1805149 - Delay the bytecode de-duplication to the instantiation step

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P1)

Description

[Tooru Fujisawa [:arai]]

Currently SharedImmutableScriptData::shareScriptData happens inside BytecodeEmitter::intoScriptStencil, but this operation uses per-runtime storage JSRuntime::scriptDataTable_.

In order to decouple frontend from JSContext/JSRuntime, while keeping the cache "per-runtime", this operation needs to be moved to instantiation step.

The downside of this change are:

• the duplicated data is kept until instantiate
• if single stencil is instantiated multiple times, the de-duplication operation will be performed for each instantiation

Comment 1

[Ted Campbell [:tcampbell]]

Unfortunately, the lock for the scriptDataTable is slightly complicated to avoid needing to take a lock on main-thread when no off-thread parse is in flight. https://searchfox.org/mozilla-central/rev/2d24d893669ad0fe8d76b0427b25369d35fcc19b/js/src/vm/Runtime.h#624-632 . You'll probably have to just try it out and measure performance impact like you suggested.

My guess is that the data size isn't likely a problem. Some mitigations we could do are when we do first instantiate, we can deduplicate in-place in the stencil so that if the stencil lives longer it doesn't waste space. Although that may be complex if a stencil is shared with Workers. It is probably possible to do a per-compilation de-duplication that catches small functions that are likelier to share code.

Computing the HashNumber off-thread might also be useful. It may even make sense to save it in ImmutableScriptData.

Comment 2

[Tooru Fujisawa [:arai]]

Attachment: Bug 1805149 - Part 1: Perform SharedImmutableScriptData::shareScriptData during instantiation. r?tcampbell!

Depends on D164333

Comment 3

[Tooru Fujisawa [:arai]]

Attachment: Bug 1805149 - Part 2: Pre-calculate ImmutableScriptData hash during compilation. r?tcampbell!

Depends on D164555

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/343425669f57
Part 1: Perform SharedImmutableScriptData::shareScriptData during instantiation. r=tcampbell
https://hg.mozilla.org/integration/autoland/rev/ac0c7c9bbd8e
Part 2: Pre-calculate ImmutableScriptData hash during compilation. r=tcampbell

Comment 5

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/343425669f57
https://hg.mozilla.org/mozilla-central/rev/ac0c7c9bbd8e