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mergesort: tighten merge loop 

llist_merge() has special inner loops for taking elements from either of
the two lists to merge.  That helps consistently preferring one over the
other, for stability.  Merge the loops, swap the lists when the other
one has the next element for the result and keep track on which one to
prefer on equality.  This results in shorter code and object text:

Before:
__TEXT	__DATA	__OBJC	others	dec	hex
412	0	0	3441	3853	f0d	mergesort.o

With this patch:
__TEXT	__DATA	__OBJC	others	dec	hex
352	0	0	3516	3868	f1c	mergesort.o

Performance doesn't get worse:

Before:
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

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):     108.4 ms ±   0.2 ms    [User: 106.7 ms, System: 1.2 ms]
  Range (min … max):   108.0 ms … 108.8 ms    27 runs

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
René Scharfe 2022-07-16 18:53:45 +02:00 committed by Junio C Hamano
parent 7a3775eeb4
commit 848afebe56
1 changed files with 6 additions and 13 deletions
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19
mergesort.c
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@ -8,10 +8,11 @@ static void *llist_merge(void *list, void *other,
int (*compare_fn)(const void *, const void *))
{
void *result = list, *tail;
int prefer_list = compare_fn(list, other) <= 0;
if (compare_fn(list, other) > 0) {
if (!prefer_list) {
result = other;
goto other;
SWAP(list, other);
}
for (;;) {
do {
@ -21,18 +22,10 @@ static void *llist_merge(void *list, void *other,
set_next_fn(tail, other);
return result;
}
} while (compare_fn(list, other) <= 0);
} while (compare_fn(list, other) < prefer_list);
set_next_fn(tail, other);
other:
do {
tail = other;
other = get_next_fn(other);
if (!other) {
set_next_fn(tail, list);
return result;
}
} while (compare_fn(list, other) > 0);
set_next_fn(tail, list);
prefer_list ^= 1;
SWAP(list, other);
}
}