File: kthread.c

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#include <pthread.h>
#include <stdlib.h>
#include <limits.h>

/************
 * kt_for() *
 ************/

struct kt_for_t;

typedef struct {
	struct kt_for_t *t;
	long i;
} ktf_worker_t;

typedef struct kt_for_t {
	int n_threads;
	long n;
	ktf_worker_t *w;
	void (*func)(void*,long,int);
	void *data;
} kt_for_t;

static inline long steal_work(kt_for_t *t)
{
	int i, min_i = -1;
	long k, min = LONG_MAX;
	for (i = 0; i < t->n_threads; ++i)
		if (min > t->w[i].i) min = t->w[i].i, min_i = i;
	k = __sync_fetch_and_add(&t->w[min_i].i, t->n_threads);
	return k >= t->n? -1 : k;
}

static void *ktf_worker(void *data)
{
	ktf_worker_t *w = (ktf_worker_t*)data;
	long i;
	for (;;) {
		i = __sync_fetch_and_add(&w->i, w->t->n_threads);
		if (i >= w->t->n) break;
		w->t->func(w->t->data, i, w - w->t->w);
	}
	while ((i = steal_work(w->t)) >= 0)
		w->t->func(w->t->data, i, w - w->t->w);
	pthread_exit(0);
}

void kt_for(int n_threads, void (*func)(void*,long,int), void *data, long n)
{
	if (n_threads > 1) {
		int i;
		kt_for_t t;
		pthread_t *tid;
		t.func = func, t.data = data, t.n_threads = n_threads, t.n = n;
		t.w = (ktf_worker_t*)alloca(n_threads * sizeof(ktf_worker_t));
		tid = (pthread_t*)alloca(n_threads * sizeof(pthread_t));
		for (i = 0; i < n_threads; ++i)
			t.w[i].t = &t, t.w[i].i = i;
		for (i = 0; i < n_threads; ++i) pthread_create(&tid[i], 0, ktf_worker, &t.w[i]);
		for (i = 0; i < n_threads; ++i) pthread_join(tid[i], 0);
	} else {
		long j;
		for (j = 0; j < n; ++j) func(data, j, 0);
	}
}