachment:
undef error - Undefined subroutine Fh::slice at data/template/template/en/default/global/hidden-fields.html.tmpl line 58 

Example program (http://pastebin.com/f3cb38cbe) should display a solid colored rectangle, but the rectangle has vertical stripes.
Patch (http://pastebin.com/f3d695d3c) changes Uint8[8] to Uint64, which forces GCC to initialize the whole value.
Blit565to565SurfaceAlphaMMX and Blit555to555SurfaceAlphaMMX also appear to be affected, but I haven't had a chance to test them before/after applying the patch.
Comment 2 Ryan C. Gordon 2009-09-13 16:33:32 UTC 
Tagging this bug with "target-1.2.14" so we can try to resolve it for SDL 1.2.14.

Please note that we may choose to resolve it as WONTFIX. This tag is largely so we have a comprehensive wishlist of bugs to examine for 1.2.14 (and so we can close bugs that we'll never fix, rather than have them live forever in Bugzilla).

--ryan.
Comment 3 Rene Dudfield 2009-09-18 11:34:06 UTC 
Created attachment 362 [details]
png from previous comment

here's the png from the above comment as an attachment.
Comment 4 Rene Dudfield 2009-09-18 11:36:55 UTC 
Created attachment 363 [details]
patch from above comment
Comment 5 Rene Dudfield 2009-09-18 11:37:52 UTC 
Created attachment 364 [details]
example problem showing the problem with the blitter
Comment 6 Sam Lantinga 2009-09-21 04:36:42 UTC 
Ryan, this looks like a good fix to me.  You want to try it out and see if the other functions mentioned need a similar fix?
Comment 7 Ryan C. Gordon 2009-09-28 21:04:20 UTC 
This patch is svn revision #4885, thanks!

--ryan.