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SubjectIn line ASM magic? What is this?
I am trying to understand what is going on in the following code.  The
reference for %2, i.e. "m"(*__xg(ptr)) seems like magic (from
.../include/i386/system.h). At the same time, the code "m" (*mem) from
the second __asm__ below (my code) seems to generate the required asm
code. Before I go with the simple version, could someone tell me why?
Inquiring minds want to know.

struct __xchg_dummy { unsigned long a[100]; };
#define __xg(x) ((struct __xchg_dummy *)(x))

__asm__ __volatile__(LOCK_PREFIX "cmpxchgl %b1,%2"
: "=a"(prev)
: "q"(new), "m"(*__xg(ptr)), "0"(old)
: "memory");

__asm__ __volatile__(
LOCK "cmpxchgl %1,%2\n\t"
:"=a" (result)
:"r" (new),
"m" (*mem),
"a0" (test)
: "memory");

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