Cross Compiling - Wasmtime

# 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