Self-unrolling macro loop in C/C++

Question

I am currently working on a project, where every cycle counts. While profiling my application I discovered that the overhead of some inner loop is quite high, because they consist of just a few machine instruction. Additionally the number of iterations in these loops is known at compile time.

So I thought instead of manually unrolling the loop with copy & paste I could use macros to unroll the loop at compile time so that it can be easily modified later.

What I image is something like this:

#define LOOP_N_TIMES(N, CODE) <insert magic here>

So that I can replace for (int i = 0; i < N, ++i) { do_stuff(); } with:

#define INNER_LOOP_COUNT 4
LOOP_N_TIMES(INNER_LOOP_COUNT, do_stuff();)

And it unrolls itself to:

do_stuff(); do_stuff(); do_stuff(); do_stuff();

Since the C preprocessor is still a mystery to me most of the time, I have no idea how to accomplish this, but I know it must be possible because Boost seems to have a BOOST_PP_REPEAT macros. Unfortunately I can't use Boost for this project.

Answer 1

You can use templates to unroll. See the disassembly for the sample Live on Godbolt

But -funroll-loops has the same effect for this sample.

---

Live On Coliru

template <unsigned N> struct faux_unroll {
    template <typename F> static void call(F const& f) {
        f();
        faux_unroll<N-1>::call(f);
    }
};

template <> struct faux_unroll<0u> {
    template <typename F> static void call(F const&) {}
};

#include <iostream>
#include <cstdlib>

int main() {
    srand(time(0));

    double r = 0;
    faux_unroll<10>::call([&] { r += 1.0/rand(); });

    std::cout << r;
}

Answer 2

You can use the pre-processor and play some tricks with token concatenation and multiple macro expansion, but you have to hard-code all possibilities:

#define M_REPEAT_1(X) X
#define M_REPEAT_2(X) X X
#define M_REPEAT_3(X) X X X
#define M_REPEAT_4(X) X X X X
#define M_REPEAT_5(X) X M_REPEAT_4(X)
#define M_REPEAT_6(X) M_REPEAT_3(X) M_REPEAT_3(X)

#define M_EXPAND(...) __VA_ARGS__

#define M_REPEAT__(N, X) M_EXPAND(M_REPEAT_ ## N)(X)
#define M_REPEAT_(N, X) M_REPEAT__(N, X)
#define M_REPEAT(N, X) M_REPEAT_(M_EXPAND(N), X)

And then expand it like this:

#define THREE 3

M_REPEAT(THREE, three();)
M_REPEAT(4, four();)
M_REPEAT(5, five();)
M_REPEAT(6, six();)

This method requires literal numbers as counts, you can't do something like this:

#define COUNT (N + 1)

M_REPEAT(COUNT, stuff();)

Answer 3

There's no standard way of doing this.

Here's a slightly bonkers approach:

#define DO_THING printf("Shake it, Baby\n")
#define DO_THING_2 DO_THING; DO_THING
#define DO_THING_4 DO_THING_2; DO_THING_2
#define DO_THING_8 DO_THING_4; DO_THING_4
#define DO_THING_16 DO_THING_8; DO_THING_8
//And so on. Max loop size increases exponentially. But so does code size if you use them. 

void do_thing_25_times(void){
    //Binary for 25 is 11001
    DO_THING_16;//ONE
    DO_THING_8;//ONE
    //ZERO
    //ZERO
    DO_THING;//ONE
}

It's not too much to ask of an optimizer to eliminate dead code. In which case:

#define DO_THING_N(N) if(((N)&1)!=0){DO_THING;}\
    if(((N)&2)!=0){DO_THING_2;}\
    if(((N)&4)!=0){DO_THING_4;}\
    if(((N)&8)!=0){DO_THING_8;}\
    if(((N)&16)!=0){DO_THING_16;}

Answer 4

You can't use a #define construct to calculate the "unroll-count". But with sufficient macros you can define this:

#define LOOP1(a) a
#define LOOP2(a) a LOOP1(a)
#define LOOP3(a) a LOOP2(a)

#define LOOPN(n,a) LOOP##n(a)

int main(void)
{
    LOOPN(3,printf("hello,world"););
}

_{Tested with VC2012}

Answer 5

You can't write real recursive statements with macros and I'm pretty sure you can't have real iteration in macros as well.

However you can take a look at Order. Although it is entirely built atop the C preprocessor it "implements" iteration-like functionalities. It actually can have up-to-N iterations, where N is some large number. I'm guessing it's similar for "recursive" macros. Any way, it is such a borderline case that few compilers support it (GCC is one of them, though).