[webkit-dev] JavaScriptCore to C++ JSLock questions

Werner Sharp wsharp at adobe.com
Fri Feb 15 14:38:52 PST 2019

Thanks Geoff.

Do you think it’s a reasonable request that if JSCallbackObject<Parent>::customHasInstance fails to find a JSObjectHasInstanceCallback that it would fall back and call the JSObject::defaultHasInstance routine?  And if so, where would I open a bug for it?

For general performance numbers, my C++ based hasInstance is about 9x slower than if I could just use the built-in one.  One written in JS doing the same prototype chain walking is about 3-5 slower depending on depth of prototype walking than the built in one.



From: <ggaren at apple.com> on behalf of Geoffrey Garen <ggaren at apple.com>
Date: Friday, February 15, 2019 at 1:46 PM
To: Werner Sharp <wsharp at adobe.com>
Cc: "webkit-dev at lists.webkit.org" <webkit-dev at lists.webkit.org>
Subject: Re: [webkit-dev] JavaScriptCore to C++ JSLock questions

Hi Werner.

There’s no API around this. If you’re sure you don’t need thread safety, you can probably manually change JavaScriptCore not to do that locking.


On Feb 15, 2019, at 6:54 AM, Werner Sharp <wsharp at adobe.com<mailto:wsharp at adobe.com>> wrote:

Hi everyone,

I’m working on a project that uses JavaScriptCore in interpreted mode along with a lot of C++ classes referenced from JS based upon the JSClassRef concept.  From doing some profiles, the transition from JSC to C++ (or vice-versa) is quite costly because of JSLockHolder, JSLock::dropAllLocks, etc.  I assume there is no way around this in a single threaded single instance of the VM?  Or no way from C++ to wrap a set of JSObjectGet* type of calls in a block somehow allowing only one lock to happen. For example, I need to query 6 properties of a pure JS object from C++.  That takes 6 round trips into JSC so 6 locks unless I do something like return a string representation of the 6 properties and parse them in my C++ code.

On a related note, one thing I’ve found slow is my own implementation of my C++ class’s hasInstance because of the locking.  I tried to leave the JSClassRef hasInstance NULL assuming that the internal instanceOf would work properly.  But that doesn’t appear to work because the JSCallbackObject always sets the ImplementsHasInstance flag but not the ImplementsDefaultHasInstance flag. I would think that “JSCallbackObject<Parent>::customHasInstance” could be enhanced to call defaultHasInstance if JSObjectHasInstanceCallback hasInstance always returns false?

Or if there is some way that defautHasInstance can be called for a JSCallbackObject I’d appreciate to hear about it.

Thanks for your time.

Werner Sharp

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