[PATCH xquartz 04/10] randr: Silence -Wshift-negative-value warnings

[Adam Jackson]

On Wed, 2015-10-14 at 15:44 -0700, Jeremy Huddleston Sequoia wrote:
> > On Oct 14, 2015, at 15:33, Alan Coopersmith  wrote:
> > 
> > On 10/14/15 03:13 PM, Jeremy Huddleston Sequoia wrote:
> > > rrtransform.c:199:23: warning: shifting a negative signed value
> > > is undefined [-Wshift-negative-value,Semantic Issue]
> > >             rot_cos = F(-1);
> > 
> > 
> > > -            rot_cos = F(-1);
> > > +            rot_cos = F(~0u);
> > 
> > Is -1 guaranteed to be ~0u on all platforms?   Or just all the ones
> > we know and care about?
> 
> I'm pretty certain that ~0u and -1 are the same bit patterns on all
> platforms.  I can't think of a reason why they wouldn't be.

On a ones' complement machine, (uint8_t)-1 is 0b11111110.  On a signed-
magnitude machine, (uint8_t)-1 is 0b10000001.  But I think you're
significantly more likely to encounter a PDP-endian machine than a non-
two's-complement machine at this point, and there's plenty of other
places in xserver where we're assuming two's complement, e.g.:

fb/fb.h:#define FB_ALLONES  ((FbBits) -1)

- ajax