561218 - [meta] JM: Improve performance of global variables

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Comment 1

[David Anderson [:dvander] - inactive, e-mail if emergency]

The plan currently is:

 1. Non-intrusively improve front-end binding of global variables.
 2. Investigate why and when functions cross globals, and find a way
    to pin scripts to a given global object.
 3. Use this information in the method JIT to improve global access.

Comment 2

[Brendan Eich [:brendan]]

[4:03pm]brendan:dmandelin, dvander: functions are statically scoped in js -- no change of global object identity for a given cloned (unjoined) function object
[4:03pm]brendan:the function's script of course is shared among all clones and the joined funobj
[4:03pm]dvander:brendan, the script's global changing is what we're worried about
[4:03pm]brendan:and not affine with any particular global in general -- but compile-n-go helps a lot
[4:04pm]dvander:we'd like to be able to JIT the script for the global it's first run on
[4:04pm]brendan:dvander: compile-n-go-created scripts (top-level and function) will all be scoped by a single global
[4:04pm]dvander:and not have to worry about invalidating
[4:04pm]brendan:dvander: the precompilation ("brutal sharing") cases are the opposite
[4:04pm]dvander:brendan, when do those cases come up?
[4:05pm]brendan:dvander: xul scripts
[4:05pm]brendan:dvander: js components too, iirc
[4:05pm]brendan:xbl
[4:05pm]brendan:bz back yet?
[4:05pm]brendan:dvander: look for COMPILE_N_GO in dxr
[4:05pm]dmandelin:what about JS_ExecuteScript being called with different globals?
[4:05pm]brendan:dmandelin: that is one of the APIs exposed for precompilation, yeah
[4:06pm]brendan:dmandelin: <script> tags are compile-n-go tho -- JS_EvaluateUCScriptForPrincipals iirc
[4:06pm]brendan:that's the case to focus on
[4:06pm]brendan:this xul stuff is second order :-P

Comment 3

[David Mandelin [:dmandelin]]

I had already summarized this discussion before the paste, so here goes:

We went over question 2 above on IRC. For posterity:

Running a script via Evaluate*Script*() compiles in COMPILE_N_GO mode, which means the caller promises to run the script with only one global. This currently enables some front-end optimizations. Browser content JS and shell JS is always run in this mode.

Our plan is that for COMPILE_N_GO scripts, we optimize globals to the max. Otherwise, we will compile the safe thing, namely to walk the scope chain up to the global object.

Comment 4

[David Anderson [:dvander] - inactive, e-mail if emergency]

This was done on JM a while ago.