opam-cross-arm

This repository contains an up-to-date Arm toolchain featuring OCaml 4.07.0, as well as some commonly used packages.

The supported build systems are 32-bit and 64-bit x86 Linux. The supported target systems are 32-bit and 64-bit x86 Arm.

If you need support for other platforms or versions, please open an issue.

Prerequisites

On 64-bit Linux build systems, 32-bit libraries must be installed. On Debian derivatives they are provided in the gcc-multilib package.

A C cross-compiler targeting the appropriate Arm platform must be installed. On Debian derivatives they are provided in the arm-linux-gnueabihf (for 32-bit Arm targets) or aarch64-linux-gnu (for 64-bit Arm targets) packages.

Installation

Add this repository to OPAM:

opam repository add arm git://github.com/ossdev07/opam-cross-arm

On 64-bit build systems, switch to 32-bit compiler when compiling for 32-bit targets:

opam switch 4.07.0+32bit
eval `opam config env`

Otherwise, use a regular compiler; its version must match the version of the cross-compiler:

opam switch 4.07.0
eval `opam config env`

Pin some prerequisite packages that don't yet have fixes merged upstream:

opam pin add topkg https://github.com/whitequark/topkg.git

If desired, request the compiler to be built with flambda optimizers:

opam install conf-flambda-arm

Install the compiler:

opam install ocaml-arm

The compiler version is selected automatically based on the current OPAM switch; either ocaml-arm32 or ocaml-arm64 can be installed in any single one.

Build some code:

echo 'let () = print_endline "Hello, world!"' >helloworld.ml
ocamlfind -toolchain arm ocamlc helloworld.ml -o helloworld.byte
ocamlfind -toolchain arm ocamlopt helloworld.ml -o helloworld.native

Run it:

wine cmd /c "set PATH=Z:/$(ocamlfind -toolchain arm printconf path)/../bin;%PATH%
             && ./helloworld.byte"
wine ./helloworld.native

Install some packages:

opam install re-arm

Write some code using them:

let () =
  let regexp = Re_pcre.regexp {|\b([a-z]+)\b|} in
  let result = Re.exec regexp "Hello, world!" in
  Format.printf "match: %s\n" (Re.get result 1)

Build it:

ocamlfind -toolchain arm ocamlopt -package re.pcre -linkpkg test_pcre.ml -o test_pcre

Make an object file out of it and link it with your native project (you'll need to call caml_startup(argv) to run OCaml code; see this article):

ocamlfind -toolchain arm ocamlopt -package re.pcre -linkpkg -output-complete-obj test_pcre.ml -o test_pcre.o

With opam-windows-cross, cross-compilation is easy!

Porting packages

OCaml packages often have components that execute at compile-time (camlp4 or ppx syntax extensions, cstubs, OASIS, ...). Thus, it is not possible to just blanketly cross-compile every package in the OPAM repository; sometimes you would even need a cross-compiled and a non-cross-compiled package at once. The package definitions also often need package-specific modification in order to work.

As a result, if you want a package to be cross-compiled, you have to copy the definition from opam-repository, rename the package to add -arm suffix while updating any dependencies it could have, and update the build script. Don't forget to add ocaml-arm as a dependency!

Findlib 1.5.4 adds a feature that makes porting packages much simpler; namely, an OCAMLFIND_TOOLCHAIN environment variable that is equivalent to the -toolchain command-line flag. Now it is not necessary to patch the build systems of the packages to select the Arm toolchain; it is often enough to add ["env" "OCAMLFIND_TOOLCHAIN=arm" make ...] to the build command in the opam file.

For projects using OASIS, the following steps will work:

build: [
  ["env" "OCAMLFIND_TOOLCHAIN=arm"
   "ocaml" "setup.ml" "-configure" "--prefix" "%{prefix}%/arm-sysroot"
                                   "--override" "ext_dll" ".dll"]
  ["env" "OCAMLFIND_TOOLCHAIN=arm"
   "ocaml" "setup.ml" "-build"]
]
install: [
  ["env" "OCAMLFIND_TOOLCHAIN=arm"
   "ocaml" "setup.ml" "-install"]
]
remove: [["ocamlfind" "-toolchain" "arm" "remove" "pkg"]]
depends: ["ocaml-arm" ...]

The output of the configure script will be entirely wrong, referring to the host configuration rather than target configuration. Thankfully, it is not actually used in the build process itself, so it doesn't matter.

For projects installing the files via OPAM's .install files (e.g. topkg), the following steps will work:

build: [["ocaml" "pkg/pkg.ml" "build" "--toolchain" "arm" ]]
install: [["opam-installer" "--prefix=%{prefix}%/arm-sysroot" "pkg.install"]]
remove: [["ocamlfind" "-toolchain" "arm" "remove" "pkg"]]
depends: ["ocaml-arm" ...]

Internals

The aim of this repository is to build a cross-compiler while altering the original codebase in the minimal possible way. (Indeed, only about 50 lines are changed.) There are no attempts to alter the configure script; rather, the configuration is provided directly. The resulting cross-compiler has several interesting properties:

• All paths to the Arm toolchain are embedded inside ocamlc and ocamlopt; thus, no knowledge of the Arm toolchain is required even for packages that have components in C, provided they use the OCaml driver to compile the C code. (This is usually the case.)
• The build system makes several assumptions that are not strictly valid while cross-compiling, mainly the fact that the bytecode the cross-compiler has just built can be ran by the ocamlrun on the build system. Thus, the requirement for a 32-bit build compiler for 32-bit targets, as well as for the matching versions.
• The .opt versions of the compiler are built using itself, which doesn't work while cross-compiling, so all provided tools are bytecode-based.

License

All files contained in this repository are licensed under the CC0 1.0 Universal license.

References

See also ocaml-cross-android and ocaml-cross-ios.