[Webkit-unassigned] [Bug 158083] LLInt should support other types of prototype GetById caching.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=158083

--- Comment #5 from Caio Lima <ticaiolima at gmail.com> ---
(In reply to comment #3)
> (In reply to comment #1)
> > Hey Keith, I am interested in work in this bug if it is possible
> > 
> > Cold you clarify some points to me?
> > 
> > 1. What are prototype misses?
> > 
> > 2. When you think in cache accessors, you mean getters and setters?
> > 
> > 3. Also, for customs
> > 
> > It could be a big help if you post some JavaScript code as example.
> > 
> > I am trying to debug the case for accessors and I noticed that the getter is
> > emitted as get_by_id op and I suspect it is already being cached (I didn't
> > check it).
> 
> Hi Caio,
> 
> It would be great if you fixed this feature and I would be happy to help.
> Feel free to message me on irc, my nick is keith_miller, if you have any
> other questions.
> 
> 1) prototype miss was the wrong terminology. I should have said that we
> should cache unset properties. Meaning, `let x = foo.bar` where `"bar" in
> foo === false`. I actually already handled this case in
> http://trac.webkit.org/changeset/201456.
> 
> 2) Yep, that's right. In JavaScriptCore we use the word Accessor to mean a
> property with a getter and/or a setter. 
> 
> For example, if you did the following: 
> 
> foo = {};
> Object.defineOwnProperty(Object.prototype, "bar", { get: () => { return 1; }
> });
> print(foo.bar);
> 
> The bytecodes will look something like:
> 
> [   9] resolve_scope     loc10, loc3, print(@id0), <GlobalProperty>, 1,
> 0x113de7900
> [  16] get_from_scope    loc6, loc10, print(@id0),
> 2048<ThrowIfNotFound|GlobalProperty|NotInitialization>, 122    predicting
> None
> [  24] resolve_scope     loc11, loc3, foo(@id1), <GlobalProperty>, 1,
> 0x113de7900
> [  31] get_from_scope    loc12, loc11, foo(@id1),
> 2048<ThrowIfNotFound|GlobalProperty|NotInitialization>, 191    predicting
> None
> [  39] get_by_id         loc9, loc12, bar(@id2)    predicting None
> [  48] call              loc5, loc6, 2, 16 status(Could Take Slow Path)   
> Original; predicting None
> 
> That get_by_id will attempt to lookup the property on foo by calling foo's
> getOwnPropertySlot function, which will set the passed PropertySlot& to
> Object.prototype's "bar" GetterSetter object. In this case, the PropertySlot
> will say that the value is an GetterSetter and we won't cache it.
> 
> 3) Customs are mostly used as a performance optimization for DOM Accessors.
> Basically, instead of allocating a JSFunction object for all accessor
> properties in the DOM we use a function pointer instead. You can see the
> typedef, GetValueFunc, in PropertySlot.h. Customs are also used in a small
> number of other places for special properties that are values but change
> might change, although that is quite rare.
> 
> I would recommend for a first pass that you implement a new opcode, I guess
> called op_get_by_id_proto_accessor, that just calls to a slow path. In the
> slow path it can load the GetterSetter object and call the getter returning
> the result. If you are feeling very ambitious, you could inline the code for
> the call in the interpreter assembly. Let me know if you plan on trying to
> inline the call since you will need to know the JavaScriptCore calling
> convention to implement it.

Thanks for the clarification Keith.

I am sending a prototype of what you described, but I definitely want to learn how can I inline a function call =).

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