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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>>
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