is an iterator allowed to reuse the same "state" object?
rossberg at google.com
Wed Apr 29 12:36:05 UTC 2015
On 29 April 2015 at 02:21, John Lenz <concavelenz at gmail.com> wrote:
> I missed it, thanks. I know things will improve in time but I'm just
> coming from a discussion with folks complaining about the performance of
> generators and GC overhead in real code with Chrome and Firefox relative to
> simple hand written loops.
Note that there is a huge difference between optimising iterators (in
particular, for-of), and optimising generators. I expect that VMs will
start getting better on the former relatively soon, for iterators that are
not generators. Making generators fast is a much more complicated problem.
On Tue, Apr 28, 2015 at 4:28 PM, Allen Wirfs-Brock <allen at wirfs-brock.com>
>> On Apr 28, 2015, at 4:21 PM, John Lenz wrote:
>> You would hope that the engines might be able to create these objects on
>> the stack but I don't think anyone does that yet and the result is a flood
>> of eden objects.
>> I would like to know I'm wrong about this.
>> did you see
>> A really good optimizing jit should be able to inline the 'next' call,
>> recognize that the IterationResult object doesn't escape (it knows this for
>> for-of loops) and not do an allocation at all.
>> On Apr 27, 2015 4:59 PM, "Allen Wirfs-Brock" <allen at wirfs-brock.com>
>>> On Apr 27, 2015, at 3:29 PM, Tab Atkins Jr. wrote:
>>> On Mon, Apr 27, 2015 at 3:11 PM, John Lenz <concavelenz at gmail.com>
>>> By which I mean the object that returns the current value and "done"
>>> IIRC, it's not supposed to. The built-in iterators will return fresh
>>> objects each time, so there's no mutation hazard. Userland iterators
>>> can of course violate this, but at their peril.
>>> Well, that's not exactly what the ES2015 spec. says. The specification
>>> of the Iterator interface (
>>> http://people.mozilla.org/~jorendorff/es6-draft.html#sec-iterator-interface )
>>> does not require that the `next` method return a fresh object each time it
>>> it called. So a userland iterator would not be violating anything by
>>> reusing a result object.
>>> However, the specifications for all ES2015 built-in iterators require
>>> that they return fresh objects.
>>> None of the built-in consumers of the Iterator interface (for-of,
>>> Array.from, etc.) retain references to IteratorResult objects after testing
>>> for `done` and accessing the `value`, so semantically they don't care
>>> whether the ResultObject is reused. However, such reuse might preclude some
>>> otherwise plausible engine level optimizations.
> es-discuss mailing list
> es-discuss at mozilla.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the es-discuss