Proposal: Abstract References

Mark S. Miller erights at google.com
Wed Oct 22 07:45:31 PDT 2014


On Tue, Oct 21, 2014 at 11:59 PM, Andreas Rossberg <rossberg at google.com>
wrote:

> On 21 October 2014 22:31, Mark S. Miller <erights at google.com> wrote:
> >> in saying that [weak maps] are not designed to work efficiently in that
> manner.
> >
> > Actually, they are. The problem is that initial implementations date from
> > before we appreciated the need for the transposed representation. The
> best
> > thing TC39 can do for the future of WeakMaps is to proceed assuming the
> > transposed representation.
>
> While I sympathise, let me clarify that this still remains a
> conjecture. AFAIK, nobody has proved it empirically in a JS
> implementation yet, we don't know in detail how complex such an
> implementation would be, and what side effects it might have on
> general performance (e.g., via added polymorphism). It's most likely
> not as easy or as clear a win as you may think it is.
>

The following code is an existence proof of sorts that, given only the
WeakMap mechanisms you've already built + one bit of magic whose
feasibility I hope we don't need to debate. I use ES5 here to make clear
that no other magic or unengineered features are assumed.


var WeakMap;

(function() {
   'use strict';

   var SlowWeakMap = WeakMap;

   function FastWeakMap() {
     var token = Object.freeze(Object.create(null));
     return Object.freeze({
       get: function(key) {
         var shadow = key.[[Shadow]];
         return shadow ? shadow.get(token) : void 0;
       },
       set: function(key, value) {
         var shadow = key.[[Transposer]]
         if (!shadow) {
           shadow = new SlowWeakMap();
           key.[[Transposer]] = shadow;
         }
         shadow.set(token, value);
       },
       clear: function() {
         token = Object.freeze(Object.create({}));
       }
     });
   }

   // Don't do this until it is a complete shim
   // WeakMap = FastWeakMap;

 }());



The magic is that each object, whether frozen or not, would need in effect
one extra internal mutable [[Shadow]] property.

Clearly, this expository implementation is suboptimal in many ways. But it
demonstrates the following:

* It provides the full complexity measure gains that a realistic
implementation would have.

* For each of these objects, an extra SlowWeakMap instance is allocated as
its shadow.

* For each access, an extra indirection through this SlowWeakMap is
therefore needed.

* Only objects that have been used as keys in FastWeakMaps would ever have
their [[Shadow]] set, so this could also be allocated on demand, given only
a bit saying whether it is present. Besides this storage of this bit, there
is no other effect or cost on any non-weakmap objects.

* Since non-weakmap code doesn't need to test this bit, there is zero
runtime cost on non-weakmap code.

* Whether an object has been used as a key or not (and therefore whether an
extra shadow has been allocated or not), normal non-weak property lookup on
the object is unaffected, and pays no additional cost.



A realistic implementation should seek to avoid allocating the extra shadow
objects. However, even if not, we are much better off with the above scheme
than we are with the current slow WeakMap.

Of course, we should proceed towards realistic implementations asap and get
actual empirical data. But the above demonstration establishes that the
issue in this thread should be considered settled.

-- 
    Cheers,
    --MarkM
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