We had some preliminary discussions around proxies, and how to potentially fix some mistakes from the past, some details here: https://github.com/tvcutsem/es-lab/issues/21 <https://github.com/tvcutsem/es-lab/issues/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: https://v8project.blogspot.com/2017/10/optimizing-proxies.html <https://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 > On Oct 5, 2017, at 5:50 AM, Alex Vincent <ajvinc...@gmail.com> wrote: > > 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: > isDataDescriptor(current) and isDataDescriptor(desc) return true > [[Configurable]] is false > [[Writable]] is false > SameValue(Desc.[[Value]], current.[[Value]]) is true > The [[GetOwnProperty]] or [[DefineOwnProperty]] internal method is executing, > indirectly calling ValidateAndApplyPropertyDescriptor for this case. > The [[Get]] or [[Set]] internal methods is probably executing, calling > [[GetOwnProperty]] or [[DefineOwnProperty]] for this case. > 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 > > -- > "The first step in confirming there is a bug in someone else's work is > confirming there are no bugs in your own." > -- Alexander J. Vincent, June 30, 2001 > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss
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