Test the true CPU frequency on Arm VMs

arm_freq.c

#include <stdio.h>
#include <stdint.h>
#include <time.h>

#define A1 "add %0, %0, #1\n\t"
#define A10 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1
#define A100 A10 A10 A10 A10 A10 A10 A10 A10 A10 A10
#define A1000 A100 A100 A100 A100 A100 A100 A100 A100 A100 A100

double get_time() {
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return ts.tv_sec + ts.tv_nsec * 1e-9;
}

int main() {
    uint64_t loops = 10000000;
    uint64_t insns_per_loop = 1000;
    uint64_t total_cycles = loops * insns_per_loop;
    
    uint64_t dummy = 0;

    double start_time = get_time();

    __asm__ __volatile__(
        "1:\n\t"
        A1000
        "subs %1, %1, #1\n\t"
        "b.ne 1b\n\t"
        : "+r" (dummy), "+r" (loops)
        :
        : "cc"
    );

    double end_time = get_time();
    double time_spent = end_time - start_time;

    double freq_Hz = (double)total_cycles / time_spent;
    double freq_GHz = freq_Hz / 1e9;

    printf("%.3f GHz\n", freq_GHz);

    return 0;
}