bswap.h: add support for built-in bswap functions

Newer compiler versions, like GCC 10 and Clang 12, have built-in
functions for bswap32 and bswap64. This comes in handy, for example,
when targeting CLANGARM64 on Windows, which would not be supported
without this logic.

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_bswap32) && __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>