Template site objects and WeakMap

Yusuke SUZUKI utatane.tea at gmail.com
Wed Jun 17 05:29:41 UTC 2015


Thanks. And sorry for the late reply.

On Wed, Jun 17, 2015 at 11:31 AM, Mark S. Miller <erights at google.com> wrote:

> Hi Yusuke, I am not sure I understood your message. Could you show some
> example code that would observe the observable difference you have in mind?
>
>
>
> On Tue, Jun 16, 2015 at 7:25 PM, Yusuke SUZUKI <utatane.tea at gmail.com>
> wrote:
>
>> Hi forks,
>>
>> In ES6 spec, template site objects are strongly referenced by the
>> realm.[[templateMap]].
>> So naive implementation leaks memory because it keeps all the site
>> objects in the realm.
>>
>
To lookup the identical template site objects, template site objects are
stored in the realm.[[templateMap]].
So they are strongly referenced and the naive implementation leaks memory.

// By writing the following code, we can leak memory that GC cannot collect.

function tag(siteObject)
{
    return siteObject;
}

for (var i = 0;; ++i) {
    eval("tag`" + i + "`");
}


>> However, we can alleviate this situation.
>> Because template site objects are frozen completely, it behaves as if
>> it's a primitive value.
>> It enables the implementation to reference it from the realm weakly. When
>> all disclosed site objects are not referenced, we can GC them because
>> nobody knows the given site object is once collected (& re-generated).
>>
>
By implementing the realm.[[templateMap]] as WeakMap, we can alleviate this
situation.

function tag(siteObject) {
    // Since siteObject is frozen, we cannot attach a property to it.
    // So if nobody has the reference to the siteObject, we can collect
this siteObject since identity can not be tested across already collected &
newly created site object.
}


>
>> But, even if the object is frozen, we can bind the property with it
>> indirectly by using WeakMap.
>> As a result, if the site objects are referenced by the realm weakly,
>> users can observe it by using WeakMap.
>>
>
var map = new WeakMap();
function tag(siteObject) {
    return siteObject;
}

var siteObject = tag`hello`;
map.set(siteObject, true);

gc();  // If realm.[[templateMap]] is implemente by the WeakMap, siteObject
will be collected.

var siteObject = tag`hello`;

map.get(siteObject);  // false, but should be true.



>
>> To avoid this situation, we need to specially handle template site
>> objects in WeakMap; WeakMap refers template site objects strongly (if we
>> choose the weak reference implementation for realm.[[templateMap]]).
>> But this may complicate the implementation and it may prevent
>> implementing WeakMap as per-object table (it can be done, but it is no
>> longer simple private symbols).
>>
>
var map = new WeakMap();
function tag(siteObject) {
    return siteObject;
}

tag`hello`;

gc();  // siteObject can be collected because there's no reference to it if
the [[templateMap]] is implemented as WeakMap.

var siteObject = tag`hello`;

map.set(siteObject, true);  // To avoid the previously described situation,
WeakMap specially handles the siteObject. It is now refereneced strongly by
the WeakMap.

gc();

var siteObject = tag`hello`;

map.get(siteObject);  // true

// And if WeakMap is collected, siteObject can be collected.


>
>> Is it intentional semantics? I'd like to hear about this.
>> (And please point it if I misunderstood)
>>
>> Best Regards,
>> Yusuke Suzuki
>>
>> _______________________________________________
>> es-discuss mailing list
>> es-discuss at mozilla.org
>> https://mail.mozilla.org/listinfo/es-discuss
>>
>>
>
>
> --
>     Cheers,
>     --MarkM
>
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