[PATCH 01/10] mergesort: unify ranks loops

From: René Scharfe <l.s.r@web.de>
Date: Jul 16, 2022, 16:52 UTC
Message-ID: <50b229e9-46b7-fdd1-3f68-de8dd4acf811@web.de>
In-Reply-To: <4d7cd286-398e-215c-f2bd-aa7e8207be4f@web.de>

llist_mergesort() has a loop for adding a new element to the ranks array and another one for rolling up said array into a single sorted list at the end. We can merge them, so that adding the last element rolls up the whole array. Handle the empty list before the main loop now because list can't be NULL anymore inside the loop.

The result is shorter code and significantly less object text:

main: __TEXT __DATA __OBJC others dec hex 652 0 0 4651 5303 14b7 mergesort.o

With this patch: __TEXT __DATA __OBJC others dec hex 412 0 0 3441 3853 f0d mergesort.o

Why is the change so big? The reduction is amplified by llist_merge() being inlined both before and after.

Performance stays basically the same:

main: 0071.12: llist_mergesort() unsorted 0.24(0.22+0.01) 0071.14: llist_mergesort() sorted 0.12(0.10+0.01) 0071.16: llist_mergesort() reversed 0.12(0.10+0.01)

Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):     109.0 ms ±   0.3 ms    [User: 107.4 ms, System: 1.1 ms]
  Range (min … max):   108.7 ms … 109.6 ms    27 runs

With this patch: 0071.12: llist_mergesort() unsorted 0.24(0.22+0.01) 0071.14: llist_mergesort() sorted 0.12(0.10+0.01) 0071.16: llist_mergesort() reversed 0.12(0.10+0.01)

Benchmark 1: t/helper/test-tool mergesort test
  Time (mean ± σ):     109.2 ms ±   0.2 ms    [User: 107.5 ms, System: 1.1 ms]
  Range (min … max):   108.9 ms … 109.6 ms    27 runs

Signed-off-by: René Scharfe <l.s.r@web.de>
---
 mergesort.c | 31 +++++++++++++++----------------
 1 file changed, 15 insertions(+), 16 deletions(-)

diff --git a/mergesort.c b/mergesort.c
index bd9c6ef8ee..92150c4101 100644
--- a/mergesort.c
+++ b/mergesort.c
@@ -57,28 +57,27 @@ void *llist_mergesort(void *list,
 {
 	void *ranks[bitsizeof(void *)];
 	size_t n = 0;
-	int i;

-	while (list) {
+	if (!list)
+		return NULL;
+
+	for (;;) {
+		int i;
+		size_t m;
 		void *next = get_next_fn(list);
 		if (next)
 			set_next_fn(list, NULL);
-		for (i = 0; n & ((size_t)1 << i); i++)
-			list = llist_merge(ranks[i], list, get_next_fn,
-					   set_next_fn, compare_fn);
+		for (i = 0, m = n;; i++, m >>= 1) {
+			if (m & 1)
+				list = llist_merge(ranks[i], list, get_next_fn,
+						   set_next_fn, compare_fn);
+			else if (next)
+				break;
+			else if (!m)
+				return list;
+		}
 		n++;
 		ranks[i] = list;
 		list = next;
 	}
-
-	for (i = 0; n; i++, n >>= 1) {
-		if (!(n & 1))
-			continue;
-		if (list)
-			list = llist_merge(ranks[i], list, get_next_fn,
-					   set_next_fn, compare_fn);
-		else
-			list = ranks[i];
-	}
-	return list;
 }
--
2.37.1