565729 - Implement a specialized array iterator

Status: RESOLVED DUPLICATE of bug 505818

(Core :: JavaScript Engine, defect)

Description

[Andreas Gal :gal]

a = [...]
for (i in a) ... a[i] ...

stringifies i, only to apply GETELEM on a using i, which turns it into an integer index again (in particular for dense arrays).

Creating the native iterator is also mildly inefficient since we make a vector of numbers, basically: iterator = [id(0), id(1), id(2) ...].

Instead, I will do the following:
- add a native ArrayIteratorClass object
- it has no malloced part, just a cursor and a length
- FOR* pull from the iterator the next id (cursor) until length, so on the trace this is a simple integer load
- the integer is stringified
- GETELEM sees the stringify call, and will peek through it, use the number directly, generating very efficient code

As a follow-up bug, we will eliminate the stringification. Nick is already working on guarded calls that can be eliminated (Nick, can you link the bug?) when they are dead. Thats needed to get read of the i2s call (its dead, but the guard keeps it alive for now).

Comment 1

[Nicholas Nethercote [inactive]]

(In reply to comment #0)
> 
> As a follow-up bug, we will eliminate the stringification. Nick is already
> working on guarded calls that can be eliminated (Nick, can you link the bug?)
> when they are dead.

Conditionally-live guards are in bug 560994.

Is eliminate-the-stringification in bug 560995?