From 51ea55190b6e72c77c96754c1bf2f149a4714848 Mon Sep 17 00:00:00 2001
From: Nicolas Pitre <nico@cam.org>
Date: Tue, 18 Aug 2009 15:26:55 -0400
Subject: [PATCH] make sure byte swapping is optimal for git

We rely on ntohl() and htonl() to perform byte swapping in many places.
However, some platforms have libraries providing really poor
implementations of those which might cause significant performance
issues, especially with the block-sha1 code.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 block-sha1/sha1.c |  4 ++--
 compat/bswap.h    | 36 ++++++++++++++++++++++++++++++++++++
 git-compat-util.h |  2 ++
 3 files changed, 40 insertions(+), 2 deletions(-)
 create mode 100644 compat/bswap.h

diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 464cb258aa..d31f2e386c 100644
--- a/block-sha1/sha1.c
+++ b/block-sha1/sha1.c
@@ -4,8 +4,8 @@
  * and to avoid unnecessary copies into the context array.
  */
 
-#include <string.h>
-#include <arpa/inet.h>
+/* this is only to get definitions for memcpy(), ntohl() and htonl() */
+#include "../git-compat-util.h"
 
 #include "sha1.h"
 
diff --git a/compat/bswap.h b/compat/bswap.h
new file mode 100644
index 0000000000..7246a12c6e
--- /dev/null
+++ b/compat/bswap.h
@@ -0,0 +1,36 @@
+/*
+ * Let's make sure we always have a sane definition for ntohl()/htonl().
+ * Some libraries define those as a function call, just to perform byte
+ * shifting, bringing significant overhead to what should be a simple
+ * operation.
+ */
+
+/*
+ * Default version that the compiler ought to optimize properly with
+ * constant values.
+ */
+static inline unsigned int default_swab32(unsigned int val)
+{
+	return (((val & 0xff000000) >> 24) |
+		((val & 0x00ff0000) >>  8) |
+		((val & 0x0000ff00) <<  8) |
+		((val & 0x000000ff) << 24));
+}
+
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+
+#define bswap32(x) ({ \
+	unsigned int __res; \
+	if (__builtin_constant_p(x)) { \
+		__res = default_swab32(x); \
+	} else { \
+		__asm__("bswap %0" : "=r" (__res) : "0" (x)); \
+	} \
+	__res; })
+
+#undef ntohl
+#undef htonl
+#define ntohl(x) bswap32(x)
+#define htonl(x) bswap32(x)
+
+#endif
diff --git a/git-compat-util.h b/git-compat-util.h
index 9f941e42b1..000859ed90 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -176,6 +176,8 @@ extern char *gitbasename(char *);
 #endif
 #endif
 
+#include "compat/bswap.h"
+
 /* General helper functions */
 extern void usage(const char *err) NORETURN;
 extern void die(const char *err, ...) NORETURN __attribute__((format (printf, 1, 2)));