bm.c

// Inspired by https://www.reddit.com/r/programming/comments/400wpy/andrei_alexandrescus_amazing_150_minutes_course/

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/resource.h>
#include <sys/times.h>
#include <time.h>

#define NUM_POWERS 19

static long long pow10[NUM_POWERS] = {
    1000000000000000000,
    100000000000000000,
    10000000000000000,
    1000000000000000,
    100000000000000,
    10000000000000,
    1000000000000,
    100000000000,
    10000000000,
    1000000000,
    100000000,
    10000000,
    1000000,
    100000,
    10000,
    1000,
    100,
    10,
    1
};

long long baseline(char *data) {
    long long result = 0;
    int i = NUM_POWERS - strlen(data);
    for (; *data; data++) {
        result += pow10[i] * (*data - '0');
        i++;
        asm("");
    }
    return result;
}

static long long correction[NUM_POWERS];

// This version avoids subtracting '0' for each digit, and instead uses a one-off correction based on the number of digits
// (It is not actually better)
long long better(char *data) {
    int i = NUM_POWERS - strlen(data);
    long long result = correction[i];
    for (; *data; data++) {
        result += pow10[i] * (*data);
        i++;
        asm("");
    }
    return result;
}

#define NUM_TRIALS 10000000000L

double get_time() {
    struct timespec ts;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts);
    return (double) ts.tv_sec + (double) ts.tv_nsec / 1000000000.0;
}

int main() {
    long long i;
    correction[NUM_POWERS-1] = -48;
    for (i = NUM_POWERS-2; i >= 0; i--) {
        correction[i] = correction[i+1] - pow10[i] * 48;
    }
    
    printf("Basic test\n");
    char *data = "123456789101112";
    printf("data = '%s', result = %lld\n", data, baseline(data));
    printf("data = '%s', result = %lld\n", data, better(data));
    
    printf("Baseline benchmark\n");
    double start, stop;
    unsigned long long result = 0;
    start = get_time();
    for (i = 0; i < NUM_TRIALS; i++) {
        result += baseline(data);
    }
    stop = get_time();
    printf("Total result %llu\n", result);
    printf("CPU time used = %lf\n", (stop - start));

    printf("Better benchmark\n");
    result = 0;
    start = get_time();
    for (i = 0; i < NUM_TRIALS; i++) {
        result += better(data);
    }
    stop = get_time();
    printf("Total result %llu\n", result);
    printf("CPU time used = %lf\n", (stop - start));
    
    return 0;
}