Bug 104654 – [CoordinatedGraphics] Use unsigned integers for UpdateAtlas IDs

• Component: WebKit2
• Status: RESOLVED FIXED

[Chris Dumez]

UpdateAtlas currently uses *signed* integer type for its identifier. Due to the way we generate those IDs, it would be safer to use *unsigned* integers. This is because the generated ID will overflow at some point and the C and C++ language standards say that overflow of a signed value is undefined behaviour. 

In the C99 standard this is in section 6.5. In the C++98 standard it is in section 5 [expr], paragraph 5. "This means that a correct C/C++ program must never generate signed overflow when computing an expression. It also means that a compiler may assume that a program will never generated signed overflow".

Note that gcc has -fwrapv flag to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. However, I still believe it is safer and more consistent to use unsigned integers here.

[Chris Dumez]

Created attachment 178761 [details]
Patch

[Kenneth Rohde Christiansen]

Comment on attachment 178761 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=178761&action=review

> Source/WebKit2/ChangeLog:14
> +        This patch uses unsigned integer type for UpdateAtlas IDs.

does unsigned have defined behavior?

[Chris Dumez]

(In reply to comment #2)
> (From update of attachment 178761 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=178761&action=review
> 
> > Source/WebKit2/ChangeLog:14
> > +        This patch uses unsigned integer type for UpdateAtlas IDs.
> 
> does unsigned have defined behavior?

Yes, unsigned integer types are guaranteed to wrap on overflow in C / C++.

[Chris Dumez]

Created attachment 178775 [details]
Patch for landing

Updated the Changelog to clarify the behavior for unsigned integers on overflow.

[WebKit Review Bot]

Comment on attachment 178775 [details]
Patch for landing

Clearing flags on attachment: 178775

Committed r137293: <http://trac.webkit.org/changeset/137293>

[WebKit Review Bot]

All reviewed patches have been landed.  Closing bug.