Improve compilation speed by taking advantage of memoization

[jonathanpoelen]

Mp11 does not take full advantage of memoization, which increases compile time and memory usage.

For example for mp_map_find_impl:

template<template<class...> class M, class... T, class K> struct mp_map_find_impl<M<T...>, K>
{
    using U = mp_inherit<mpmf_wrap<T>...>;

    template<template<class...> class L, class... U> static mp_identity<L<K, U...>> f( mp_identity<L<K, U...>>* );
    static mp_identity<void> f( ... );

    using type = mpmf_unwrap< decltype( f((U*)0) ) >;
};

Here, the function f is duplicated as many times as there are instantiations of mp_map_find_impl. but, f depends only on K, not on M, nor on T. Moving the functions into a separate structure that takes K allows it not to be duplicated.

Example:

template<class K> struct mp_map_find_key
{
    template<template<class...> class L, class... U> static mp_identity<L<K, U...>> f( mp_identity<L<K, U>
    static mp_identity<void> f( ... );
};

template<template<class...> class M, class... T, class K> struct mp_map_find_impl<M<T...>, K>
{
    using U = mp_inherit<mpmf_wrap<T>...>;

    using type = mpmf_unwrap< decltype( mp_map_find_key<K>::f((U*)0) ) >;
};

Here is the before/after result (measured with /usr/bin/time --format='%Es - %MK' on Linux)

compiler	gcc-12	clang-15
before	0:00.46s - 200544K	0:00.45s - 159492K
after	0:00.37s - 172132K	0:00.43s - 156836K

The test code:

#include <boost/mp11/map.hpp>

using namespace boost::mp11;

using numbers = mp_iota_c<100>;
using l1 = mp_transform<mp_list, mp_append<numbers, numbers>>;
using l2 = mp_append<mp_list<mp_list<mp_size_t<2>>>, l1>;
using l3 = mp_append<mp_list<mp_list<mp_size_t<3>>>, l1>;
using l4 = mp_append<mp_list<mp_list<mp_size_t<4>>>, l1>;
using l5 = mp_append<mp_list<mp_list<mp_size_t<5>>>, l1>;

using m1 = mp_fold<l1, mp_list<>, mp_map_insert>;
using m2 = mp_fold<l2, mp_list<>, mp_map_insert>;
using m3 = mp_fold<l3, mp_list<>, mp_map_insert>;
using m4 = mp_fold<l4, mp_list<>, mp_map_insert>;
using m5 = mp_fold<l5, mp_list<>, mp_map_insert>;

I noticed this pattern several times, but the changes are minor compared to the benefit (especially with gcc).

[pdimov]

Would you be willing to submit this change as a pull request?