// http://stackoverflow.com/questions/37637935/how-can-i-execute-lua-functions-from-c-without-having-to-find-them-each-time


#include "lua.hpp"
#include <iostream>
#include <chrono>

int main(int argc, char **argv) {
    lua_State* L = luaL_newstate();
    luaL_openlibs(L);

    {
        auto start = std::chrono::steady_clock::now();

        for (int i = 0; i < 1000000000; ++i)
        {
            lua_getglobal(L, "tonumber");
            lua_pop(L, 1);
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> diff = end - start;
        std::cout << (end - start).count() << " " << diff.count() << std::endl;
    }

    {
        lua_getglobal(L, "tonumber");
        int tonumber = luaL_ref(L, LUA_REGISTRYINDEX);
        auto start = std::chrono::steady_clock::now();

        for (int i = 0; i < 1000000000; ++i)
        {
            lua_rawgeti(L, LUA_REGISTRYINDEX, tonumber);
            lua_pop(L, 1);
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> diff = end - start;
        std::cout << (end - start).count() << " " << diff.count() << std::endl;
    }

    {
        lua_State* L2 = lua_newthread(L);
        lua_getglobal(L2, "tonumber");
        int tonumber = lua_gettop(L2);
        auto start = std::chrono::steady_clock::now();

        for (int i = 0; i < 1000000000; ++i)
        {
            lua_pushvalue(L2, tonumber);
            lua_xmove(L2, L, 1);
            lua_pop(L, 1);
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> diff = end - start;
        std::cout << (end - start).count() << " " << diff.count() << std::endl;
    }

    {
        lua_getglobal(L, "tonumber");
        auto start = std::chrono::steady_clock::now();

        for (int i = 0; i < 1000000000; ++i)
        {
            lua_pushvalue(L, -1);
            lua_pop(L, 1);
        }

        auto end = std::chrono::steady_clock::now();
        std::chrono::duration<double> diff = end - start;
        std::cout << (end - start).count() << " " << diff.count() << std::endl;
    }

    lua_close(L);
    return 1;
}

Results (time passed in seconds on right side):
25093427537 25.0934
12223462792 12.2235
11023562360 11.0236
5823560132 5.82356