570955 - VM could optimize activation objects

Status: RESOLVED WONTFIX

(Tamarin Graveyard :: Baseline JIT (CodegenLIR), defect, P3)

Description

[Edwin Smith]

ASC creates activation objects eagerly when:
  - there are nested functions
  - there are try catch blocks
  - there are with blocks.

Typically activation objects only escape via OP_newclass or OP_newfunction.  They are either accessed implicitly by getslot/setslot or implicitly via OP_findproperty.

If all property access is identifiable, then
 - some properties could be scalarized if not accessed by OP_findprop* or nested functions
 - if all properties are scalarized then the object doesn't need to exist.

Comment 1

[Edwin Smith]

In some cases we might be able to delay/defer activation object creation, if the object is only accessed along some control flow paths.  need data on whether this is frequent in relevant apps.

Comment 2

[Edwin Smith]

Also see the conservative analyzer for REST arguments.  Something like that could work for activation objects.  ASC creates empty activation objects for code with try/catch blocks, and that pattern should be easy to recognize.  (even maybe only running the analyzer for methods with exceptions, to not impact jit time much).

See bug 571468 and dependents for the rest analyzer discussion.

Comment 3

[Tom Harwood]

There is an important trivial case: an empty catch block generates an activation object, and this generates noticeable load in Flex startup.

Comment 4

[Scott Petersen]

Jeff Dyer has a prototype implementation that deals with the empty catch block case. Handling that case alone speeds up launch time of a Flex app I've been experimenting with by about 2.5%

Comment 5

[Scott Petersen]

In case it wasn't clear, Jeff's prototype is in ASC (as opposed to the VM.)

Comment 6

[Jeff Dyer]

related ASC bug: https://bugs.adobe.com/jira/browse/ASC-4109

Comment 7

[Lars T Hansen]

Closure optimization of functions not used for their value: https://bugs.adobe.com/jira/browse/ASC-4122.

Comment 8

[Lars T Hansen]

Avoiding creating activation objects in the presence of closure creation, when those activation objects are not captured: https://bugs.adobe.com/jira/browse/ASC-4123

Comment 9

[Lars T Hansen]

Avoiding placing non-captured variables in activation records: https://bugs.adobe.com/jira/browse/ASC-4124

Comment 10

[Sylvestre Ledru [:Sylvestre]]

Tamarin is a dead project now. Mass WONTFIX.

Comment 11

[Sylvestre Ledru [:Sylvestre]]

Tamarin isn't maintained anymore. WONTFIX remaining bugs.