• Matt Turner's avatar
    dix: Assert noPanoramiXExtension is false in PanoramiX code · 61aa40ae
    Matt Turner authored
    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: xorg/xserver#590
    
    Signed-off-by: Matt Turner's avatarMatt Turner <mattst88@gmail.com>
    61aa40ae
events.c 191 KB