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Pre-Compiling and Cross-Compiling WebAssembly Programs

docs/examples-pre-compiling-wasm.md

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Pre-Compiling and Cross-Compiling WebAssembly Programs

Wasmtime can compile a WebAssembly program to native code on one machine, and then run it on a different machine. This has a number of benefits:

  • Faster start up: Compilation is removed from the critical path. When a new HTTP request comes into your function-as-a-service platform, for example, you do not have to wait for the associated Wasm program to compile before it can start handling the request. Similarly, when a new update for your embedded device's Wasm application logic comes in, you do not need to compile the update on the under-powered device before it can begin running new updated logic.

  • Less Memory Usage: Pre-compiled Wasm programs can be lazily mmaped from disk, only paging their code into memory as those code paths are executed. If none of the code on a page is ever executed, the OS will never make the page resident. This means that running pre-compiled Wasm programs lowers overall memory usage in the system.

  • Smaller Code Size for Embedded: Wasmtime can be built such that it can only run Wasm programs that were pre-compiled elsewhere. These builds will not include the executable code for Wasm compilation. This is done by disabling the cranelift and winch cargo features at build time. These builds are useful for embedded devices, where programs must be small and fit within the device's constrained environment.

  • Smaller Attack Surfaces: Similarly, building Wasmtime without a compiler, and with only support for running pre-compiled Wasm programs, can be useful for security-minded embeddings to reduce the potential attack surface exposed to untrusted and potentially hostile Wasm guests. Compilation, triggered by the control plane, can happen inside a Wasmtime build that can compile but not run Wasm programs. Execution, in the data plane, can happen inside a Wasmtime build that can run but not compile new Wasm programs. Exposing a minimal attack surface to untrusted code is good security practice.

Note that these benefits are applicable regardless which Wasm execution strategy you've configured: Cranelift, Winch, or Pulley.

Pre-Compile the Wasm on One Machine

This must be done with a Wasmtime build that has a Wasm execution strategy enabled, e.g. was built with the cranelift or winch cargo features. It does not require the ability to run Wasm programs, so the runtime cargo feature can be disabled at build time.

rust,ignore
{{#include ../examples/pre_compile.rs}}

Run the Pre-Compiled Wasm on Another Machine

This must be done with a Wasmtime build that can run pre-compiled Wasm programs, that is a Wasmtime built with the runtime cargo feature. It does not need to compile new Wasm programs, so the cranelift and winch cargo features can be disabled.

rust,ignore
{{#include ../examples/run_pre_compiled.rs}}

See Also