1784156 - Add constant GC instructions to wasm::InitExpr

Status: RESOLVED FIXED

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

Description

[Ryan Hunt [:rhunt]]

The GC proposal includes several instructions as 'constant instructions' which means they're valid as initializer expressions in globals [1].

Constant Expressions

In order to allow RTTs to be initialised as globals, the following extensions are made to the definition of constant expressions:

    i31.new is a constant instruction
    struct.new_canon and struct.new_canon_default are constant instructions
    array.new_canon, array.new_canon_default, and array.new_canon_fixed are constant instructions
        Note: array.new_canon_data and array.new_canon_elem are not for the time being, see above
    global.get is a constant instruction and can access preceding (immutable) global definitions, not just imports as in the MVP

So we'll need to update wasm::InitExpr to be able to validate and interpret these instructions.

One interesting addition is that 'global.get' can get preceding immutable global definitions, not just imports.

[1] https://github.com/WebAssembly/gc/blob/main/proposals/gc/MVP.md#constant-expressions

Comment 1

[Ryan Hunt [:rhunt]]

See bug 1784497 for how this may affect tables.

Comment 2

[Ryan Hunt [:rhunt]]

i31.new is blocked on adding i31 support, which we're deferring for now.
array.new_canon_fixed is blocked on the advanced array work Julian is doing in bug 1774840.

The rest should be available to do.

I think "global.get is a constant instruction and can access preceding (immutable) global definitions" should be separated into a different bug, as there is some subtlety to it from a discussion in the last CG meeting.

Comment 3

[Yury Delendik (:yury)]

Attachment: Bug 1784156 - Add struct.new and array.new instructions to wasm::InitExpr. r?rhunt

• add struct.new, .new_default instructions
• add array.new, .new_default, .new_fixed instructions

Comment 4

[Yury Delendik (:yury)]

(In reply to Ryan Hunt [:rhunt] from comment #2)

I think "global.get is a constant instruction and can access preceding (immutable) global definitions" should be separated into a different bug, as there is some subtlety to it from a discussion in the last CG meeting.

Tracking that in bug 1789698

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/0da02291b4a7
Add struct.new and array.new instructions to wasm::InitExpr. r=rhunt

Comment 6

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/0da02291b4a7