921813 - Arrow functions: Remove "this" binding when "this" not observable in the body.

Status: RESOLVED DUPLICATE of bug 989204

(Core :: JavaScript Engine, defect)

Description

[Asaf Romano (gone)]

Attachment: scratchpad.js

Try the attached script in Scratchpad. I see a constant 10%+ difference (but I'm not sure it actually scales).

Comment 1

[Till Schneidereit [:till]]

Attachment: shell benchmark loosely based on original test

I can confirm the speed difference. However, there are quite a few other constructs in the test that we don't yet fully optimize (i.e., array comprehensions and generators).

I created a version that calculates the same results in a very different way, getting rid of everything but a single list.filter call in the functions under test. This version has a speed difference of 2-3%, when run in the shell.

Comment 2

[Nicolas B. Pierron [:nbp]]

Note that
  x => set.has(x)

is literally equivalent to:
  function (x) { set.has(x) }.bind(this)

and not:
  function (x) { set.has(x) }

We should optimize the arrow functions when "this" is not observable in the body of arrow function.

Comment 3

[Guilherme Lima]

Is this still valid? I get a difference of less than 1% between the two on the second testcase.

Comment 4

[Jan de Mooij [:jandem]]

This has been fixed yes.

http://www.jandemooij.nl/blog/2014/04/11/fast-arrow-functions-in-firefox-31/