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>
diff --git a/Makefile b/Makefile
index 92341c4..fb82d14 100644
--- a/Makefile
+++ b/Makefile
@@ -137,6 +137,10 @@
# 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 @@
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
