Merge branch 'bd/qsort'

* bd/qsort:
  compat: Add simplified merge sort implementation from glibc

Makefile

@@ -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

compat/qsort.c

@@ -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);
+	}
+}

git-compat-util.h

@@ -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