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dix: Assert noPanoramiXExtension is false in PanoramiX code

Merged Matt Turner requested to merge mattst88/xserver:noPanoramiXExtension into master

When compiling with link time optimization, GCC thinks it's discovered undefined behavior:

events.c: In function 'XineramaConfineCursorToWindow':
events.c:609:13: warning: iteration 2147483647 invokes undefined behavior [-Waggressive-loop-optimizations]
events.c:609:11: note: within this loop
events.c:605:49: warning: array subscript -1 is below array bounds of 'struct _Window *[16]' [-Warray-bounds]
events.c:606:31: warning: array subscript -1 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]
events.c:610:39: warning: array subscript -2 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]
events.c:617:38: warning: array subscript -2 is below array bounds of 'struct _Window *[16]' [-Warray-bounds]
events.c:619:35: warning: array subscript -2 is below array bounds of 'struct _Screen *[16]' [-Warray-bounds]

This results from

i = PanoramiXNumScreens - 1;

RegionCopy(&pSprite->Reg1, &pSprite->windows[i]->borderSize);
off_x = screenInfo.screens[i]->x;
off_y = screenInfo.screens[i]->y;

where GCC believes that PanoramiXNumScreens might be 0. Unfortunately GCC is just smart enough to be an annoyance because this case is not actually possible: XineramaConfineCursorToWindow() is only called when noPanoramiXExtension is false, and if noPanoramiXExtension is false then PanoramiXNumScreens must be >1 (see PanoramiXExtensionInit()).

So, add an assert(!noPanoramiXExtension), which to my surprise provides GCC with information even in release builds and lets GCC understand that the code is not doing anything that is undefined behavior.

I chose this solution instead of the proposed assert(i >= 0) because the same pattern occurs in CheckVirtualMotion() but is inside an 'if (!noPanoramiXExtension)' and does not generate any warnings.

Fixes: #590 (closed) Signed-off-by: Matt Turner mattst88@gmail.com

Edited by Matt Turner