505898 - TM: Remove local root scopes and NewGCThing pigeonhole

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[Andreas Gal :gal]

We currently have 3 ways of temporary rooting newborn JSGCThings:

- pigeon hole (we might rely on this in some places, unclear)
- LocalRootStack (used in JSXML only)
- JSAutoTempValueRooter (the new shiny thing we want to keep)

We currently pay a price of the pigeon hole and the LRS in the allocator path, and we should get rid of that overhead. Ideally we should do some static analysis that proves that we have all the right JSAutoTempValueRooters in place, and then rip out the old constructs.

Comment 1

[Brendan Eich [:brendan]]

Newborn roots (pigeon hole) are going, they are broken as designed.

LocalRootStack protects newborns but not oldborns who may become disconnected while a native is running, and then a GC nests.

Ergo JSAutoTempValueRooter (new name, public API if possible) is the way.

See bug 421934 and bug 353231 for static analysis work to find all extant rooting bugs. The analysis AFAIK would ignore newborn roots, but until we have it, we cannot assume newborns are not needed, even though they're hazardous as hell.

/be

Comment 2

[Igor Bukanov]

(In reply to comment #0)
> We currently pay a price of the pigeon hole and the LRS in the allocator path,
> and we should get rid of that overhead.

For compatibility the rooting can be done when JS API returns GC things.

Comment 3

[Andreas Gal :gal]

Yeah, #2 is exactly on point I think. We can take the overhead in JSAPI and that would probably help static analysis a lot since we can restrict it to the engine itself.

Comment 4

[Andreas Gal :gal]

This bug was obsoleted by 516832. With 516832 fixed we can now eliminate all 3 rooting methods.

Comment 5

[Igor Bukanov]

(In reply to comment #4)
> This bug was obsoleted by 516832. With 516832 fixed we can now eliminate all 3
> rooting methods.

Both newborns and local roots cannot be just eliminated since the conservative scanning covers just the stack. Thus any elimination of these roots must ensure that the code does not rely on these roots for GC things stored in malloced structures, not on the native stack.