#include <math.h>
#include "lfsr.h"
+#include "../compiler/compiler.h"
/*
* LFSR taps retrieved from:
* this switch.
*/
switch (spin) {
- case 16: __LFSR_NEXT(fl, fl->last_val);
case 15: __LFSR_NEXT(fl, fl->last_val);
case 14: __LFSR_NEXT(fl, fl->last_val);
case 13: __LFSR_NEXT(fl, fl->last_val);
}
}
+/*
+ * lfsr_next does the following:
+ *
+ * a. Return if the number of max values has been exceeded.
+ * b. Check if we have a spin value that produces a repeating subsequence.
+ * This is previously calculated in `prepare_spin` and cycle_length should
+ * be > 0. If we do have such a spin:
+ *
+ * i. Decrement the calculated cycle.
+ * ii. If it reaches zero, add "+1" to the spin and reset the cycle_length
+ * (we have it cached in the struct fio_lfsr)
+ *
+ * In either case, continue with the calculation of the next value.
+ * c. Check if the calculated value exceeds the desirable range. In this case,
+ * go back to b, else return.
+ */
int lfsr_next(struct fio_lfsr *fl, uint64_t *off, uint64_t last)
{
- int repeat;
- unsigned int spin;
-
- repeat = fl->num_vals % fl->cycle_length;
- if (repeat == 0)
- spin = fl->spin + 1;
- else
- spin = fl->spin;
-
- if (fl->num_vals > fl->max_val)
+ if (fl->num_vals++ > fl->max_val)
return 1;
- fl->num_vals++;
-
do {
- __lfsr_next(fl, spin);
- } while (fl->last_val > fl->max_val);
+ if (fl->cycle_length && !--fl->cycle_length) {
+ __lfsr_next(fl, fl->spin + 1);
+ fl->cycle_length = fl->cached_cycle_length;
+ } else
+ __lfsr_next(fl, fl->spin);
+ } while (fio_unlikely(fl->last_val > fl->max_val));
*off = fl->last_val;
return 0;
{
int i;
+ /*
+ * For an LFSR, there is always a prohibited state (all ones).
+ * Thus, if we need to find the proper LFSR for our size, we must take that
+ * into account.
+ */
for (i = 3; i < 64; i++)
- if ((1UL << i) > size) /* TODO: Explain why. */
+ if ((1UL << i) > size)
return taps[i];
return NULL;
* Thus, [1] is equivalent to (y * i) % (spin + 1) == 0;
* Also, the cycle's length will be (x * i) + (y * i) / (spin + 1)
*/
-int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
+static int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
{
uint64_t max = (fl->cached_bit << 1) - 1;
uint64_t x, y;
x = max / (spin + 1);
y = max % (spin + 1);
- fl->cycle_length = max; /* This is the expected cycle */
+ fl->cycle_length = 0; /* No cycle occurs, other than the expected */
fl->spin = spin;
for (i = 1; i <= spin; i++) {
break;
}
}
+ fl->cached_cycle_length = fl->cycle_length;
+
+ /*
+ * Increment cycle length for the first time only since the stored value
+ * will not be printed otherwise.
+ */
+ fl->cycle_length++;
return 0;
}