Explicit compile hints

[marjakh]

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WebKittens

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Title of the spec

Explicit compile hints

URL to the spec

https://explainers-by-googlers.github.io/explicit-javascript-compile-hints-file-based/

URL to the spec's repository

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Issue Tracker URL

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Explainer URL

https://docs.google.com/document/d/19xTAM4A75tz0xUq_velMzGA4JHEgXpyflUxXTcuNiyE

TAG Design Review URL

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Mozilla standards-positions issue URL

https://github.com/mozilla/standards-positions/issues/780

WebKit Bugzilla URL

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Radar URL

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Description

Knowing which JavaScript functions to parse & compile upfront can speed up web page loading.

Some example numbers from our experiments:

• Data set of 4 big web pages, local cold runs: Parse + compile time decreases 4 - 13 %, foreground parse + compile time decreases 59 - 86%.
• Internal web devs report 5% improvement in their page load time metrics w/ PIFEing all functions in core file.

Currently, V8 uses the PIFE heuristic to direct which functions to compile. This heuristic has existed for a long time and is well known to web developers. V8 and SpiderMonkey follow the PIFE heuristic, but JavaScriptCore doesn't.

Using PIFEs for transmitting information about which functions should be eager-compiled has downsides, though. Especially:

• using it forces using function expressions instead of function declarations; this might regress in all browsers, and for browsers which don't follow the PIFE hint there's no upside
• it cannot be applied to ES6 class methods

Thus, we want to experiment with other ways to transmit compile hints.

We don't want to cause extra work for browsers who don't want to support compile hints; we'll only explore options where ignoring the hints happens naturally.

We're currently starting early experiments with potential users, and will create the actual spec based on feedback.