Some simplifications and improvements to the configure script

[xavierleroy]

• Remove host detection and config.guess script. We no longer need it to select between assembly files.
• Use pkg-config to find GMP. This works better on Mac with Homebrew, for example. Fall back to the old method if pkg-config is not available or doesn't know about GMP.

[xavierleroy]

Also, I wonder about MPIR support. This library seems to be abandoned, e.g. http://mpir.org is down and can only be found on archive.org. Likewise, there are few packages for MPIR around (none in Debian and Ubuntu, for example), and some are marked obsolete (Homebrew).

[antoinemine]

The rationale for MPIR was the better support for Microsoft's compilers on Windows. I understand that there is still some activity on a Windows-specific fork of MPIR https://github.com/wbhart/mpir (github are the new web pages !)

[ddeclerck]

Hi,

Could I suggest making the configure script more cross-compilation-friendly ? (i.e. using ocamlfind to look for ocaml* binaries and allow specifying a C toolchain prefix, to be used when calling pkg-config)

[antoinemine]

Could I suggest making the configure script more cross-compilation-friendly ? (i.e. using ocamlfind to look for ocaml* binaries and allow specifying a C toolchain prefix, to be used when calling pkg-config)

I think this should belong to a different issue.

Would switching to dune (#73) help ?

[ddeclerck]

Would switching to dune (#73) help ?

That would work too, indeed.

[xavierleroy]

(i.e. using ocamlfind to look for ocaml* binaries and allow specifying a C toolchain prefix, to be used when calling pkg-config)

I have no experience with cross-compiling OCaml libraries. Can you point me at examples of OCaml packages that use the cross-compilation tricks you mention?

[xavierleroy]

I understand that there is still some activity on a Windows-specific fork of MPIR https://github.com/wbhart/mpir

Last commit in 2020. Last (and first) release in 2017. I'm not reassured.

[ddeclerck]

(i.e. using ocamlfind to look for ocaml* binaries and allow specifying a C toolchain prefix, to be used when calling pkg-config)

I have no experience with cross-compiling OCaml libraries. Can you point me at examples of OCaml packages that use the cross-compilation tricks you mention?

There are a few packages from opam-cross-windows that perform some of these tricks (though none of them do precisely what we want here) :
https://github.com/ocaml-cross/opam-cross-windows/blob/main/packages/ocurl-windows/ocurl-windows.0.9.2/opam
https://github.com/ocaml-cross/opam-cross-windows/blob/main/packages/camlzip-windows/camlzip-windows.1.07/files/patches/cross-compilation.patch
https://github.com/ocaml-cross/opam-cross-windows/blob/main/packages/ctypes-windows/ctypes-windows.0.17.1/opam

However it should be sufficient to change the first lines of your configure script as follows:

ocaml='ocamlfind ocaml'
ocamlc='ocamlfind ocamlc'
ocamlopt='ocamlfind ocamlopt'
ocamlmklib='ocamlfind ocamlmklib'
ocamldep='ocamlfind ocamldep'
ocamldoc='ocamlfind ocamldoc'

And prepend a variable named PREFIX (or anything you like) to each occurrence of pkg-config :

    if ${PREFIX}pkg-config gmp 2>/dev/null; then
        echo 'package gmp: found'
        gmp='OK'
        cclib="$cclib $(${PREFIX}pkg-config --libs gmp)"
        ccinc="$ccinc $(${PREFIX}pkg-config --cflags gmp)"
        ccdef="-DHAS_GMP $ccdef"

ocamlfind uses the OCAMLFIND_TOOLCHAIN environment variable to look for the proper ocaml* binaries. As for the PREFIX variable, it is meant to be fed to the configure script by an opam file modified for cross-compilation (and should be left blank otherwise). Here, we could add zarith to the opam-cross-windows repository simply by tweaking its build command in the opam file as follows:

build: [
  [ "sh" "-exc" "OCAMLFIND_TOOLCHAIN=windows PREFIX=%{conf-gcc-windows:prefix}% ./configure" ]
]

[xavierleroy]

This PR hasn't been reviewed, but I think it's good stuff and I'm going to merge it nonetheless.

The discussion side-stepped in two directions:
1- Support for MPIR: OK, let's keep it unchanged, even though the MPIR project looks completely dead.
2- Support for cross-compilation: a PR along the lines mentioned by @ddeclerck would be welcome, but it's a separate topic.