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

Adam Barth abarth at webkit.org
Sun Aug 2 01:45:37 PDT 2009

On Sun, Aug 2, 2009 at 1:41 AM, Adam Barth<abarth at webkit.org> wrote:
> 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).

Actually, that change just moved code that was actually introduced in
http://trac.webkit.org/changeset/38063 by Alexey Proskuryakov
(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
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