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>

diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 464cb258aaa11786d3615a55c0ae8dff95150667..d31f2e386c392e7250a08969c750b3e3c91512b1 100644 (file)
--- 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 (file)
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 9f941e42b16d927f738b4911b41344dd2f3ed0da..000859ed909a286e9caa8b703a2fd10e9c6dc76f 100644 (file)
--- 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)));