[DEMO][PATCH v2 6/5] compat: add a qsort_s() implementation based on GNU's qsort_r(1)

["René Scharfe" <l.s.r@web.de>]

Implement qsort_s() as a wrapper to the GNU version of qsort_r(1) and
use it on Linux.  Performance increases slightly:

Test                         HEAD^             HEAD
--------------------------------------------------------------------
0071.2: sort(1)              0.10(0.20+0.02)   0.10(0.21+0.01) +0.0%
0071.3: string_list_sort()   0.17(0.15+0.01)   0.16(0.15+0.01) -5.9%

Additionally the unstripped size of compat/qsort_s.o falls from 24576
to 16544 bytes in my build.

IMHO these savings aren't worth the increased complexity of having to
support two implementations.
---
 Makefile         |  6 ++++++
 compat/qsort_s.c | 18 ++++++++++++++++++
 config.mak.uname |  1 +
 3 files changed, 25 insertions(+)

diff --git a/Makefile b/Makefile
index 53ecc84e28..46db1c773f 100644
--- a/Makefile
+++ b/Makefile
@@ -282,6 +282,9 @@ all::
 # Define HAVE_ISO_QSORT_S if your platform provides a qsort_s() that's
 # compatible with the one described in C11 Annex K.
 #
+# Define HAVE_GNU_QSORT_R if your platform provides a qsort_r() that's
+# compatible with the one introduced with glibc 2.8.
+#
 # Define UNRELIABLE_FSTAT if your system's fstat does not return the same
 # information on a not yet closed file that lstat would return for the same
 # file after it was closed.
@@ -1426,6 +1429,9 @@ ifdef HAVE_ISO_QSORT_S
 else
 	COMPAT_OBJS += compat/qsort_s.o
 endif
+ifdef HAVE_GNU_QSORT_R
+	COMPAT_CFLAGS += -DHAVE_GNU_QSORT_R
+endif
 ifdef RUNTIME_PREFIX
 	COMPAT_CFLAGS += -DRUNTIME_PREFIX
 endif
diff --git a/compat/qsort_s.c b/compat/qsort_s.c
index 52d1f0a73d..763ee1faae 100644
--- a/compat/qsort_s.c
+++ b/compat/qsort_s.c
@@ -1,5 +1,21 @@
 #include "../git-compat-util.h"
 
+#if defined HAVE_GNU_QSORT_R
+
+int git_qsort_s(void *b, size_t n, size_t s,
+		int (*cmp)(const void *, const void *, void *), void *ctx)
+{
+	if (!n)
+		return 0;
+	if (!b || !cmp)
+		return -1;
+
+	qsort_r(b, n, s, cmp, ctx);
+	return 0;
+}
+
+#else
+
 /*
  * A merge sort implementation, simplified from the qsort implementation
  * by Mike Haertel, which is a part of the GNU C Library.
@@ -67,3 +83,5 @@ int git_qsort_s(void *b, size_t n, size_t s,
 	}
 	return 0;
 }
+
+#endif
diff --git a/config.mak.uname b/config.mak.uname
index 447f36ac2e..a1858f54ff 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -37,6 +37,7 @@ ifeq ($(uname_S),Linux)
 	NEEDS_LIBRT = YesPlease
 	HAVE_GETDELIM = YesPlease
 	SANE_TEXT_GREP=-a
+	HAVE_GNU_QSORT_R = YesPlease
 endif
 ifeq ($(uname_S),GNU/kFreeBSD)
 	HAVE_ALLOCA_H = YesPlease
-- 
2.11.0