Allow atomic operations on unshared memories #147
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As discussed in the CG on November 12, 2019 and in WebAssembly#144. Specifically, all atomic accesses are now allowed to validate and execute normally on unshared memories and wait operations trap when used with unshared memories. The PR updates Overview.md and makes a best-effort attempt at updating the spec, making no changes when there is already a TODO for updating spec text. It also adds new TODO comments to the reference interpreter where changes will have to be made.
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Technically, the changes look good to me, modulo comment. Personally, I'm fine with these semantics, but did the CG actually declare consensus on it? The Nov meeting was a bit inconclusive. Now that you re-clarified the options on the issue and everyone had enough time to dwell on it, it may be appropriate to bring it back to the CG to get official sign-off via a poll.
| @@ -440,8 +440,6 @@ Atomic Memory Instructions | |||
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| * Let :math:`\limits~\share` be the :ref:`memory type <syntax-memtype>` :math:`C.\CMEMS[0]`. | |||
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| * The sharedness :math:`\share` must be |MSHARED|. | |||
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You'll also need to adjust the respective premise in the formal rule below to keep it in sync, e.g., by changing C.\CMEMS[0] = \limits~\MSHARED to C.\CMEMS[0] = \memtype (here and elsewhere).
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From previous discussions, my conclusions were
- Allowing plain atomic operations on non-shared memory was uncontroversial
- There are still open questions about Atomics.wait semantics
- It's unclear to me from the notes/discussions whether Atomics.notify always returning 0 was already agreed upon
The semantics as laid out in this PR lgtm, +1 to @rossberg's suggestion of adding explicit polls in an upcoming meeting.
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Sounds good, I'll add a vote to the agenda for the next meeting 👍 |
The proposed semantics can be found in WebAssembly/threads#147
The proposed semantics can be found in WebAssembly/threads#147
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@rossberg I just want to confirm that the latest changes are what you had in mind. |
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@rossberg ping! |
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I'll merge this now, but of course I am happy to fix any mistakes or holes in follow-up PRs. |
Adjust atomics.wait, atomics.notify semantics for when they are used with non-shared Wasm memory to mirror the spec change introduced in: WebAssembly/threads#147. This does not need to be gated by the flag here, as this will only decode if the flag is enabled. Bug: v8:9921 Change-Id: I7f2e018fed6bd131ad4c386def1e838626c28a4d Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2256863 Reviewed-by: Ben Smith <[email protected]> Commit-Queue: Deepti Gandluri <[email protected]> Cr-Commit-Position: refs/heads/master@{#68468}
As discussed in the CG on November 12, 2019 and in #144. Specifically,
all atomic accesses are now allowed to validate and execute normally
on unshared memories and wait operations trap when used with unshared
memories.
The PR updates Overview.md and makes a best-effort
attempt at updating the spec, making no changes when there is
already a TODO for updating spec text. It also adds new TODO comments
to the reference interpreter where changes will have to be made.