Quick way to count number of instructions executed in a C program

Question

Is there an easy way to quickly count the number of instructions executed (x86 instructions - which and how many each) while executing a C program ?

I use gcc version 4.7.1 (GCC) on a x86_64 GNU/Linux machine.

Answer 1

Linux perf_event_open system call with config = PERF_COUNT_HW_INSTRUCTIONS

This Linux system call appears to be a cross architecture wrapper for performance events, including both hardware performance counters from the CPU and software events from the kernel.

Here's an example adapted from the man perf_event_open page:

perf_event_open.c

#define _GNU_SOURCE
#include <asm/unistd.h>
#include <linux/perf_event.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <unistd.h>

#include <inttypes.h>
#include <sys/types.h>

static long
perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                int cpu, int group_fd, unsigned long flags)
{
    int ret;

    ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                    group_fd, flags);
    return ret;
}

int
main(int argc, char **argv)
{
    struct perf_event_attr pe;
    long long count;
    int fd;

    uint64_t n;
    if (argc > 1) {
        n = strtoll(argv[1], NULL, 0);
    } else {
        n = 10000;
    }

    memset(&pe, 0, sizeof(struct perf_event_attr));
    pe.type = PERF_TYPE_HARDWARE;
    pe.size = sizeof(struct perf_event_attr);
    pe.config = PERF_COUNT_HW_INSTRUCTIONS;
    pe.disabled = 1;
    pe.exclude_kernel = 1;
    // Don't count hypervisor events.
    pe.exclude_hv = 1;

    fd = perf_event_open(&pe, 0, -1, -1, 0);
    if (fd == -1) {
        fprintf(stderr, "Error opening leader %llx\n", pe.config);
        exit(EXIT_FAILURE);
    }

    ioctl(fd, PERF_EVENT_IOC_RESET, 0);
    ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

    /* Loop n times, should be good enough for -O0. */
    __asm__ (
        "1:;\n"
        "sub $1, %[n];\n"
        "jne 1b;\n"
        : [n] "+r" (n)
        :
        :
    );

    ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
    read(fd, &count, sizeof(long long));

    printf("Used %lld instructions\n", count);

    close(fd);
}

Compile and run:

g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o perf_event_open.out perf_event_open.c
./perf_event_open.out

Output:

Used 20016 instructions

So we see that the result is pretty close to the expected value of 20000: 10k * two instructions per loop in the __asm__ block (sub, jne).

If I vary the argument, even to low values such as 100:

./perf_event_open.out 100

it gives:

Used 216 instructions

maintaining that constant + 16 instructions, so it seems that accuracy is pretty high, those 16 must be just the ioctl setup instructions after our little loop.

Now you might also be interested in:

• prevent reordering of the syscalls: Enforcing statement order in C++
• prevent the test loop from being optimized out: How to prevent GCC from optimizing out a busy wait loop?

Other events of interest that can be measured by this system call:

• cycle counts: How to get the CPU cycle count in x86_64 from C++?

Tested on Ubuntu 20.04 amd64, GCC 9.3.0, Linux kernel 5.4.0, Intel Core i7-7820HQ CPU.