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Subject[PATCH 4/5] lib/list_sort: Simplify and remove MAX_LIST_LENGTH_BITS
Rather than a fixed-size array of pending sorted runs, use the ->prev
links to keep track of things. This reduces stack usage, eliminates
some ugly overflow handling, and reduces the code size.

Also:
* merge() no longer needs to handle NULL inputs, so simplify.
* The same applies to merge_and_restore_back_links(), which is renamed
to the less ponderous merge_final(). (It's a static helper function,
so we don't need a super-descriptive name; comments will do.)

x86-64 code size 1086 -> 740 bytes (-346)

(Yes, I see checkpatch complaining about no space after comma in
"__attribute__((nonnull(2,3,4,5)))". Checkpatch is wrong.)

Signed-off-by: George Spelvin <lkml@sdf.org>
---
include/linux/list_sort.h | 1 +
lib/list_sort.c | 152 ++++++++++++++++++++++++--------------
2 files changed, 96 insertions(+), 57 deletions(-)

diff --git a/include/linux/list_sort.h b/include/linux/list_sort.h
index ba79956e848d..20f178c24e9d 100644
--- a/include/linux/list_sort.h
+++ b/include/linux/list_sort.h
@@ -6,6 +6,7 @@

struct list_head;

+__attribute__((nonnull(2,3)))
void list_sort(void *priv, struct list_head *head,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b));
diff --git a/lib/list_sort.c b/lib/list_sort.c
index 85759928215b..e4819ef0426b 100644
--- a/lib/list_sort.c
+++ b/lib/list_sort.c
@@ -7,33 +7,47 @@
#include <linux/list_sort.h>
#include <linux/list.h>

-#define MAX_LIST_LENGTH_BITS 20
+/*
+ * By declaring the compare function with the __pure attribute, we give
+ * the compiler more opportunity to optimize. Ideally, we'd use this in
+ * the prototype of list_sort(), but that would involve a lot of churn
+ * at all call sites, so just cast the function pointer passed in.
+ */
+typedef int __pure __attribute__((nonnull(2,3))) (*cmp_func)(void *,
+ struct list_head const *, struct list_head const *);

/*
* Returns a list organized in an intermediate format suited
* to chaining of merge() calls: null-terminated, no reserved or
* sentinel head node, "prev" links not maintained.
*/
-static struct list_head *merge(void *priv,
- int (*cmp)(void *priv, struct list_head *a,
- struct list_head *b),
+__attribute__((nonnull(2,3,4)))
+static struct list_head *merge(void *priv, cmp_func cmp,
struct list_head *a, struct list_head *b)
{
- struct list_head head, *tail = &head;
+ struct list_head *head, **tail = &head;

- while (a && b) {
+ for (;;) {
/* if equal, take 'a' -- important for sort stability */
- if ((*cmp)(priv, a, b) <= 0) {
- tail->next = a;
+ if (cmp(priv, a, b) <= 0) {
+ *tail = a;
+ tail = &a->next;
a = a->next;
+ if (!a) {
+ *tail = b;
+ break;
+ }
} else {
- tail->next = b;
+ *tail = b;
+ tail = &b->next;
b = b->next;
+ if (!b) {
+ *tail = a;
+ break;
+ }
}
- tail = tail->next;
}
- tail->next = a?:b;
- return head.next;
+ return head;
}

/*
@@ -43,44 +57,52 @@ static struct list_head *merge(void *priv,
* prev-link restoration pass, or maintaining the prev links
* throughout.
*/
-static void merge_and_restore_back_links(void *priv,
- int (*cmp)(void *priv, struct list_head *a,
- struct list_head *b),
- struct list_head *head,
- struct list_head *a, struct list_head *b)
+__attribute__((nonnull(2,3,4,5)))
+static void merge_final(void *priv, cmp_func cmp, struct list_head *head,
+ struct list_head *a, struct list_head *b)
{
struct list_head *tail = head;
u8 count = 0;

- while (a && b) {
+ for (;;) {
/* if equal, take 'a' -- important for sort stability */
- if ((*cmp)(priv, a, b) <= 0) {
+ if (cmp(priv, a, b) <= 0) {
tail->next = a;
a->prev = tail;
+ tail = a;
a = a->next;
+ if (!a)
+ break;
} else {
tail->next = b;
b->prev = tail;
+ tail = b;
b = b->next;
+ if (!b) {
+ b = a;
+ break;
+ }
}
- tail = tail->next;
}
- tail->next = a ? : b;

+ /* Finish linking remainder of list b on to tail */
+ tail->next = b;
do {
/*
- * In worst cases this loop may run many iterations.
+ * If the merge is highly unbalanced (e.g. the input is
+ * already sorted), this loop may run many iterations.
* Continue callbacks to the client even though no
* element comparison is needed, so the client's cmp()
* routine can invoke cond_resched() periodically.
*/
- if (unlikely(!(++count)))
- (*cmp)(priv, tail->next, tail->next);
-
- tail->next->prev = tail;
- tail = tail->next;
- } while (tail->next);
+ if (unlikely(!++count))
+ cmp(priv, b, b);
+ b->prev = tail;
+ tail = b;
+ b = b->next;
+ } while (b);

+ /* And the final links to make a circular doubly-linked list */
tail->next = head;
head->prev = tail;
}
@@ -91,55 +113,71 @@ static void merge_and_restore_back_links(void *priv,
* @head: the list to sort
* @cmp: the elements comparison function
*
- * This function implements "merge sort", which has O(nlog(n))
- * complexity.
+ * This function implements a bottom-up merge sort, which has O(nlog(n))
+ * complexity. We use depth-first order to take advantage of cacheing.
+ * (I.e. when we get to the fourth element, we immediately merge the
+ * first two 2-element lists.)
*
* The comparison function @cmp must return a negative value if @a
* should sort before @b, and a positive value if @a should sort after
* @b. If @a and @b are equivalent, and their original relative
* ordering is to be preserved, @cmp must return 0.
+ *
+ * (Actually, it is always called with @a being the element which was
+ * originally first, so it is not necessary to to distinguish the @a < @b
+ * and @a == @b cases; the return value may be a simple boolean. But if
+ * you ever *use* this freedom, be sure to update this comment to document
+ * that code now depends on preserving this property!)
*/
+__attribute__((nonnull(2,3)))
void list_sort(void *priv, struct list_head *head,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b))
{
- struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
- -- last slot is a sentinel */
- int lev; /* index into part[] */
- int max_lev = 0;
- struct list_head *list;
+ struct list_head *list = head->next, *pending = NULL;
+ size_t count = 0; /* Count of pending */

- if (list_empty(head))
+ if (list == head->prev) /* Zero or one elements */
return;

- memset(part, 0, sizeof(part));
-
+ /* Convert to a null-terminated singly-linked list. */
head->prev->next = NULL;
- list = head->next;

- while (list) {
+ /*
+ * Data structure invariants:
+ * - All lists are singly linked and null-terminated; prev
+ * pointers are not maintained.
+ * - pending is a prev-linked "list of lists" of sorted
+ * sublists awaiting further merging.
+ * - Sublists are sorted by size and age, smallest & newest at front.
+ * - All of the sorted sublists are power-of-two in size,
+ * corresponding to bits set in "count".
+ */
+ do {
+ size_t bit;
struct list_head *cur = list;
+
+ /* Extract the head of "list" as a single-element list "cur" */
list = list->next;
cur->next = NULL;

- for (lev = 0; part[lev]; lev++) {
- cur = merge(priv, cmp, part[lev], cur);
- part[lev] = NULL;
+ /* Do merges corresponding to set lsbits in count */
+ for (bit = 1; count & bit; bit <<= 1) {
+ cur = merge(priv, (cmp_func)cmp, pending, cur);
+ pending = pending->prev; /* Untouched by merge() */
}
- if (lev > max_lev) {
- if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
- printk_once(KERN_DEBUG "list too long for efficiency\n");
- lev--;
- }
- max_lev = lev;
- }
- part[lev] = cur;
+ /* And place the result at the head of "pending" */
+ cur->prev = pending;
+ pending = cur;
+ count++;
+ } while (list->next);
+
+ /* Now merge together last element with all pending lists */
+ while (pending->prev) {
+ list = merge(priv, (cmp_func)cmp, pending, list);
+ pending = pending->prev;
}
-
- for (lev = 0; lev < max_lev; lev++)
- if (part[lev])
- list = merge(priv, cmp, part[lev], list);
-
- merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
+ /* The final merge, rebuilding prev links */
+ merge_final(priv, (cmp_func)cmp, head, pending, list);
}
EXPORT_SYMBOL(list_sort);
--
2.20.1
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