From 51ea55190b6e72c77c96754c1bf2f149a4714848 Mon Sep 17 00:00:00 2001 From: Nicolas Pitre Date: Tue, 18 Aug 2009 15:26:55 -0400 Subject: 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 Signed-off-by: Junio C Hamano --- compat/bswap.h | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 compat/bswap.h (limited to 'compat') diff --git a/compat/bswap.h b/compat/bswap.h new file mode 100644 index 000000000..7246a12c6 --- /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 -- cgit v1.2.1