LibTomMath backend support

[antoinemine]

This PR adds LibTomMath support as alternative backend to GMP and MPIR.
This is especially useful for compilation under Windows.

Using LibTomMath is enabled using the -tommath option in configure.

Tested under Linux with LibTomMath versions 1.2.1 and 1.3.0.

Caveats:

• not all Zarith functions are supported with LibTomMath (they raise a failure instead); all main arithmetic, Boolean, and bit operations are supported though
• the binding is not as well optimized for memory and time as the GMP one
• hash functions give different results between GMP and LibTomMath
• the marshalling formats for GMP and LibTomMath are not compatible
• compilation with 1.3.0 gives a depreciation warning for mp_expt_u32; however, 1.2.1 does not have the replacement function and I did not find any convenient way to check the library version number!

[antoinemine]

I'd be interested in some (even light) feedback on this PR, as we have interested users for a LibTomMath backend (this is available on Windows, unlike GMP, and still developed, unlike MPIR). @xavierleroy, what do you think?

[xavierleroy]

Well, you're adding a 2500-line C file that is mostly a cut-and-paste from a 3500-line C file (caml_z.c), so it's going to be hard to maintain.

Also, GMP is available on Windows, see e.g.
https://packages.msys2.org/package/mingw-w64-x86_64-gmp
https://github.com/gx/gmp
https://stackoverflow.com/questions/47359417/how-to-compile-gmp-for-windows-using-visual-studio
https://gmplib.org/manual/Notes-for-Particular-Systems#Notes-for-Particular-Systems

So, I need to better understand the Windows problem this PR is trying to solve.

[hhugo]

(The CI is running the testsuite on windows, with the mingw OCaml port and gmp)

[antoinemine]

Well, I know some users that rely on Visual Studio on Windows, for better or worse. LibTomMath has an official and maintained support for compilation with VS. As far as I understand, this is not really the case for GMP nor MPIR (what I see currently are mainly one-off efforts that are a bit hackey, and repositories stuck to an old version of GMP).

[hhugo]

Well, I know some users that rely on Visual Studio on Windows, for better or worse. LibTomMath has an official and maintained support for compilation with VS. As far as I understand, this is not really the case for GMP nor MPIR (what I see currently are mainly one-off efforts that are a bit hackey, and repositories stuck to an old version of GMP).

Maybe we should setup the CI to test the msvc port of OCaml then.

[xavierleroy]

I asked @dra27 about where to find a GMP package after his talk on OPAM for Windows, and the short answer was: opam install conf-gmp should quite certainly work for the Mingw64 port of OCaml, but more work is needed for the MSVC port.

Relatedly, there seems to be a binary package of GMP for MSVC provided by vcpkg: https://vcpkg.io/en/package/gmp . With luck, that may be enough to build Zarith with ocaml-msvc.

At any rate. I'd like the people who really want Zarith with MSVC to explore these possibilities, before we embark into adding 2500 lines of C code to support an obscure alternative to GMP.

[dra27]

With #157, I was able to build a tests-passing Zarith using MSVC 5.2.0 from opam and this vcpkg.json:

{
  "dependencies": [ "gmp" ],
  "builtin-baseline": "68d349964cb4e8da561fd849d9491e6ba11c5681",
  "overrides": []
}

Having done vcpkg install, I then ran

$ PKG_CONFIG_PATH=/cygdrive/c/Devel/Zarith/vcpkg_installed/x64-windows/lib/pkgconfig ./configure

and then edited the resulting Makefile.config to change /cygdrive/c to C:/, then one can run:

$ make
$ PATH="/cygdrive/c/Devel/Zarith/vcpkg_installed/x64-windows/bin:$PATH" make tests

The good bit is that including vcpkg in opam shouldn't take as long as I thought it might (i.e. it won't need opam 3.0...), but it will take a bit longer than this train journey...

[xavierleroy]

Very nice! Thanks a lot for the experiment and for #157. Long train journeys are great :-)