1863609 - Optimize away a ref.cast that is dominated by a ref.test

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Ryan Hunt [:rhunt]]

For a snippet like:

if (x instanceof Foo) {
  (Foo)x
}

Compilers might generate a ref.test for the conditional and a ref.cast for the interior. We should be able to optimize away the second type check. We may already do this in MIR, but it'd be good to double check.

Comment 1

[Ben Visness [:bvisness]]

Attachment: Bug 1863609: Enable GVN of wasm subtype checks. r=rhunt

This patch also enables GVN of redundant MTest instructions, where an
MTest is dominated by either the true or false path of a previous MTest
on the same value. This can remove some tests entirely, such as the
trapping path from the cast instruction suggested in the bug.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/41ece3527411
Enable GVN of wasm subtype checks. r=rhunt

Comment 3

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

https://hg.mozilla.org/mozilla-central/rev/41ece3527411