664784 - Iterators are very slow compared to doing a for loop on an array

Status: NEW

(Core :: JavaScript Engine, defect)

Description

[Boris Zbarsky [:bzbarsky]]

Consider the attached testcase, which tests various ways of enumerating an array.  I get numbers like so:

Wrapped iterator took   147 ms
Iterator took            93 ms
Self-hosted foreach took 16 ms
Array.foreach took       16 ms
'for each' loop took     37 ms

I profiled the wrapped iterator case, and it looks like we spend:

 13% pushing and popping generator frames
 23% in js::Interpret called from js::RunScript called from SendToGenerator
 10% snapshotting for that inner iterator (?)
  5% in SendToGenerator
  3% in generator_op
  3% under pushInvokeArgs
  8% under js_GetMethod called from js_IteratorMore
  3% in js_IteratorMore itself
  9% in JM jitcode

and some smaller bits.

Basically, lots of slow stuff.  Given that chrome code is trying to use iterators, anything we can do to speed things up here?

Comment 1

[Boris Zbarsky [:bzbarsky]]

Attachment: Shell testcase

Comment 2

[Luke Wagner [:luke]]

We haven't really focused on generator perf at all; as in, we don't compile JSOP_YIELD/JSOP_GENERATOR.  About equally important (for small generator bodies) I think would be building a generator IC akin to the Function.prototype.{call,apply} ICs.

I think those are the high-order perf bits, but we could do better by changing the stack layout so that generators don't copy so much state back and forth on each activation/deactivation.