[PATCH] make sure byte swapping is optimal for git

From

Nicolas Pitre <nico@cam.org>

Date

Aug 18, 2009, 19:26 UTC

Message-ID

<alpine.LFD.2.00.0908181523430.6044@xanadu.home>

In-Reply-To

<alpine.LFD.2.00.0908181516510.6044@xanadu.home>

Signed-off-by: Nicolas Pitre <nico@cam.org>
---

On Tue, 18 Aug 2009, Nicolas Pitre wrote:

> Well, I gave in and added a comment to the patch anyway, with more 
> improvements in the case of constant values.  Patch follows.

diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 464cb25..d31f2e3 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 0000000..7246a12
--- /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 9f941e4..000859e 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)));