#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>

#if !defined(__GNUC__) && !defined(__i386__)
#error This code only works with GCC/i386.
#endif

/* The reason this only works under GNU C and the x86 is we're using the
 * rdtsc instruction.
 */
static inline unsigned long long rdtsc() {
	unsigned long long rval;

	asm volatile ("rdtsc" : "=A" (rval));
	return rval;
}

static inline unsigned long read_and_clear(unsigned long * ptr) {
    unsigned long rval;

	asm volatile ("lock xchgl %0, %1" : "=r" (rval), "+m" (*ptr) : "0" (0));
    return rval;
}


int main(void) {
	int i;
    volatile unsigned long trash;
    unsigned long val = 1;
	unsigned long long start, stop, time, min;

	/* Time how long a rdtsc takes- we do this ten times and take the 
	 * cheapest run.
	 */
	min = ~0ull;
	for (i = 0; i < 10; ++i) {
		start = rdtsc();
		trash = 0;
		stop = rdtsc();
		time = stop - start;
		if (time < min) {
			min = time;
		}
	}
	printf("Minimum time for a rdtsc instruction (in clocks): %llu\n", min);

	min = ~0ull;
	for (i = 0; i < 10; ++i) {
		val = 1;
		start = rdtsc();
		trash = read_and_clear(&val);
		stop = rdtsc();
		time = stop - start;
		if (time < min) {
			min = time;
		}
	}
	printf("Minimum time for a read_and_clear() + rdtsc (in clocks): %llu\n", min);
	return 0;
}