threads: add thread.spawn_indirect
#2042
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As discussed in [bytecodealliance#89], this adds support for a new intrinsic, `thread.spawn_indirect`. This new operation would allow spawning a shared function stored in a table via a table index. This leaves some future work undone: - `thread.spawn` could/should be renamed to `thread.spawn_ref` - `thread.spawn_indirect` could/should take the encoding byte from `thread.hw_concurrency`--swap `0x07` for `0x06` - importantly, `thread.spawn_indirect` should gain a field indicating which type to expect in the indirect table; due to current limitations in `wasm-tools`, the locations to check once this is possible are marked with `TODO: spawn indirect types`. [bytecodealliance#89]: WebAssembly/shared-everything-threads#89
[bytecodealliance#447] tries to make the built-in naming a bit more consistent; this change propagates that here. [bytecodealliance#447]: WebAssembly/component-model#447
This encoding change is necessary due to recent additions to the component model; see [bytecodealliance#447]. [bytecodealliance#447]: WebAssembly/component-model#447
Initially I implemented `thread.spawn_indirect` without the ability to check the type of the function to spawn out of an abundance of caution (see the implementation issues described in [bytecodealliance#89]). In the process of writing out the specification, we convinced ourselves that these problems should not apply to `thread.spawn_indirect`. This change adds the function type index necessary for doing some extra validation of `thread.spawn_indirect` and adds some tests related to this. One unimplemented TODO is what to do about shared tables: technically, the table used by a `thread.spawn_indirect` should be shared but we have so far prevented this in the component model; this can be resolved later, though. [bytecodealliance#89]: WebAssembly/shared-everything-threads#89
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@alexcrichton, I rebased this on top of |
| // TODO: check that the table is shared once components allow shared | ||
| // tables. |
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Is this TODO still applicable?
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Yeah, I added it because we still prevent shared tables from being used in components. At some point we will have to relax that but it seems like a separate PR. Speaking of: when can or should we relax that? We have this not-accepted.wast file that gates some of this but I can't fully remember why we added it:
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Oh that was due to #1970. What I added there was a very coarse version of "you can't lower into a 64-bit memory".
What really needs to happen is that every location that uses a memory (or table) needs to perform a subtype check on what the actual table/memory received is. For example all canonical options referring to a memory have to refer to a non-shared 32-bit linear memory.
Basically #1970 was a coarse over-approximation that didn't break anyone "on stable". To remove the logic from #1970 we'll need to audit usage of memory_at and table_at to ensure everything is doing a subtype check as appropriate.
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(sorry about all the interim changes but thanks for rebasing over them!) |
This propagates the upstream spec changes to the component model in #447 here. See commit messages for more details.