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. Cross compile MLton and bootstrap 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 you are porting 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)

    • Compile and install the GnuMP. This varies from platform to platform. In platform/<os>.h, you need to include the appropriate gmp.h.

  • 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:


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/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.


Once you’ve got all the regressions working, you can build MLton for the new target. As with the regressions, the idea for bootstrapping is to generate the C and assembly on a working machine, copy it to the target machine, and then compile and link there. Here’s the sequence of steps.

  1. On a working machine, with the newly rebuilt compiler, in the mlton directory, do:

    mlton -stop g -target <target> mlton.mlb
  2. Copy to the target machine.

  3. On the target machine, move the libraries to the right place. That is, in build/lib/targets, do:

    rm -rf self
    mv <target> self

Also make sure you have all the header files in build/lib/include. You can copy them from a host machine that has run make runtime.

  1. On the target machine, compile and link MLton. That is, in the mlton directory, do something like:

    gcc -c -Ibuild/lib/include -Ibuild/lib/targets/self/include -O1 -w mlton/mlton.*.[cs]
    gcc -o build/lib/mlton-compile \
            -Lbuild/lib/targets/self \
            -L/usr/local/lib \
            mlton.*.o \
            -lmlton -lgmp -lgdtoa -lm
  2. At this point, MLton should be working and you can finish the rest of a usual make on the target machine.

    make basis-no-check script mlbpathmap constants libraries tools
  3. Making the last tool, mlyacc, will fail, because mlyacc cannot bootstrap its own yacc.grm.* files. On the host machine, run make -C mlyacc src/yacc.grm.sml. Then copy both files to the target machine, and compile mlyacc, making sure to supply the path to your newly compile mllex: make -C mlyacc MLLEX=mllex/mllex.

There are other details to get right, like making sure that the tools directories were clean so that the tools are rebuilt on the new platform, but hopefully this structure works. Once you’ve got a compiler on the target machine, you should test it by running all the regressions normally (i.e. without the -cross flag) and by running a couple rounds of self compiles.

Also see

The above description is based on the following emails sent to the MLton list.