Even if you want to store weak-map specific meta data per object, nobody would store that directly in the object. Thats needlessly cruel on the cache since >>99.9% of objects never end up in a weak map. Instead one would locate that meta data outside the object in a direct mapped dense area (like mark bitmaps), which is on its own page that is not write protected.
Andreas On May 16, 2011, at 10:39 AM, Brendan Eich wrote: > On May 16, 2011, at 12:47 AM, Erik Corry wrote: > >> I think the objects used as keys in weak maps need to be somehow >> annotated with this information so that the GC can clean up the weak >> maps when the keys die. This means that if you take an object that is >> frozen and use it as a key in a weak map then it will need to be >> mutated in some way and can't be on a read-only page. > > That's already false in Firefox nightlies. We support Object.freeze. We have > a WeakMap implementation. We do not mutate the frozen object. Its GC metadata > does not reside in a header for it, or even in the same OS page. > > >> Perhaps you have a different, efficient, implementation. I can't see >> us gaining much from putting frozen objects on read-only pages, thus I >> can't accept it as a very strong argument about the way that frozen >> objects should work together with a new feature. > > This is a bit too subjective an argument, sorry. > > My point about 50+ years of OS and MMU firewalling is important. Chrome > (recently hacked by French spook-types, but also hacked over a year ago with > a two-step attack) is a convincing example. > > Sure, we have user-code isolation tools in our belts, including fancy > compiler/runtime pairs. But it's hard to beat processes if you want to be > sure. No silver bullet, simply "stronger isolation". > > >>> Weak maps are in Firefox nightlies. We're playing with page protection too >>> (not for freezing, yet). This seems like a dare, but it also seems to be >>> dodging my point in replying again: that private names cannot be used to >>> extend frozen objects in the "[[Extensible]] = true" sense of the spec. >> >> Is there a description anywhere about how you have implemented GC of weak >> maps? > > http://hg.mozilla.org/tracemonkey/rev/7dcd0d16cc08 > > Look for WeakMap::mark... names. There's no need to mutate a key object. > There should not be, either. > > Yes, this GC can iterate. A lot, but a "fix" doesn't obviously require > mutating (possibly frozen) key objects. Also, since POITROAE we are going to > measure twice, Optimize once. > > /be _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
