#include <stdio.h>

/* Initialize the cycle counter */

static unsigned cyc_hi = 0;
static unsigned cyc_lo = 0;


/* Set *hi and *lo to the high and low order bits of the cycle counter.
   Implementation requires assembly code to use the rdtsc instruction. */

void access_counter(unsigned *hi, unsigned *lo)
{
     asm("rdtsc; movl %%edx, %0; movl %%eax, %1" /* read cycle counter */
         : "=r" (*hi), "=r" (*lo)                /* and move results to */
         : /* No input */                        /* the two outputs */
         : "%edx", "%eax");
}

/* Record the current value of the cycle counter. */
void start_counter()
{
     access_counter(&cyc_hi,&cyc_lo);
}

/* Return the number of cycles since the last call to start_counter. */

double get_counter()
{
     unsigned ncyc_hi, ncyc_lo;
     unsigned hi, lo, borrow;
     double result;

     /* get cycle counter */
     access_counter(&ncyc_hi, &ncyc_lo);

     /*  double precision subtraction */
     lo = ncyc_lo - cyc_lo;
     borrow = lo > ncyc_lo;
     hi = ncyc_hi - cyc_hi - borrow;
     result = (double) hi * (1 << 30) * 4 + lo;
     if (result < 0) {
        fprintf(stderr,"Error: counter returns neg value: %.0f\n",result);
     }
     return result;
}