Use cases for WeakMap

Andreas Gal gal at mozilla.com
Mon May 16 10:52:51 PDT 2011


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



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