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Subject[PATCH 1/1] Lib: sort.c: replaced heap sort algorithm with introspective sort
Date
Replaced heap sort algorithm with faster introspective sort algorithm.

Signed-off-by: Andrey Abramov <st5pub@yandex.ru>
---
v1: The introspective sort algorithm is faster the heap sort (for example on my machine on a 100MB of random data it was consistently almost twice faster) and it doesn't have the worst case, unlike qsort.

lib/sort.c | 127 ++++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 112 insertions(+), 15 deletions(-)

diff --git a/lib/sort.c b/lib/sort.c
index d6b7a202b0b6..edc09287e572 100644
--- a/lib/sort.c
+++ b/lib/sort.c
@@ -1,8 +1,9 @@
// SPDX-License-Identifier: GPL-2.0
/*
- * A fast, small, non-recursive O(nlog n) sort for the Linux kernel
+ * A fast, recursive O(nlog n) sort for the Linux kernel
*
* Jan 23 2005 Matt Mackall <mpm@selenic.com>
+ * Feb 5 2019 Andrey Abramov <st5pub@yandex.ru> (introspective sort)
*/

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
@@ -48,33 +49,22 @@ static void generic_swap(void *a, void *b, int size)
* @num: number of elements
* @size: size of each element
* @cmp_func: pointer to comparison function
- * @swap_func: pointer to swap function or NULL
+ * @swap_func: pointer to swap function
*
- * This function does a heapsort on the given array. You may provide a
- * swap_func function optimized to your element type.
+ * This function does a heapsort on the given array.
*
* Sorting time is O(n log n) both on average and worst-case. While
* qsort is about 20% faster on average, it suffers from exploitable
* O(n*n) worst-case behavior and extra memory requirements that make
* it less suitable for kernel use.
*/
-
-void sort(void *base, size_t num, size_t size,
+void heapsort(void *base, size_t num, size_t size,
int (*cmp_func)(const void *, const void *),
void (*swap_func)(void *, void *, int size))
{
/* pre-scale counters for performance */
int i = (num/2 - 1) * size, n = num * size, c, r;

- if (!swap_func) {
- if (size == 4 && alignment_ok(base, 4))
- swap_func = u32_swap;
- else if (size == 8 && alignment_ok(base, 8))
- swap_func = u64_swap;
- else
- swap_func = generic_swap;
- }
-
/* heapify */
for ( ; i >= 0; i -= size) {
for (r = i; r * 2 + size < n; r = c) {
@@ -103,4 +93,111 @@ void sort(void *base, size_t num, size_t size,
}
}

+void introsort(void *base, size_t num, size_t size,
+ int (*cmp_func)(const void *, const void *),
+ void (*swap_func)(void *, void *, int size),
+ unsigned int max_depth, unsigned int depth)
+{
+
+ void *last = base + (num - 1) * size;
+ void *pivot = base + ((num - 1) / 2) * size;
+ void *i = base;
+ void *j = last;
+
+ if (num <= 1)
+ return;
+
+ /* switching to heapsort */
+ if (depth >= max_depth) {
+ heapsort(base, num, size, cmp_func, swap_func);
+ return;
+ }
+
+ /* making pivot be the median of middle, first and last elements */
+ if ((cmp_func(pivot, base) >= 0 && cmp_func(base, last) >= 0)
+ || (cmp_func(last, base) >= 0 && cmp_func(base, pivot) >= 0)) {
+ pivot = base;
+ } else if ((cmp_func(pivot, last) >= 0 && cmp_func(last, base) >= 0)
+ || (cmp_func(base, last) >= 0 && cmp_func(last, pivot) >= 0)) {
+ pivot = last;
+ }
+
+ /* split array */
+ while (true) {
+ while (cmp_func(i, pivot) < 0 && i < last)
+ i += size;
+ while (cmp_func(j, pivot) > 0 && j > base)
+ j -= size;
+
+ if (i >= j)
+ break;
+
+ swap_func(i, j, size);
+
+ if (i == pivot)
+ pivot = j;
+ else if (j == pivot)
+ pivot = i;
+
+ j -= size;
+ i += size;
+ }
+
+ /* continue for smaller parts */
+ if (i < last)
+ introsort(i, ((size_t)last - (size_t)i) / size + 1,
+ size, cmp_func, swap_func, max_depth, depth + 1);
+ if (base < j)
+ introsort(base, ((size_t)j - (size_t)base) / size + 1,
+ size, cmp_func, swap_func, max_depth, depth + 1);
+}
+
+unsigned int log2_up(size_t val)
+{
+ unsigned int log = 0;
+ size_t current = 1;
+
+ unsigned int max_reachable_log = sizeof(val) * 8 - 1;
+
+ while (current < val) {
+ current <<= 1;
+ log++;
+ if (log == max_reachable_log && current < val)
+ return max_reachable_log + 1;
+ }
+
+ return log;
+}
+
+
+/**
+ * sort - sort an array of elements
+ * @base: pointer to data to sort
+ * @num: number of elements
+ * @size: size of each element
+ * @cmp_func: pointer to comparison function
+ * @swap_func: pointer to swap function or NULL
+ *
+ * This function does a introspective sort on the given array. You may provide a
+ * swap_func function optimized to your element type.
+ *
+ * The introspective sort use both qsort and heapsort,
+ * so it is faster than heapsort on average,
+ * but it doesn't have the worst case unlike qsort
+ */
+void sort(void *base, size_t num, size_t size,
+ int (*cmp_func)(const void *, const void *),
+ void (*swap_func)(void *, void *, int size))
+{
+ if (!swap_func) {
+ if (size == 4 && alignment_ok(base, 4))
+ swap_func = u32_swap;
+ else if (size == 8 && alignment_ok(base, 8))
+ swap_func = u64_swap;
+ else
+ swap_func = generic_swap;
+ }
+
+ introsort(base, num, size, cmp_func, swap_func, log2_up(num) * 2, 1);
+}
EXPORT_SYMBOL(sort);
--
2.20.1
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