In 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");

** Reply to message from George Anzinger <[email protected]> on Wed, 15 Nov 2000 12:55:46 -0800

Quoted 10 lines

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))

Quoted 13 lines

              __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");

I've been a lot of gcc inline asm recently, and I still consider it a black art. There are times when I just throw in what I think makes sense, and then look at the code the compiler generated. If it's wrong, I try something else.

Both versions look correct to me. The "m" simply tells the compiler that __xg(ptr) is a memory location, and the contents of that memory location should NOT be copied to a register. The confusion occurs because its unintuitive that the "*" is required. Otherwise, it would have been "r", which basically tells the compiler to copy the contents to a register first.