compat: Add simplified merge sort implementation from glibc

qsort in Windows 2000 (and various other C libraries) is a Quicksort
with the usual O(n^2) worst case.  Unfortunately, sorting Git trees
seems to get very close to that worst case quite often:

    $ /git/gitbad runstatus
    # On branch master
    qsort, nmemb = 30842
    done, 237838087 comparisons.

This patch adds a simplified version of the merge sort that is glibc's
qsort(3).  As a merge sort, this needs a temporary array equal in size
to the array that is to be sorted, but has a worst-case performance of
O(n log n).

The complexity that was removed is:

* Doing direct stores for word-size and -aligned data.
* Falling back to quicksort if the allocation required to perform the
  merge sort would likely push the machine into swap.

Even with these simplifications, this seems to outperform the Windows
qsort(3) implementation, even in Windows XP (where it is "fixed" and
doesn't trigger O(n^2) complexity on trees).

[jes: moved into compat/qsort.c, as per Johannes Sixt's suggestion]
[bcd: removed gcc-ism, thanks to Edgar Toernig.  renamed make variable
      per Junio's comment.]

Signed-off-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Brian Downing 2008-02-05 15:10:44 -06:00 committed by Junio C Hamano
parent 7a2078b4b0
commit 43fe901b71
3 changed files with 76 additions and 0 deletions

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@ -137,6 +137,10 @@ all::
# Define THREADED_DELTA_SEARCH if you have pthreads and wish to exploit
# parallel delta searching when packing objects.
#
# Define INTERNAL_QSORT to use Git's implementation of qsort(), which
# is a simplified version of the merge sort used in glibc. This is
# recommended if Git triggers O(n^2) behavior in your platform's qsort().
#
GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
@$(SHELL_PATH) ./GIT-VERSION-GEN
@ -722,6 +726,10 @@ ifdef NO_MEMMEM
COMPAT_CFLAGS += -DNO_MEMMEM
COMPAT_OBJS += compat/memmem.o
endif
ifdef INTERNAL_QSORT
COMPAT_CFLAGS += -DINTERNAL_QSORT
COMPAT_OBJS += compat/qsort.o
endif
ifdef THREADED_DELTA_SEARCH
BASIC_CFLAGS += -DTHREADED_DELTA_SEARCH

62
compat/qsort.c Normal file
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@ -0,0 +1,62 @@
#include "../git-compat-util.h"
/*
* A merge sort implementation, simplified from the qsort implementation
* by Mike Haertel, which is a part of the GNU C Library.
*/
static void msort_with_tmp(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *),
char *t)
{
char *tmp;
char *b1, *b2;
size_t n1, n2;
if (n <= 1)
return;
n1 = n / 2;
n2 = n - n1;
b1 = b;
b2 = (char *)b + (n1 * s);
msort_with_tmp(b1, n1, s, cmp, t);
msort_with_tmp(b2, n2, s, cmp, t);
tmp = t;
while (n1 > 0 && n2 > 0) {
if (cmp(b1, b2) <= 0) {
memcpy(tmp, b1, s);
tmp += s;
b1 += s;
--n1;
} else {
memcpy(tmp, b2, s);
tmp += s;
b2 += s;
--n2;
}
}
if (n1 > 0)
memcpy(tmp, b1, n1 * s);
memcpy(b, t, (n - n2) * s);
}
void git_qsort(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *))
{
const size_t size = n * s;
char buf[1024];
if (size < sizeof(buf)) {
/* The temporary array fits on the small on-stack buffer. */
msort_with_tmp(b, n, s, cmp, buf);
} else {
/* It's somewhat large, so malloc it. */
char *tmp = malloc(size);
msort_with_tmp(b, n, s, cmp, tmp);
free(tmp);
}
}

View File

@ -426,4 +426,10 @@ static inline int strtol_i(char const *s, int base, int *result)
return 0;
}
#ifdef INTERNAL_QSORT
void git_qsort(void *base, size_t nmemb, size_t size,
int(*compar)(const void *, const void *));
#define qsort git_qsort
#endif
#endif