what is the proper asm spec for xchg

[Sean McNeil]

I'm looking at why audio/sdl_mixer will not compile and I see that
devel/sdl12 has an include with

#elif defined(__GNUC__) && defined(__x86_64__)
static __inline__ Uint16 SDL_Swap16(Uint16 x)
{
        __asm__("xchgb %b0,%h0" : "=q" (x) :  "0" (x));
        return x;
}

This appears to be incorrect as it is giving errors when compiled.  Does
anyone more conversant with asm directives for amd64 have an idea what
this should be?

Thanks,
Sean

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