1559965 - Threads: Support new fence instruction

Status: RESOLVED FIXED

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

Description

[Lars T Hansen [:lth]]

The Wasm CG meeting in A Coruña in June 2019 voted to add an atomics.fence instruction to the threads proposal, to guard against a compatibility problem that may appear once we add weaker atomics in the future. Issue here: https://github.com/WebAssembly/threads/issues/140

Comment 1

[Lars T Hansen [:lth]]

Spec text: https://github.com/WebAssembly/threads/pull/141

Comment 2

[Lars T Hansen [:lth]]

We want to be sure to get this into ff70.

Comment 3

[Ryan Hunt [:rhunt]]

I've got a first attempt at implementing this working here [1].

The actual semantics of the operation hook into MacroAssembler::memoryBarrier. I had to hook into GlobalValueNumbering a bit to make sure that MWasmFence wasn't discarded from not being used with Ion, and need to verify there's no other places that need to be updated as well. I confirmed from looking at 'codegen' spew that mfence is actually being emitted on x86-64.

There's also a really basic test attached. I'm not sure what a great testing plan looks like at this time.

[1] https://treeherder.mozilla.org/#/jobs?repo=try&revision=ee9af92ff683766c57f1dbfcb902a922b18b29fb

Comment 5

[Lars T Hansen [:lth]]

It shouldn't be necessary to futz with GVN, it should be enough to flag the MIR node as guarded, I think? (Call setGuard() from the constructor.) Curious to know more details.

Comment 6

[Ryan Hunt [:rhunt]]

(In reply to Lars T Hansen [:lth] from comment #5)

It shouldn't be necessary to futz with GVN, it should be enough to flag the MIR node as guarded, I think? (Call setGuard() from the constructor.) Curious to know more details.

Ah, that sounds better. It was late, and I wanted to get it working :) I'll try that out soon.

Comment 7

[Ryan Hunt [:rhunt]]

Attachment: Bug 1559965 - Wasm: Implement new atomic.fence instruction. r?bbouvier

This commit implements the 'atomic.fence' Wasm instruction.

Issue: https://github.com/WebAssembly/threads/issues/140
Overview: https://github.com/WebAssembly/threads/pull/141

The instruction is encoded as, 0xFE 0x03, with a reserved byte trailing for a future
memory order immediate. The instruction is implemented by emitting a memoryBarrier
through the macro assembler.

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/d32c27f13f21
Wasm: Implement new atomic.fence instruction. r=bbouvier

Comment 9

[Cristina Coroiu [:ccoroiu]]

https://hg.mozilla.org/mozilla-central/rev/d32c27f13f21