572031 - JM: With fatvals+tracing, specialize unboxing numbers from getelem when it helps

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[David Mandelin [:dmandelin]]

See bug 572029 for background. Even after the fix for that bug, we can still get another 15-20ms win on crypto and fannkuch if we are smart about doing specialized unboxing from getelem. The key is being able to predict when specializing for int or double is a good idea. Ideas we've come up with:

1. Start by fully specializing. Keep a bit per trace tree (or maybe script) that indicates if we've misspeculated after a getelem. Once it is set, use the common number path instead. This limits the damage to 1 per tree (or script).

2. Try to predict by looking ahead in the bytecode stream. If we can find the consumer of the result of the getelem, we can make a good guess. If the consumer is a setelem, we should use the common path, because that enables the box-after-unbox optimization. If the consumer is a bitop and the value is an int, we can guess that that is stable. Further empirical observation would tell us what to do for other cases.

3. Wait for type-specialized arrays, which may just make this problem go away.

Comment 1

[Nicholas Nethercote [inactive]]

(In reply to comment #0)
> 
> 3. Wait for type-specialized arrays, which may just make this problem go away.

Yeah, I was about to pipe up and suggest that... :)

Comment 2

[Ryan VanderMeulen [:RyanVM]]

Obsolete with the removal of tracejit.