[x86_64] Provide __bswap_{16,32,64}s()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86_64/include/bits/byteswap.h

 #ifndef _BITS_BYTESWAP_H
 #define _BITS_BYTESWAP_H
 
+/** @file
+ *
+ * Byte-order swapping functions
+ *
+ */
+
+#include <stdint.h>
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static inline __attribute__ (( always_inline, const )) uint16_t
 __bswap_variable_16 ( uint16_t x ) {
 	__asm__ ( "xchgb %b0,%h0" : "=Q" ( x ) : "0" ( x ) );
 	return x;
 }
 
+static inline __attribute__ (( always_inline )) void
+__bswap_16s ( uint16_t *x ) {
+	__asm__ ( "rorw $8, %0" : "=g" ( *x ) : "0" ( *x ) );
+}
+
 static inline __attribute__ (( always_inline, const )) uint32_t
 __bswap_variable_32 ( uint32_t x ) {
 	__asm__ ( "bswapl %k0" : "=r" ( x ) : "0" ( x ) );
 	return x;
 }
 
+static inline __attribute__ (( always_inline )) void
+__bswap_32s ( uint32_t *x ) {
+	__asm__ ( "bswapl %k0" : "=r" ( *x ) : "0" ( *x ) );
+}
+
 static inline __attribute__ (( always_inline, const )) uint64_t
 __bswap_variable_64 ( uint64_t x ) {
 	__asm__ ( "bswapq %q0" : "=r" ( x ) : "0" ( x ) );
 	return x;
 }
 
+static inline __attribute__ (( always_inline )) void
+__bswap_64s ( uint64_t *x ) {
+	__asm__ ( "bswapq %q0" : "=r" ( *x ) : "0" ( *x ) );
+}
+
 #endif /* _BITS_BYTESWAP_H */