Cross Compiling

When contributing to Wasmtime and Cranelift you may run into issues that only reproduce on a different architecture from your development machine. Luckily, cargo makes cross compilation and running tests under QEMU pretty easy.

This guide will assume you are on an x86-64 with Ubuntu/Debian as your OS. The basic approach (with commands, paths, and package names appropriately tweaked) applies to other Linux distributions as well.

On Windows you can install build tools for AArch64 Windows, but targeting platforms like Linux or macOS is not easy. While toolchains exist for targeting non-Windows platforms you'll have to hunt yourself to find the right one.

On macOS you can install, through Xcode, toolchains for iOS but the main x86_64-apple-darwin is really the only easy target to install. You'll need to hunt for toolchains if you want to compile for Linux or Windows.

Install Rust Targets

First, use rustup to install Rust targets for the other architectures that Wasmtime and Cranelift support:

$ rustup target add \
    s390x-unknown-linux-gnu \
    riscv64gc-unknown-linux-gnu \
    aarch64-unknown-linux-gnu

Install GCC Cross-Compilation Toolchains

Next, you'll need to install a gcc for each cross-compilation target to serve as a linker for rustc.

$ sudo apt install \
    gcc-s390x-linux-gnu \
    gcc-riscv64-linux-gnu \
    gcc-aarch64-linux-gnu

Install `qemu`

You will also need to install qemu to emulate the cross-compilation targets.

$ sudo apt install qemu-user

Configure Cargo

The final bit to get out of the way is to configure cargo to use the appropriate gcc and qemu when cross-compiling and running tests for other architectures.

Add this to .cargo/config.toml in the Wasmtime repository (or create that file if none already exists).

[target.aarch64-unknown-linux-gnu]
linker = "aarch64-linux-gnu-gcc"
runner = "qemu-aarch64 -L /usr/aarch64-linux-gnu -E LD_LIBRARY_PATH=/usr/aarch64-linux-gnu/lib -E WASMTIME_TEST_NO_HOG_MEMORY=1"

[target.riscv64gc-unknown-linux-gnu]
linker = "riscv64-linux-gnu-gcc"
runner = "qemu-riscv64 -L /usr/riscv64-linux-gnu -E LD_LIBRARY_PATH=/usr/riscv64-linux-gnu/lib -E WASMTIME_TEST_NO_HOG_MEMORY=1"

[target.s390x-unknown-linux-gnu]
linker = "s390x-linux-gnu-gcc"
runner = "qemu-s390x -L /usr/s390x-linux-gnu -E LD_LIBRARY_PATH=/usr/s390x-linux-gnu/lib -E WASMTIME_TEST_NO_HOG_MEMORY=1"

Cross-Compile Tests and Run Them!

Now you can use cargo build, cargo run, and cargo test as you normally would for any crate inside the Wasmtime repository, just add the appropriate --target flag!

A few examples:

Build the wasmtime binary for aarch64:

$ cargo build --target aarch64-unknown-linux-gnu

Run the tests under riscv emulation:

$ cargo test --target riscv64gc-unknown-linux-gnu

Run the wasmtime binary under s390x emulation:

$ cargo run --target s390x-unknown-linux-gnu -- compile example.wasm

Wasmtime