X-Git-Url: https://git.kernel.dk/?a=blobdiff_plain;f=lib%2Flfsr.c;h=0c0072ccb39b15e970eeb6bd92342b67a85cd8bb;hb=92330695e64a42fd5dc54a6970a4b8122d3cd80e;hp=4c15c6256f9a1d41ab95a1bfdb0d8e59fb92796a;hpb=a64ea63f648e77d583b5287634f44bea7da79b4b;p=fio.git

diff --git a/lib/lfsr.c b/lib/lfsr.c
index 4c15c625..0c0072cc 100644
--- a/lib/lfsr.c
+++ b/lib/lfsr.c
@@ -2,6 +2,7 @@
 #include <math.h>
 
 #include "lfsr.h"
+#include "../compiler/compiler.h"
 
 /*
  * LFSR taps retrieved from:
@@ -10,7 +11,7 @@
  * The memory overhead of the following tap table should be relatively small,
  * no more than 400 bytes.
  */
-static uint8_t taps[64][FIO_MAX_TAPS] =
+static uint8_t lfsr_taps[64][FIO_MAX_TAPS] =
 {
 	{0}, {0}, {0},		//LFSRs with less that 3-bits cannot exist
 	{3, 2},			//Tap position for 3-bit LFSR
@@ -87,7 +88,6 @@ static inline void __lfsr_next(struct fio_lfsr *fl, unsigned int spin)
 	 * 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);
@@ -112,27 +112,30 @@ static inline void __lfsr_next(struct fio_lfsr *fl, unsigned int spin)
  * lfsr_next does the following:
  *
  * a. Return if the number of max values has been exceeded.
- * b. Check if the next iteration(s) produce a cycle (due to spin) and add "1"
- *    where necessary.
- * c. Calculate the next value and return.
+ * 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 lfsr_next(struct fio_lfsr *fl, uint64_t *off)
 {
-	int repeat;
-	unsigned int spin;
-
 	if (fl->num_vals++ > fl->max_val)
 		return 1;
 
-	repeat = fl->num_vals % fl->cycle_length;
-	if (repeat == 0)
-		spin = fl->spin + 1;
-	else
-		spin = fl->spin;
-
 	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;
@@ -153,9 +156,14 @@ static uint8_t *find_lfsr(uint64_t size)
 {
 	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. */
-			return taps[i];
+		if ((1UL << i) > size)
+			return lfsr_taps[i];
 
 	return NULL;
 }
@@ -178,7 +186,7 @@ static uint8_t *find_lfsr(uint64_t size)
  * 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;
@@ -189,7 +197,7 @@ int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
 
 	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++) {
@@ -198,6 +206,13 @@ int prepare_spin(struct fio_lfsr *fl, unsigned int spin)
 			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;
 }
@@ -219,15 +234,15 @@ int lfsr_reset(struct fio_lfsr *fl, unsigned long seed)
 int lfsr_init(struct fio_lfsr *fl, uint64_t nums, unsigned long seed,
 		unsigned int spin)
 {
-	uint8_t *lfsr_taps;
+	uint8_t *taps;
 
-	lfsr_taps = find_lfsr(nums);
-	if (!lfsr_taps)
+	taps = find_lfsr(nums);
+	if (!taps)
 		return 1;
 
 	fl->max_val = nums - 1;
-	fl->xormask = lfsr_create_xormask(lfsr_taps);
-	fl->cached_bit = 1UL << (lfsr_taps[0] - 1);
+	fl->xormask = lfsr_create_xormask(taps);
+	fl->cached_bit = 1UL << (taps[0] - 1);
 
 	if (prepare_spin(fl, spin))
 		return 1;