MLton 20180207

# GnuMP

The GnuMP library (GNU Multiple Precision arithmetic library) is a library for arbitrary precision integer arithmetic. MLton uses the GnuMP library to implement the Basis Library IntInf module.

## Known issues

- There is a known problem with the GnuMP library (prior to version 4.2.x), where it requires a lot of stack space for some computations, e.g. IntInf.toString of a million digit number. If you run with stack size limited, you may see a segfault in such programs. This problem is mentioned in the GnuMP FAQ, where they describe two solutions.

  - Increase (or unlimit) your stack space. From your program, use setrlimit, or from the shell, use ulimit.

  - Configure and rebuild libgmp with --disable-alloca, which will cause it to allocate temporaries using malloc instead of on the stack.

- On some platforms, the GnuMP library may be configured to use one of multiple ABIs (Application Binary Interfaces). For example, on some 32-bit architectures, GnuMP may be configured to represent a limb as either a 32-bit long or as a 64-bit long long. Similarly, GnuMP may be configured to use specific CPU features.

  In order to efficiently use the GnuMP library, MLton represents an IntInf.int value in a manner compatible with the GnuMP library’s representation of a limb. Hence, it is important that MLton and the GnuMP library agree upon the representation of a limb.

  - When using a source package of MLton, building will detect the GnuMP library’s representation of a limb.

  - When using a binary package of MLton that is dynamically linked against the GnuMP library, the build machine and the install machine must have the GnuMP library configured with the same representation of a limb. (On the other hand, the build machine need not have the GnuMP library configured with CPU features compatible with the install machine.)

  - When using a binary package of MLton that is statically linked against the GnuMP library, the build machine and the install machine need not have the GnuMP library configured with the same representation of a limb. (On the other hand, the build machine must have the GnuMP library configured with CPU features compatible with the install machine.)

    However, MLton will be configured with the representation of a limb from the GnuMP library of the build machine. Executables produced by MLton will be incompatible with the GnuMP library of the install machine. To reconfigure MLton with the representation of a limb from the GnuMP library of the install machine, one must edit:

        /usr/lib/mlton/self/sizes

    changing the

        mplimb = ??

    entry so that ?? corresponds to the bytes in a limb; and, one must edit:

        /usr/lib/mlton/sml/basis/config/c/arch-os/c-types.sml

    changing the

        (* from "gmp.h" *)
        structure C_MPLimb = struct open Word?? type t = word end
        functor C_MPLimb_ChooseWordN (A: CHOOSE_WORDN_ARG) = ChooseWordN_Word?? (A)

    entries so that ?? corresponds to the bits in a limb.