array_values should be a no-op for packed layout arrays

I noticed this commit
<https://github.com/facebook/hhvm/commit/4167d66fcfa81a1c2a1
9e853282fac968d9cd454>
recently in HHVM, which makes array_values() return the same array back if
it's packed instead of building a copy.

My understanding is PHP 7 also has packed arrays like HHVM, and my reading
of PHP's array_values() implementation (here
<https://github.com/php/php-src/blob/c6982995504b6e21e8a5ade
29cfb16a55196dc43/ext/standard/array.c#L4000>)
is that it always creates a copy of the array, even if it's packed.

Would this be a worthwhile optimisation to make in PHP as well?

That would be nice if array_values() wouldn't reindex the array.

But it does.

$a = [12=> 'foo', 42 => 'bar']; // packed array
$b = array_values($a);

$b // [0 => 'foo', 42 => 'bar']; // keys have been changed and are not
kept from source

So, array_values() must allocate a new array, and fill it in with the
values of source, because array_values() reindexes the source array from
key 0 ; whatever was the original array.

Note that the allocation is very light, in PHP 7, we allocate a full buffer
at once , and not one buffer for each slot. The performance inpact of such
an allocation is really really tiny.

Minor nit: [12=>'foo', 42=>'bar'] is not a packed array.

[0=>'foo', 2=>'bar'] is however, so your primary point stands.
However it should be simple enough to detect when a packed array is
also vector-like (indexed from 0 to n-1) and make this minor
optimization.

-Sara