## MLton 20241230

# PortingMLton

Porting MLton to a new target platform (architecture or OS) involves the following steps.

1. Make the necessary changes to the scripts, runtime system, Basis Library implementation, and compiler.

2. Get the regressions working using a cross compiler.

3. Bootstrap MLton on the target.

MLton has a native code generator only for AMD64 and X86, so, if you are porting to another architecture, you must use the C code generator. These notes do not cover building a new native code generator.

Some of the following steps will not be necessary if MLton already supports the architecture or operating system being ported to.

## What code to change

- Scripts.

  - In `bin/platform`, add new cases to define `$HOST_OS` and `$HOST_ARCH`.

- Runtime system.

  The goal of this step is to be able to successfully run `make` in the `runtime` directory on the target machine.

  - In `platform.h`, add a new case to include `platform/<arch>.h` and `platform/<os>.h`.

  - In `platform/<arch>.h`:

    - define `MLton_Platform_Arch_host`.

  - In `platform/<os>.h`:

    - include platform-specific includes.

    - define `MLton_Platform_OS_host`.

    - define all of the `HAS_*` macros.

  - In `platform/<os>.c` implement any platform-dependent functions that the runtime needs.

  - Add rounding mode control to `basis/Real/IEEEReal.c` for the new arch (if not `HAS_FEROUND`)

  - Build and install the GMP development library. This varies from platform to platform.

- Basis Library implementation (`basis-library/*`)

  - In `primitive/prim-mlton.sml`:

    - Add a new variant to the `MLton.Platform.Arch.t` datatype.

    - modify the constants that define `MLton.Platform.Arch.host` to match with `MLton_Platform_Arch_host`, as set in `runtime/platform/<arch>.h`.

    - Add a new variant to the `MLton.Platform.OS.t` datatype.

    - modify the constants that define `MLton.Platform.OS.host` to match with `MLton_Platform_OS_host`, as set in `runtime/platform/<os>.h`.

  - In `mlton/platform.{sig,sml}` add a new variant.

  - In `sml-nj/sml-nj.sml`, modify `getOSKind`.

  - Look at all the uses of `MLton.Platform` in the Basis Library implementation and see if you need to do anything special. You might use the following command to see where to look.

        find basis-library -type f | xargs grep 'MLton\.Platform'

    If in doubt, leave the code alone and wait to see what happens when you run the regression tests.

- Compiler.

  - In `lib/stubs/mlton-stubs/platform.sig` add any new variants, as was done in the Basis Library.

  - In `lib/stubs/mlton-stubs/mlton.sml` add any new variants in `MLton.Platform`, as was done in the Basis Library.

The string used to identify a particular architecture or operating system must be the same (except for possibly case of letters) in the scripts, runtime, Basis Library implementation, and compiler (stubs). In `mlton/main/main.fun`, MLton itself uses the conversions to and from strings:

    MLton.Platform.{Arch,OS}.{from,to}String

If the there is a mismatch, you may see the error message `strange arch` or `strange os`.

## Running the regressions with a cross compiler

When porting to a new platform, it is always best to get all (or as many as possible) of the regressions working before moving to a self compile. It is easiest to do this by modifying and rebuilding the compiler on a working machine and then running the regressions with a cross compiler. It is not easy to build a gcc cross compiler, so we recommend generating the C and assembly on a working machine (using MLton’s `-target` and `-stop g` flags, copying the generated files to the target machine, then compiling and linking there.

1. Remake the compiler on a working machine.

2. Use `bin/add-cross` to add support for the new target. In particular, this should create `build/lib/mlton/targets/<target>/` with the platform-specific necessary cross-compilation information.

3. Run the regression tests with the cross-compiler. To cross-compile all the tests, do

       bin/regression -cross <target>

   This will create all the executables. Then, copy `bin/regression` and the `regression` directory to the target machine, and do

       bin/regression -run-only <target>

   This should run all the tests.

Repeat this step, interleaved with appropriate compiler modifications, until all the regressions pass.

## Bootstrap

The idea for bootstrapping MLton on a new platform is as follows:

- send the current sources to a remote machine (using ssh)

- build the MLton runtime system on the remote machine

- receive the built runtime system from the remote machine as a new target on the host machine

- build bootstrap compiler sources on the host machine for the new target

- send the boostrap sources to the remote machine

- build the boostrap compiler on the remote machine using the boostrap compiler sources

- complete the MLton build on the remote machine with the boostrap compiler to obtain a boot package

- build MLton on the remote machine from clean sources using the boot package

- receive the built binary release from the remote machine

The `remote-bootstrap` goal of the root `Makefile` automates this process; see comments in the root `Makefile`. Here is an example bootstrapping on an OpenBSD machine (named `thunder`):

    $ make REMOTE_MACHINE=thunder REMOTE_MAKE=gmake REMOTE_MAKEFLAGS=WITH_GMP_DIR=/usr/local remote-bootstrap

Once you’ve got a compiler on the target machine, you should test it by running all the regressions normally and by running a couple rounds of self compiles.

## Also see

- http://www.mlton.org/pipermail/mlton/2002-October/013110.html

- http://www.mlton.org/pipermail/mlton/2004-July/016029.html

- https://github.com/MLton/mlton/issues/296