1804310 - Implement memory.discard instruction

Status: RESOLVED FIXED

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

Description

[Ryan Hunt [:rhunt] (on leave until early May)]

Implement the memory.discard [1] in Ion/Baseline. Both of them will likely just perform a call to a builtin to do the real work.

The semantics of this instruction are not firmly pinned down yet, we can make something up or coordinate with V8 to see if they've chosen something yet. A reasonable start would be memory.discard $memIndex : [i32:offset, i32:length] -> [].

The idea for the instruction is roughly a semantic zero'ing of some range of memory, with the hint provided to the OS that this memory is not likely needed anytime soon. The OS would likely free the backing pages then, and provide a fresh zero-page back on-demand if the memory is touched in the future.

For Linux (OSX too?), this would be done with madvise(MADV_DONTNEED)
For Windows, this might be VirtualFree(MEM_DECOMMIT);VirtualAlloc(MEM_COMMIT). We'll need to double check the atomicity of that for threading.

[1] https://github.com/WebAssembly/memory-control/blob/main/proposals/memory-control/Overview.md#proposed-changes

Comment 1

[Ben Visness [:bvisness]]

Attachment: Bug 1804310: Implement memory.discard wasm instruction. r=rhunt

memory.discard stuff from windows

Rearrange discard code (and scaffold for M64)

Get the JS API working, bounds checks and all

Visual cleanup

Comment 2

[Ben Visness [:bvisness]]

Attachment: Bug 1804310: Add wasm memory control pref. r=rhunt

Depends on D167324

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/73c05bcb4c1f
Implement memory.discard wasm instruction. r=rhunt,supply-chain-reviewers
https://hg.mozilla.org/integration/autoland/rev/893e8d9462ad
Add wasm memory control pref. r=rhunt

Comment 4

[Cristina Horotan [:chorotan]]

https://hg.mozilla.org/mozilla-central/rev/73c05bcb4c1f
https://hg.mozilla.org/mozilla-central/rev/893e8d9462ad