perf: Context independent, shared null && undefined

[obastemur]

Currently null and undefined are created per JavascriptLibrary.

• We can't compare a JsVar to null or undefined because we don't know its' origin.
• JsVar -> ToRecyc..Object -> Compare... is unnecessarily more complicated
• There are some places we know JsVar is actually a Recyc..Object hence things are easier (this improvement is already merged to master). Yet, those places are very limited

Any objections to making null, undefined values are allocated per framework only once?

Is there anything else that we could use the same approach?

[MikeHolman]

true and false are potentially useful additions.

I like the idea of sharing more static data between contexts, but I'm concerned that there may be a number of places where the ScriptContext is considered and will lead to us going down slow paths, and that you may run into a bunch of asserts.

[obastemur]

but I'm concerned that there may be a number of places where the ScriptContext is considered and will lead to us going down slow paths, and that you may run into a bunch of asserts.

@MikeHolman could you provide more context on this?

[MikeHolman]

The example I'm thinking of is that for property caches, if an object's scriptContext isn't the current context we won't cache it. I think there are a number of places where we will do these type of comparisons.

[obastemur]

I see. How your scenario could prevent us from having a single undefined or null framework wide? (this is the part I didn't understand though)

[MikeHolman]

I'm having a hard time coming up with anything that will concretely be a problem, so maybe there isn't anything. I just have a feeling that there will be a number of subtle issues.