Bug 83295

Summary:	difference of non negative enums is never negative
Product:	cairo	Reporter:	Massimo <sixtysix>
Component:	general	Assignee:	Chris Wilson <chris>
Status:	RESOLVED MOVED	QA Contact:	cairo-bugs mailing list <cairo-bugs>
Severity:	normal
Priority:	medium
Version:	unspecified
Hardware:	Other
OS:	All
Whiteboard:
i915 platform:		i915 features:

Description Massimo 2014-08-31 13:53:41 UTC

Gcc treats enum with non negative values as unsigned,
so using the difference in a comparison function is
questionable:

http://cgit.freedesktop.org/cairo/tree/src/cairo-polygon-intersect.c#n794

for example adding a 

assert (a->type - b->type >= 0);

gcc prints

src/cairo-polygon-intersect.c|795 col 15| warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
      assert (a->type - b->type >= 0);

this means that three events (START, STOP, INTERSECTION) in the same
place will be potentially each one after each other, making difficult
to understand whether equally located STOP/INTER/START events should be
really processed in that order or not.

Comment 1 Chris Wilson 2014-08-31 14:00:21 UTC

But that is not equivalent. The equivalent assert would be assert((int)(a->type - b->type) >= 0);

Comment 2 Massimo 2014-08-31 14:22:22 UTC

(In reply to comment #1)
> But that is not equivalent. The equivalent assert would be
> assert((int)(a->type - b->type) >= 0);

but that means that you are assigning/converting UINT_MAX - 1 or 2
to an int and gcc knows it cannot happen and I think after 
inlining and only checking the sign of the int gcc translates it
to a->type != b->type

Comment 3 GitLab Migration User 2018-08-25 13:36:56 UTC

-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been closed from further activity.

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