[webkit-dev] JavaScript bindings changed to call scriptExecutionContext virtual function?

[Adam Barth]

On Sun, Aug 2, 2009 at 1:13 AM, Darin Adler<darin at apple.com> wrote:
> I noticed that many JavaScript binding implementations are now calling the
> virtual function scriptExecutionContext on DOM nodes. This should never be
> done! That's a virtual function, so it's not as fast as calling document().
> The virtual Node::scriptExecutionContext() function should be made private
> so that we can catch at least some cases of this programming mistake at
> compile time.
>
> When this change was made, this presumably slowed down DOM bindings, since
> document() is an inline function that expands to a single memory read and
> scriptExecutionContext() is a virtual function call.
>
> When was this done? Can the person who did it fix it?

Do you mean the two calls to scriptExecutionContext() in
JSNodeCustom.cpp?  Those appear to have been added in
http://trac.webkit.org/changeset/40675 by Sam Weinig (reviewed by
Darin Adler).  If you mean one of the other 36 occurrences of
scriptExecutionContext, I'm not sure, but we can certainly track it
down.

Is there a benchmark that shows the performance issue?  That will be
helpful in making sure we've fixed the problem.

> There are various techniques that can be used to fix this.
>
> One is to only call scriptExecutionContext in generic code, and use
> functions named document() in less generic code that's working with a Node
> rather than an EventTarget.
>
> Another is to break scriptExecutionContext into virtual and non-virtual
> parts as we do with functions like Node::prefix and Node::virtualPrefix.
>
>    -- Darin
>
> _______________________________________________
> webkit-dev mailing list
> webkit-dev at lists.webkit.org
> http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
>