Different loop unrolling strategies

unroll.c

// CFLAGS="-Wall -Wextra -Wno-implicit-fallthrough"
#include <assert.h>
#include <stdio.h>

// 1) (n - (n % 4)) / 4 iterations
// 2) n % 4 iterations
void loop1(int n)
{
    int i, j;

    for (i = 0; i < n - 3; i += 4) {
        printf("loop1-main: %d\n", i + 0);
        printf("loop1-main: %d\n", i + 1);
        printf("loop1-main: %d\n", i + 2);
        printf("loop1-main: %d\n", i + 3);
    }

    assert(n - 3 <= i && i <= n);
    // Post loop iterates between 0 and n - (n - 3) == 3 times
    assert(n - 3 <= n - (n % 4));

    assert(i % 4 == 0);
    assert(i == n - (n % 4));
    // n - i == n - (n - (n % 4)) == n % 4
    assert(n - i == n % 4);

    for (j = i; j < n; j++) {
        printf("loop1-post: %d\n", j);
    }

    assert(j == n);
}

// 1) (n - (n % 4)) / 4 iterations
// 2) n % 4 iterations
void loop2(int n)
{
    int i, j;

    for (i = 0; i < n - (n % 4); i += 4) {
        printf("loop2-main: %d\n", i + 0);
        printf("loop2-main: %d\n", i + 1);
        printf("loop2-main: %d\n", i + 2);
        printf("loop2-main: %d\n", i + 3);
    }

    assert(i % 4 == 0);
    assert(i == n - (n % 4));
    // n - i == n - (n - (n % 4)) == n % 4
    assert(n - i == n % 4);

    for (j = i; j < n; j++) {
        printf("loop2-post: %d\n", j);
    }

    assert(j == n);
}

// 1) n % 4 iterations
// 2) (n - (n % 4)) / 4 iterations
void loop3(int n)
{
    int i, j;

    for (i = 0; i < n % 4; i++) {
        printf("loop3-pre: %d\n", i);
    }

    assert(i == n % 4);
    // n - i == n - (n % 4)
    assert(n - i == n - (n % 4));

    for (j = i; j < n; j += 4) {
        printf("loop3-main: %d\n", j + 0);
        printf("loop3-main: %d\n", j + 1);
        printf("loop3-main: %d\n", j + 2);
        printf("loop3-main: %d\n", j + 3);
    }

    assert(j == n);
}

// Duff's device
void loop4(int n)
{
    int i = 0;

    if (n > 0) {
        switch (n % 4) {
        case 0: do { printf("loop4-main: %d\n", i++);
        case 3:      printf("loop4-main: %d\n", i++);
        case 2:      printf("loop4-main: %d\n", i++);
        case 1:      printf("loop4-main: %d\n", i++);
                } while (i < n);
        }
    }

    assert(i == n);
}

void test(int n)
{
    printf(" n | n - 3 | n - (n %% 4)\n");
    printf("---+-------+-------------\n");

    int i;

    for (i = 0; i <= n; i++) {
        printf("%2d | %5d | %10d\n", i, i - 3, i - (i % 4));
    }
}

int main(void)
{
    int i;

    for (i = 0; i <= 10; i++) {
        loop1(i);
        printf("\n");
    }

    for (i = 0; i <= 10; i++) {
        loop2(i);
        printf("\n");
    }

    for (i = 0; i <= 10; i++) {
        loop3(i);
        printf("\n");
    }
    
    for (i = 0; i <= 10; i++) {
        loop4(i);
        printf("\n");
    }

    //test(10);

    return 0;
}