bswap.h: Add ntohs() and htons() functions

Since commit 70aab459, the msvc and MinGW builds have relied on
the built-in implementation of ntohl() and htonl(), rather than
linking the wsock32 library. The new index manipulation code now
calls ntohs()/htons() in addition to ntohl()/htonl(), so we need
to provide a built-in implementation of the 16-bit functions.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>

src/bswap.h

@@ -21,7 +21,15 @@ GIT_INLINE(uint32_t) default_swab32(uint32_t val)
 
 #undef bswap32
 
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+GIT_INLINE(uint16_t) default_swab16(uint16_t val)
+{
+	return (((val & 0xff00) >> 8) |
+		((val & 0x00ff) << 8));
+}
+
+#undef bswap16
+
+#if defined(__GNUC__) && defined(__i386__)
 
 #define bswap32(x) ({ \
 	uint32_t __res; \
@@ -32,11 +40,41 @@ GIT_INLINE(uint32_t) default_swab32(uint32_t val)
 	} \
 	__res; })
 
+#define bswap16(x) ({ \
+	uint16_t __res; \
+	if (__builtin_constant_p(x)) { \
+		__res = default_swab16(x); \
+	} else { \
+		__asm__("xchgb %b0,%h0" : "=q" (__res) : "0" ((uint16_t)(x))); \
+	} \
+	__res; })
+
+#elif defined(__GNUC__) && defined(__x86_64__)
+
+#define bswap32(x) ({ \
+	uint32_t __res; \
+	if (__builtin_constant_p(x)) { \
+		__res = default_swab32(x); \
+	} else { \
+		__asm__("bswapl %0" : "=r" (__res) : "0" ((uint32_t)(x))); \
+	} \
+	__res; })
+
+#define bswap16(x) ({ \
+	uint16_t __res; \
+	if (__builtin_constant_p(x)) { \
+		__res = default_swab16(x); \
+	} else { \
+		__asm__("xchgb %b0,%h0" : "=Q" (__res) : "0" ((uint16_t)(x))); \
+	} \
+	__res; })
+
 #elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64))
 
 #include <stdlib.h>
 
 #define bswap32(x) _byteswap_ulong(x)
+#define bswap16(x) _byteswap_ushort(x)
 
 #endif
 
@@ -48,3 +86,12 @@ GIT_INLINE(uint32_t) default_swab32(uint32_t val)
 #define htonl(x) bswap32(x)
 
 #endif
+
+#ifdef bswap16
+
+#undef ntohs
+#undef htons
+#define ntohs(x) bswap16(x)
+#define htons(x) bswap16(x)
+
+#endif