queue.c

#include <assert.h>
#include <pthread.h>
#include <stdint.h>
#include <stdlib.h>

#define lenof(a)    (int)(sizeof(a) / sizeof(*(a)))

typedef uint16_t QueueUint;

typedef struct {
    QueueUint head;
    QueueUint tail;
    uint64_t  slots[1<<3];
} Queue;

static void *consumer(void *arg)
{
    Queue *q = arg;
    for (uint64_t v = 0;;) {
        QueueUint head = q->head;
        #if ACQUIRE
        QueueUint tail = __atomic_load_n(&q->tail, __ATOMIC_ACQUIRE);
        #else
        QueueUint tail = q->tail;
        #endif
        if (tail != head) {
            QueueUint len = lenof(q->slots);
            uint64_t e = q->slots[head%len];
            __atomic_store_n(&q->head, head+1, __ATOMIC_RELEASE);
            assert(e == v++);
        }
    }
}

int main()
{
    Queue *q = calloc(1, sizeof(*q));
    pthread_create(&(pthread_t){0}, 0, consumer, q);
    for (uint64_t v = 0;;) {
        #if ACQUIRE
        QueueUint head = __atomic_load_n(&q->head, __ATOMIC_ACQUIRE);
        #else
        QueueUint head = q->head;
        #endif
        QueueUint tail = q->tail;
        QueueUint len = lenof(q->slots);
        if (tail != (QueueUint)(head + len)) {
            q->slots[tail%len] = v++;
            __atomic_store_n(&q->tail, tail+1, __ATOMIC_RELEASE);
        }
    }
}