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# Wasm Proposals
This document is intended to describe the current status of WebAssembly
proposals in Wasmtime. For information about implementing a proposal in Wasmtime
see the [associated
documentation](./contributing-implementing-wasm-proposals.md).
WebAssembly proposals that want to be [tier 2 or above](./stability-tiers.md)
are required to check all boxes in this matrix. An explanation of each matrix
column is below.
## On-by-default proposals
| Proposal | Phase 4 | Tests | Finished | Fuzzed | API | C API |
|--------------------------|---------|-------|----------|--------|-----|-------|
| [`mutable-globals`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`sign-extension-ops`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`nontrapping-fptoint`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`multi-value`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`bulk-memory`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`reference-types`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`simd`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`component-model`] | ❌[^1] | ✅ | ✅ | ⚠️[^2] | ✅ | ❌[^5]|
| [`relaxed-simd`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`multi-memory`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`threads`] | ✅ | ✅ | ✅[^9] | ❌[^3] | ✅ | ✅ |
| [`tail-call`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`extended-const`] | ✅ | ✅ | ✅ | ✅ | ✅ | ✅ |
[^1]: The `component-model` proposal is not at phase 4 in the standardization
process but it is still enabled-by-default in Wasmtime.
[^2]: Various shapes of components are fuzzed but full-on fuzzing along the
lines of `wasm-smith` are not implemented for components.
[^3]: Fuzzing with threads is an open implementation question that is expected
to get fleshed out as the [`shared-everything-threads`] proposal advances.
[^5]: Support for the C API for components is desired by many embedders but
does not currently have anyone lined up to implement it.
[^9]: There are [known
issues](https://github.com/bytecodealliance/wasmtime/issues/4245) with
shared memories and the implementation/API in Wasmtime, for example they
aren't well integrated with resource-limiting features in `Store`.
Additionally `shared` memories aren't supported in the pooling allocator.
## Off-by-default proposals
| Proposal | Phase 4 | Tests | Finished | Fuzzed | API | C API |
|--------------------------|---------|-------|----------|--------|-----|-------|
| [`memory64`] | ❌ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`function-references`] | ✅ | ✅ | ❌ | ❌ | ✅ | ❌ |
| [`gc`] [^6] | ✅ | ✅ | ❌[^7] | ❌ | ✅ | ❌ |
| [`wide-arithmetic`] | ❌ | ✅ | ✅ | ✅ | ✅ | ✅ |
| [`custom-page-sizes`] | ❌ | ✅ | ✅ | ✅ | ✅ | ❌ |
[^6]: There is also a [tracking
issue](https://github.com/bytecodealliance/wasmtime/issues/5032) for the
GC proposal.
[^7]: The implementation of GC has [known performance
issues](https://github.com/bytecodealliance/wasmtime/issues/9351) which can
affect non-GC code when the GC proposal is enabled.
## Unimplemented proposals
| Proposal | Tracking Issue |
|-------------------------------|----------------|
| [`branch-hinting`] | [#9463](https://github.com/bytecodealliance/wasmtime/issues/9463) |
| [`exception-handling`] | [#3427](https://github.com/bytecodealliance/wasmtime/issues/3427) |
| [`flexible-vectors`] | [#9464](https://github.com/bytecodealliance/wasmtime/issues/9464) |
| [`memory-control`] | [#9467](https://github.com/bytecodealliance/wasmtime/issues/9467) |
| [`stack-switching`] | [#9465](https://github.com/bytecodealliance/wasmtime/issues/9465) |
| [`shared-everything-threads`] | [#9466](https://github.com/bytecodealliance/wasmtime/issues/9466) |
[`mutable-globals`]: https://github.com/WebAssembly/mutable-global/blob/master/proposals/mutable-global/Overview.md
[`sign-extension-ops`]: https://github.com/WebAssembly/spec/blob/master/proposals/sign-extension-ops/Overview.md
[`nontrapping-fptoint`]: https://github.com/WebAssembly/spec/blob/master/proposals/nontrapping-float-to-int-conversion/Overview.md
[`multi-value`]: https://github.com/WebAssembly/spec/blob/master/proposals/multi-value/Overview.md
[`bulk-memory`]: https://github.com/WebAssembly/bulk-memory-operations/blob/master/proposals/bulk-memory-operations/Overview.md
[`reference-types`]: https://github.com/WebAssembly/reference-types/blob/master/proposals/reference-types/Overview.md
[`simd`]: https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md
[`tail-call`]: https://github.com/WebAssembly/tail-call/blob/main/proposals/tail-call/Overview.md
[`branch-hinting`]: https://github.com/WebAssembly/branch-hinting
[`exception-handling`]: https://github.com/WebAssembly/exception-handling
[`extended-const`]: https://github.com/WebAssembly/extended-const
[`flexible-vectors`]: https://github.com/WebAssembly/flexible-vectors
[`memory-control`]: https://github.com/WebAssembly/memory-control
[`stack-switching`]: https://github.com/WebAssembly/stack-switching
[`shared-everything-threads`]: https://github.com/WebAssembly/shared-everything-threads
[`memory64`]: https://github.com/WebAssembly/memory64/blob/master/proposals/memory64/Overview.md
[`multi-memory`]: https://github.com/WebAssembly/multi-memory/blob/master/proposals/multi-memory/Overview.md
[`threads`]: https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md
[`component-model`]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md
[`relaxed-simd`]: https://github.com/WebAssembly/relaxed-simd/blob/main/proposals/relaxed-simd/Overview.md
[`function-references`]: https://github.com/WebAssembly/function-references/blob/main/proposals/function-references/Overview.md
[`wide-arithmetic`]: https://github.com/WebAssembly/wide-arithmetic/blob/main/proposals/wide-arithmetic/Overview.md
[`gc`]: https://github.com/WebAssembly/gc
[`custom-page-sizes`]: https://github.com/WebAssembly/custom-page-sizes
## Feature requirements
For each column in the above tables, this is a further explanation of its meaning:
* **Phase 4** - The proposal must be in phase 4, or greater, of [the
WebAssembly standardization process][phases].
* **Tests** - All spec tests must be passing in Wasmtime and where appropriate
Wasmtime-specific tests, for example for the API, should be passing. Tests
must pass at least for Cranelift on all [tier 1](./stability-tiers.md)
platforms, but missing other platforms is otherwise acceptable.
* **Finished** - No open questions, design concerns, or serious known bugs. The
implementation should be complete to the extent that is possible. Support
must be implemented for all [tier 1](./stability-tiers.md) targets and
compiler backends.
* **Fuzzed** - Has been fuzzed for at least a week minimum. We are also
confident that the fuzzers are fully exercising the proposal's functionality.
The `module_generation_uses_expected_proposals` test in the `wasmtime-fuzzing`
crate must be updated to include this proposal.
> For example, it would *not* have been enough to simply enable reference
> types in the `compile` fuzz target to enable that proposal by
> default. Compiling a module that uses reference types but not instantiating
> it nor running any of its functions doesn't exercise any of the GC
> implementation and does not run the inline fast paths for `table` operations
> emitted by the JIT. Exercising these things was the motivation for writing
> the custom fuzz target for `table.{get,set}` instructions.
One indication of the status of fuzzing is [this
file](https://github.com/bytecodealliance/wasmtime/blob/main/crates/fuzzing/src/generators/module.rs#L16)
which controls module configuration during fuzzing.
* **API** - The proposal's functionality is exposed in the `wasmtime` crate's
API. At minimum this is `Config::wasm_the_proposal` but proposals such as
[`gc`] also add new types to the API.
* **C API** - The proposal's functionality is exposed in the C API.
[phases]: https://github.com/WebAssembly/meetings/blob/master/process/phases.md
|
