Intent to unship: asm.js optimizations

[Ryan Hunt]

As of Firefox 148 I intend to disable asm.js optimizations for all platforms.

asm.js [1] is the predecessor to WebAssembly, and has been supported since

Firefox 22 [2].

Removing asm.js optimizations will simplify our compilers and runtime,

reducing our attack surface for security bugs and improve developer velocity.

asm.js is a subset of JS code, and so all asm.js content will continue to work

using our normal JS optimizations. Performance of asm.js content may change

as a result.

We observe about 0.2% of page loads contain an asm.js script. We've conducted

manual tests and a release A/B experiment on the performance impact and

believe it to be acceptable.

Users of asm.js can upgrade to use WebAssembly for improved performance,

code size, and stability.

Bug to disable: https://bugzilla.mozilla.org/show_bug.cgi?id=2002635

Bug to remove code (for a later release): https://bugzilla.mozilla.org/show_bug.cgi?id=ragnarok

[1] asmjs.org

[2] https://website-archive.mozilla.org/www.mozilla.org/firefox_releasenotes/en-us/firefox/22.0/releasenotes/?redirect_source=mozilla-org