On Thu, Mar 3, 2016 at 7:48 PM, 'Robert Eisele' via v8-users <
v8-users@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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