1471879 - Consider to use nsAutoString in those dictionaries which are only used as params

Status: NEW

(Core :: DOM: Bindings (WebIDL), enhancement, P3)

Description

[Olli Pettay [:smaug][[e-mail]]]

See https://perfht.ml/2lComBg as an example. We allocate the string, and allocation is slow.

Comment 1

[Olli Pettay [:smaug][[e-mail]]]

Or could we use FakeStrings?

Comment 2

[Boris Zbarsky [:bzbarsky]]

Hmm.

So we don't want to expose FakeString too much to consumers; it's a binding implementation detail that has to be used a bit carefully.  I guess we already do via unions to some extent...

This used to not be ok because FakeString was a dependent string, but I guess nowadays it typically isn't when coming from JS.

Doing this for dictionaries in general is not too hard.  Doing this only for ones used as args would require a bit more work; we don't track that right now.

Comment 3

[Boris Zbarsky [:bzbarsky]]

So why the "only used as params" limitation?  Is the idea to exclude return values?  Or exclude members of other dictionaries, or sequence elements, or something else?

Comment 4

[Olli Pettay [:smaug][[e-mail]]]

Just thinking if return value objects might be stored as member variables or something.
Basically want to somehow try avoid higher memory usage, if the object might be heap allocated.
Not sure what would be the best way for that.