linux/arch/x86/include/asm/byteorder.h

#ifndef _ASM_X86_BYTEORDER_H
#define _ASM_X86_BYTEORDER_H

#include <asm/types.h>
#include <linux/compiler.h>

#ifdef __GNUC__

#ifdef __i386__

static inline __attribute_const__ __u32 ___arch__swab32(__u32 x)
{
#ifdef CONFIG_X86_BSWAP
	asm("bswap %0" : "=r" (x) : "0" (x));
#else
	asm("xchgb %b0,%h0\n\t"	/* swap lower bytes	*/
	    "rorl $16,%0\n\t"	/* swap words		*/
	    "xchgb %b0,%h0"	/* swap higher bytes	*/
	    : "=q" (x)
	    : "0" (x));
#endif
	return x;
}

static inline __attribute_const__ __u64 ___arch__swab64(__u64 val)
{
	union {
		struct {
			__u32 a;
			__u32 b;
		} s;
		__u64 u;
	} v;
	v.u = val;
#ifdef CONFIG_X86_BSWAP
	asm("bswapl %0 ; bswapl %1 ; xchgl %0,%1"
	    : "=r" (v.s.a), "=r" (v.s.b)
	    : "0" (v.s.a), "1" (v.s.b));
#else
	v.s.a = ___arch__swab32(v.s.a);
	v.s.b = ___arch__swab32(v.s.b);
	asm("xchgl %0,%1"
	    : "=r" (v.s.a), "=r" (v.s.b)
	    : "0" (v.s.a), "1" (v.s.b));
#endif
	return v.u;
}

#else /* __i386__ */

static inline __attribute_const__ __u64 ___arch__swab64(__u64 x)
{
	asm("bswapq %0"
	    : "=r" (x)
	    : "0" (x));
	return x;
}

static inline __attribute_const__ __u32 ___arch__swab32(__u32 x)
{
	asm("bswapl %0"
	    : "=r" (x)
	    : "0" (x));
	return x;
}

#endif

/* Do not define swab16.  Gcc is smart enough to recognize "C" version and
   convert it into rotation or exhange.  */

#define __arch__swab64(x) ___arch__swab64(x)
#define __arch__swab32(x) ___arch__swab32(x)

#define __BYTEORDER_HAS_U64__

#endif /* __GNUC__ */

#include <linux/byteorder/little_endian.h>

#endif /* _ASM_X86_BYTEORDER_H */