Simple test for faster strlen implementation

strlen.c

#include <stdint.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>


size_t strlen_simple(const char *str)
{
    size_t i = 0;
    while (str[i] != '\0') {
        i++;
    }
    return i;
}

size_t strlen_check_8(const char *str)
{
    // we want to make sure addr is in a 8 bytes boundary
    const char *cstr;
    for (cstr = str; ((uintptr_t)cstr & 7) != 0; cstr++) {
        if (*cstr == '\0')
            return cstr - str;
    }

    size_t len = cstr - str;
    uint64_t n;
    while (1) {
        n = *((uint64_t *)&str[len]);

        if ((n - 0x0101010101010101) & (~n) & 0x8080808080808080)
            break;

        len += 8;
    }

    /*
     * check against NULL-termination position
     * only available in little-endian machine
     * */
    /* if ((n & 0xff) == 0x00) */ 
    /*     return len; */
    /* else if (((n >> 8) & 0xff) == 0x00) */
    /*     return len + 1; */
    /* else if (((n >> 16) & 0xff) == 0x00) */
    /*     return len + 2; */
    /* else if (((n >> 24) & 0xff) == 0x00) */
    /*     return len + 3; */
    /* else if (((n >> 32) & 0xff) == 0x00) */
    /*     return len + 4; */
    /* else if (((n >> 40) & 0xff) == 0x00) */
    /*     return len + 5; */
    /* else if (((n >> 48) & 0xff) == 0x00) */
    /*     return len + 6; */
    /* else if (((n >> 56) & 0xff) == 0x00) */
    /*     return len + 7; */
    str = &str[len];
    if (str[0] == '\0')
        return len;
    if (str[1] == '\0')
        return len + 1;
    if (str[2] == '\0')
        return len + 2;
    if (str[3] == '\0')
        return len + 3;
    if (str[4] == '\0')
        return len + 4;
    if (str[5] == '\0')
        return len + 5;
    if (str[6] == '\0')
        return len + 6;
    if (str[7] == '\0')
        return len + 7;
}

int main(int argc, char **argv) {
    char str[] = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbbbbbbb";

    if (argc < 2) {
        fprintf(stderr, "must provide fp ID 1 ~ 3\n");
        exit(2);
    }

    size_t (*fp)(const char *);
    switch (argv[1][0]) {
        case '1':
            fp = &strlen;
            break;
        case '2':
            fp = &strlen_check_8;
            break;
        case '3':
            fp = &strlen_simple;
            break;
    }

    for (int i = 0; i < 0xfffffff; i++) {
        fp(str);
    }
}

Test environment:
Linux jackyyin-MacBookPro 5.13.0-30-generic #33~20.04.1-Ubuntu SMP Mon Feb 7 14:25:10 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

compile under O0:
./test 1 0.76s user 0.00s system 99% cpu 0.761 total
./test 2 8.38s user 0.00s system 99% cpu 8.383 total
./test 3 79.59s user 0.00s system 99% cpu 1:19.60 total

compile under O3:
./test 1 0.63s user 0.00s system 99% cpu 0.634 total
./test 2 3.61s user 0.00s system 99% cpu 3.607 total
./test 3 18.55s user 0.00s system 99% cpu 18.565 total