test.c

#include <pthread.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

static uint64_t loop_counter;
static bool drop_out;

uint32_t xbegin(void)
{
    uint32_t result;

    __asm__ __volatile__ ("xbegin 1f; mov $0xffffffff,%%eax; 1:" : "=r"(result) :: "memory");

    return result;
}

void xend(void)
{
    __asm__ __volatile__ ("xend" ::: "memory");
}


void inc_loop_counter(void) __attribute__((noinline))
{
    loop_counter += 1;
}


void *test(void *ptr)
{
    (void)ptr;

    uint32_t status;

    while (!drop_out) {
        if ((status = xbegin()) == 0xffffffffu) {
            inc_loop_counter();

            xend();
        } else {
            drop_out = true;
            return (void *)(uintptr_t)status;
        }
    }

    return NULL;
}


int main(int argc, char *argv[])
{
    int thread_counter = 2;

    if (argc > 1) {
        thread_counter = strtoul(argv[1], NULL, 0);
        if (!thread_counter) {
            fprintf(stderr, "Usage: %s [thread count]\n", argv[0]);
            return 1;
        }
    }

    pthread_t threads[thread_counter];
    uint32_t result = 0;

    for (int i = 0; i < thread_counter; i++) {
        pthread_create(&threads[i], NULL, &test, NULL);
    }

    for (int i = 0; i < thread_counter; i++) {
        void *tres;
        pthread_join(threads[i], &tres);
        if (tres) {
            result = (uintptr_t)tres;
        }
    }

    printf("%lu iterations; abort result: %x\n", loop_counter, result);
}