bswap.h: use built-in bswap32 and bswap64 when available

Clang 12 and GCC 10 added support for built-in bswap32 and bswap64. As bswap.h doesn't support ARM64/aarch64 when using GCC/Clang, this is a good opportunity to start using those built-ins. In the future, we might want to remove the custom git_bswap32 and git_bswap64 altogether and leave that up to the compiler to handle.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>

Author: dennisameling

compat/bswap.h

@@ -74,6 +74,12 @@ static inline uint64_t git_bswap64(uint64_t x)
 }
 #endif
 
+/* available since Clang 12 and GCC 10, i.a. needed for ARM64/aarch64 on Windows */
+#elif defined(__has_builtin) && __has_builtin(__builtin_bswap64)
+
+#define bswap32(x)     __builtin_bswap32((x))
+#define bswap64(x)     __builtin_bswap64((x))
+
 #elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64) || defined(_M_ARM64))
 
 #include <stdlib.h>