Messages in this thread |  | | Date | Wed, 15 Nov 2000 16:17:37 -0600 | From | Timur Tabi <> | Subject | Re: In line ASM magic? What is this? |
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** 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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