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SubjectRe: In line ASM magic? What is this?
** Reply to message from George Anzinger <george@mvista.com> on Wed, 15 Nov
2000 12:55:46 -0800


> 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");

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.



--
Timur Tabi - ttabi@interactivesi.com
Interactive Silicon - http://www.interactivesi.com

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