rust/docs/usage.md

# Usage

rustc_codegen_cranelift can be used as a near-drop-in replacement for `cargo build` or `cargo run` for existing projects.

Assuming `$cg_clif_dir` is the directory you cloned this repo into and you followed the instructions (`y.rs prepare` and `y.rs build` or `test.sh`).

## Cargo

In the directory with your project (where you can do the usual `cargo build`), run:

```bash
$ $cg_clif_dir/build/cargo.sh build
```

This will build your project with rustc_codegen_cranelift instead of the usual LLVM backend.

## Rustc

> You should prefer using the Cargo method.

```bash
$ $cg_clif_dir/build/bin/cg_clif my_crate.rs
```

## Jit mode

In jit mode cg_clif will immediately execute your code without creating an executable file.

> This requires all dependencies to be available as dynamic library.
> The jit mode will probably need cargo integration to make this possible.

```bash
$ $cg_clif_dir/build/cargo.sh jit
```

or

```bash
$ $cg_clif_dir/build/bin/cg_clif -Cllvm-args=mode=jit -Cprefer-dynamic my_crate.rs
```

There is also an experimental lazy jit mode. In this mode functions are only compiled once they are
first called. It currently does not work with multi-threaded programs. When a not yet compiled
function is called from another thread than the main thread, you will get an ICE.

```bash
$ $cg_clif_dir/build/cargo.sh lazy-jit
```

## Shell

These are a few functions that allow you to easily run rust code from the shell using cg_clif as jit.

```bash
function jit_naked() {
    echo "$@" | $cg_clif_dir/build/bin/cg_clif - -Cllvm-args=mode=jit -Cprefer-dynamic
}

function jit() {
    jit_naked "fn main() { $@ }"
}

function jit_calc() {
    jit 'println!("0x{:x}", ' $@ ');';
}
```