Performance of Lua calling C function

Question

I noticed a big difference in performance of my C program depending on the -fPIC flag. When I use it my program is about 30% slower than without it. I am comparing it with a Lua program which calls a C function (where all the heavy calculation is done). Firstly I created a shared object with the C function, so had to use the -fPIC flag. The performance is very similar to the C code with -fPIC flag. So now I tried to the same without the .so: I called Lua from C:

int main()
{
    lua_State* L = luaL_newstate();
    luaL_openlibs(L);
    lua_register(L, "my_c_function", my_c_function);
    luaL_dofile(L, "my_lua_program.lua");
    lua_close(L);
    return 0;
}

But here performance is the same regardless if I use the -fPIC flag or not (and the same as the approach with .so). I was expecting some improvement without the -fPIC flag... Any advice on how can I investigate it further? Is the second approach creating position independent code anyway and that's why the performance is similar? Thanks!

More information, as suggested by the comment: I use the -O3 flag, gcc 4.7.2, Ubuntu 12.04.2, x86_64. Yes, I was quite surprised with so big overhead... My program is calculating Mandelbrot fractal. So there are two loops iterating over x and y and the function I have in C is isMandelbrot: it takes the number of iterations and returns bool: belongs to Mandelbrot set or not. I use the shared object with 'require'.

Answer 1

I think the code you are running on is x86. This platform has performance issues with -fPIC, where the location of any imported function requires the local eip to be found. The code to do this adds a small overhead to the function. Unfortunately lua is full of very small functions, and it will increase the relative overhead.

On x64 -fPIC doesn't have this overhead.