Also ES6 is mostly a burden for JS implementations, not much of an
opportunity. We have to invest some serious effort just to *maintain* previous
performance as we're implementing ES6, and in some cases not even that is
possible.
On Fri, Mar 4, 2016 at 3:49 PM, 'Andreas Rossberg' via v8-users <
v8-users@googlegroups.com> wrote:
> There is no refcount in JS implementation, and no affordable way to
> discover aliasing dynamically. And given the gazillion intersession and
> mutation points in JavaScript's semantics, it is also practically
> impossible to analyse aliasing at compile time, in all but the most trivial
> cases. Note also that the callback itself may have or acquire references to
> the same object as A and look at them during the execution of map itself.
>
> On 4 March 2016 at 15:34, 'Robert Eisele' via v8-users <
> v8-users@googlegroups.com> wrote:
>
>> Sounds reasonable, but wouldn't it make sense to make this pattern
>> matching based on internal references (plus refcount) instead of
>> variable-name matching? That would solve the issue probably.
>>
>> Above that, I agree that the limiting factors are based on function calls
>> if it's implemented in a naive way. However, I think ES6 gives a huge
>> opportunity introducing a lot of high-level semantic information, which can
>> be used to infer what the user wants and execute it much faster. So even if
>> the user writes down a function call, it can be a simple loop inside of v8.
>> I thought it's already done this way, when I heard ES6 became much faster
>> recently.
>>
>> On Friday, March 4, 2016 at 11:34:51 AM UTC+1, Jakob Kummerow wrote:
>>>
>>> On Thu, Mar 3, 2016 at 7:48 PM, 'Robert Eisele' via v8-users <
>>> v8-u...@googlegroups.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> I wonder if v8 is able to optimize the pattern
>>>>
>>>> A = A.map(x => ~x)
>>>>
>>>> In this case v8 can work on the array instead of creating a new object
>>>> and replacing it with the original array.
>>>>
>>>
>>> Counter-example:
>>> var A = B;
>>> A = A.map(...);
>>> // B must not have been modified
>>>
>>> You can make these as complicated as you want:
>>> var B = [1, 2, 3];
>>> (function(A) {
>>> A = A.map(...); // How can this .map() call know that B exists?
>>> })(B);
>>>
>>> In general, it's pretty much impossible to tell whether there are any
>>> additional references to an object somewhere; the only exception is when
>>> the object has just been allocated in the current function.
>>>
>>> Also, on the scale of machine instructions, the overhead of a function
>>> call is pretty large, which affects many of the Array builtins like .map()
>>> and .forEach(). In many real usage scenarios, that might not matter,
>>> because the arrays in question are small enough and/or the callbacks
>>> themselves expensive enough that call overhead doesn't make much of a
>>> difference. But when you micro-benchmark simple cases like "A.map(x =>
>>> ~x)" vs. "for (var i = 0; i < A.length; i++) { A[i] = ~A[i]; }" for a
>>> couple million elements, you'll see a pretty big difference, and the
>>> majority of that will be caused by the function calls.
>>>
>>>
>>>> Is such a behavior already implemented?
>>>>
>>>
>>> No, because it's complicated, and the benefit is somewhat unclear. We
>>> might be able to optimize some patterns in the (distant) future.
>>>
>>>
>>>> Thanks!
>>>>
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