Subject: Re: [PATCH] block-sha1: Windows declares ntohl() in winsock2.h

From: Junio C Hamano <[email-masked]>
Date: Tue, 18 Aug 2009 13:17:57 -0700

Nicolas Pitre <[email-masked]> writes:

> On Tue, 18 Aug 2009, Junio C Hamano wrote:
>
>> To reduce confusion, you may want to rename compat/bswap.h to something
>> like compat/ntohl-htonl-fix.h ;-)
>
> Bah.  If you wish, you can edit the patch directly for this, unless you 
> really prefer me to repost.  Maybe we might want to add a 8-byte 
> versions of those as well eventually, which is why I chose a more 
> generic name.

Ok, here is what I came up with after many squashing...

-- >8 --
From: Nicolas Pitre <[email-masked]>
Date: Tue, 18 Aug 2009 12:43:08 -0400
Subject: [PATCH] bswap: avoid potentially inefficient ntohl/htonl on i386/x86-64

Johannes Sixt reports that on Windows ntohl()/htonl() are not found in
<arpa/inet.h>, and as a minimal fix we need to include <winsock2.h>
instead.  Sebastian Schuberth points out that they are implemented as
out-of-line functions on Windows, which defeats these byteorder "macros"
used for performance.

Use bswap instruction through gcc inline asm instead on i386/x86-64
as a generic solution to this.

Signed-off-by: Johannes Sixt <[email-masked]>
Signed-off-by: Linus Torvalds <[email-masked]>
Signed-off-by: Nicolas Pitre <[email-masked]>
Signed-off-by: Junio C Hamano <[email-masked]>
---
 block-sha1/sha1.c |    4 +---
 compat/bswap.h    |   19 +++++++++++++++++++
 git-compat-util.h |    2 ++
 3 files changed, 22 insertions(+), 3 deletions(-)
 create mode 100644 compat/bswap.h

diff --git a/block-sha1/sha1.c b/block-sha1/sha1.c
index 464cb25..51a27c1 100644
--- a/block-sha1/sha1.c
+++ b/block-sha1/sha1.c
@@ -4,9 +4,7 @@
  * and to avoid unnecessary copies into the context array.
  */
 
-#include <string.h>
-#include <arpa/inet.h>
-
+#include "../git-compat-util.h"
 #include "sha1.h"
 
 #if defined(__i386__) || defined(__x86_64__)
diff --git a/compat/bswap.h b/compat/bswap.h
new file mode 100644
index 0000000..78fd2df
--- /dev/null
+++ b/compat/bswap.h
@@ -0,0 +1,19 @@
+/*
+ * 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
+ * swapping, bringing significant overhead to what should be a simple
+ * operation.
+ */
+
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+
+#define bswap32(x) ({ \
+	unsigned int __res; \
+	__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)));
-- 
1.6.4.245.g50659