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0.5.0 Jul 25, 2022
0.4.2 Jul 24, 2022
0.4.1 Sep 23, 2021
0.4.0 Apr 15, 2021
0.1.0 Apr 19, 2019

#65 in WebAssembly

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Wasmi - WebAssembly (Wasm) Interpreter

Wasmi is an efficient and lightweight WebAssembly interpreter with a focus on constrained and embedded systems.

Version 0.31.0 has been audited by SRLabs.

Announcement: Transfer of Ownership

As of 2024-02-01, the original owner and maintainer of the Wasmi project, Parity Technologies, has officially transferred ownership of the project to me, Robin Freyler. Read more about this transfer here.

Distinct Features

The following list states some of the distinct features of Wasmi.

  • Simple, correct and deterministic execution of WebAssembly.
  • Low-overhead and cross-platform WebAssembly runtime for embedded environments.
  • JIT bomb resisting translation.
  • Loosely mirrors the Wasmtime API.
  • 100% WebAssembly spec testsuite compliance.
  • Built-in support for fuel metering.

WebAssembly Proposals

The new Wasmi engine supports a variety of WebAssembly proposals and will support even more of them in the future.

WebAssembly Proposal Status Comment
mutable-global Since version 0.14.0.
saturating-float-to-int Since version 0.14.0.
sign-extension Since version 0.14.0.
multi-value Since version 0.14.0.
bulk-memory Since version 0.24.0. (#628)
reference-types Since version 0.24.0. (#635)
simd Unlikely to be supported.
tail-calls Since version 0.28.0. (#683)
extended-const Since version 0.29.0. (#707)
function-references 📅 Planned but not yet implemented. (#774)
gc 📅 Planned but not yet implemented. (#775)
multi-memory 📅 Planned but not yet implemented. (#776)
threads 📅 Planned but not yet implemented. (#777)
relaxed-simd Unlikely to be supported since simd is unlikely to be supported.
component-model 📅 Planned but not yet implemented. (#897)
WASI 👨‍🔬 Experimental support via the wasmi_wasi crate or the Wasmi CLI application.


As CLI Application

Install the newest Wasmi CLI version:

cargo install wasmi_cli

Run wasm32-unknown-unknown or wasm32-wasi Wasm binaries:

wasmi_cli <WASM_FILE> --invoke <FUNC_NAME> [<FUNC_ARGS>]*

As Rust Library

Refer to the Wasmi crate docs to learn how to use the Wasmi crate as library.


Build & Test

Clone the Wasmi repository and build using cargo:

git clone https://github.com/wasmi-labs/wasmi.git --recursive
cd wasmi
cargo build
cargo test


In order to benchmark Wasmi use the following command:

cargo bench

Use translate, instantiate, execute or overhead filters to only run benchmarks that test performance of Wasm translation, instantiation, execution or miscellaneous overhead respectively, e.g. cargo bench execute.

We maintain a timeline for benchmarks of every commit to master that can be viewed here.

Supported Platforms

Wasmi supports a wide variety of architectures and platforms.

  • Fore more details see this list of supported platforms for Rust.
  • Note: Wasmi can be used in no_std embedded environments, thus not requiring the standard library (std).
  • Only some platforms are checked in CI and guaranteed to be fully working by the Wasmi maintainers.

Production Builds

In order to reap the most performance out of Wasmi we highly recommended to compile the Wasmi crate using the following Cargo profile:

lto = "fat"
codegen-units = 1

When compiling for the WebAssembly target we highly recommend to post-optimize Wasmi using Binaryen's wasm-opt tool since our experiments displayed a 80-100% performance improvements when executed under Wasmtime and also slightly smaller Wasm binaries.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.