Suggestion: Proxy.[[GetOwnProperty]] caching non-configurable, non-writable data descriptors?
Instead of storing the properties in a map, I think you could first call [[GetOwnProperty]] on the target, and if it's not configurable nor writable, return the same descriptor without calling the proxy trap.
However, I disagree with this kind of things. Proxy traps are not only useful because of the value they return, they can also have desirable side-effects.
We had some preliminary discussions around proxies, and how to potentially fix some mistakes from the past, some details here: tvcutsem/es-lab#21, tvcutsem/es-lab#21
But this is very tricky, specially because Proxies were designed with the idea that the handler could also be a Proxy, that’s why we do all the gymnastics in the spec.
V8 has started some effort to improve proxies: v8project.blogspot.com/2017/10/optimizing-proxies.html, v8project.blogspot.com/2017/10/optimizing-proxies.html
My suggestion is to wait and see how far they can go, and we can attempt to change the spec if there is anything that prevent them from achieving a great deal of improvements based on the current spec.
./caridy
---------- Forwarded message ---------- From: Oriol _ <oriol-bugzilla at hotmail.com> Instead of storing the properties in a map, I think you could first call [[GetOwnProperty]] on the target, and if it's not configurable nor writable, return the same descriptor without calling the proxy trap.
I believe that's what my original suggestion was.
However, I disagree with this kind of things. Proxy traps are not only
useful because of the value they return, they can also have desirable side-effects.
--Oriol
You make an annoyingly good point here... :-) I made a similar point a few months ago to someone who wanted to create a "hybrid" object which hid the proxy as the prototype of a regular object, for speed. shudder
That said, I still believe the algorithm I pointed out goes through a lot of steps that can be unnecessary in the scenario I laid out... except that the creator of the proxy has no way of telling the implementing engine that. There's no "proxy configuration options" that I can modify to pass these optimizing hints to the engine. Maybe that's something we might need.
I know, this is quite a stretch, suggesting more API for proxies. (I still want Reflect.parse to happen, after all, and unlike kai zhu, I think operator overloading is a good thing...) I "blame" (actually, credit) Tom van Cutsem for that, as he made a suggestion about optional properties being an object in my own Membrane API recently. It was a good idea there, and for other algorithmic shortcuts in proxy traps, it might be a good idea here.
I also take Caridy's point about Google Chrome 62's optimizations that are coming. I didn't know that was actually happening until Caridy posted about it here.
Thanks for your response, though: it's nice to know someone cares about this.
Alex
Proxies are a little slow right now, and maybe the rules of ECMAScript can safely allow for caching a non-configurable, non-writable data descriptor.
Specifically, in ValidateAndApplyPropertyDescriptor from ES8 (section 9.1.6.3, step 7a-iii), we've already shown current and desc both have the following traits:
That is a very specific set of traits. As I understand it, it means the proxy target's property named P in Proxy.[[GetOwnProperty]] is permanently locked and can never change - and so neither can the proxy ever report a different value. Therefore, calling on the proxy handler for that property a second time is, at least in my view, either redundant or unnecessarily expensive.
I would suggest that implementers in section 9.5 (Proxy internals) optionally have a private Map where values that are non-configurable and non-writable are stored. Then [[GetOwnProperty]] could insert a couple of new steps into its algorithm. Between steps 4 and 5 of the current algorithm, I would add: "If the optional [[Map]] object exists and [[Map]].has(P) returns true, return [[Map]].get(P)." In step 17 of the current algorithm, I would add a substep: "If isDataDescriptor(resultDesc) and resultDesc.[[Writable]] is false and the optional [[Map]] object exists, call [[Map]].set(P, resultDesc). (If the [[Map]] internal slot exists but does not contain a Map, the [[Map]] may be filled with a Map at this time.)"
I would also suggest similar changes for [[DefineOwnProperty]]. (Section 9.5.6)
Alternatively, if storing a specific descriptor in the Map is unpalatable, the specification could request storing resultDesc.[[Value]] in step 17, and create a new non-configurable, non-writable descriptor before the current step 5.
Also, if a Map is too expensive, the spec could allow for an equivalent native map<string, JSValue> or whatever other appropriate data structure
fills the need.
Counter-point (1): if the intent of the proxy is to treat all property look-ups equally, then caching the non-writable, non-configurable descriptors goes against that intent - because now the trap is invoked only once for properties returning this type of descriptor. But there's a lot of steps, including invoking a custom proxy handler trap with an unknown number of steps, and a lot of complexity in that trap to ensure what it returns will pass all the assertions in the spec.
Counter-point (2): I don't know how common it is to have a property descriptor that meets all six criteria at the beginning of this post, in particular that both [[Configurable]] and [[Writable]] are false. So this does add another pointer, at least to null initially, for the [[Map]] slot. This is why I am proposing the [[Map]] slot as optional for implementers. Let the engines decide whether it's warranted or not as an optimization.
Thoughts?
Alex Vincent
Hayward, CA, USA