810715 - De-lazify a JSFunction's script before querying its ndefaults value

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Till Schneidereit [:till]]

To support default parameters in functions with lazily created bytecode, the ndefaults field has to move to JSFunction. This is because fooFunction.length can be called before fooFunction is invoked the first time and thus de-lazified.

From a quick look at all the sites that set ndefaults, it looks like they all have access to the containing JSFunction and could set the field on that just as well.

Comment 1

[Mozilla RelEng Bot]

Try run for b38b02bc6579 is complete.
Detailed breakdown of the results available here:
    https://tbpl.mozilla.org/?tree=Try&rev=b38b02bc6579
Results (out of 1 total builds):
    failure: 1
Builds (or logs if builds failed) available at:
http://ftp.mozilla.org/pub/mozilla.org/firefox/try-builds/[e-mail]-b38b02bc6579

Comment 2

[Till Schneidereit [:till]]

Attachment: Add ndefaults to JSFunction to enable using Function.length on lazy functions. r=?

This adds an ndefaults field to JSFunction and keeps it in sync with the one on JSScript. Removing ndefaults from the script isn't feasible since, as opposed to what I said in comment 0, we don't always have a JSFunction available when setting ndefaults, after all.

Comment 3

[Luke Wagner [:luke]]

Could you remind me why foo.length couldn't first clone the script?

Comment 4

[Till Schneidereit [:till]]

That'd certainly be an option, too. You think that the case of foo.length being queried without foo being called will be too rare to bother about the needless clone?

Comment 5

[Luke Wagner [:luke]]

Yeah, I wouldn't worry about trying to delay the clone; it stands to reason that if they are querying the length they are about to (or just did) call the function.

Comment 6

[Till Schneidereit [:till]]

Attachment: De-lazify a JSFunction's script before querying its ndefaults value. r=?

You're right, of course: Going to any lengths at all to prevent cloning here is just plain silly.

This makes the new patch rather boring. To spice it up a little, I fixed an unrelated call to getOrCreateScript where the result wasn't checked. Hope that's ok.

Comment 7

[Luke Wagner [:luke]]

Comment on attachment 707524 [details] [diff] [review]
De-lazify a JSFunction's script before querying its ndefaults value. r=?

Review of attachment 707524 [details] [diff] [review]:
-----------------------------------------------------------------

Great!

Comment 8

[Till Schneidereit [:till]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/ec019c0fc63a

Comment 9

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/ec019c0fc63a