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");
George
** Reply to message from George Anzinger <[email protected]> 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 - [email protected]
Interactive Silicon - http://www.interactivesi.com
When replying to a mailing-list message, please direct the reply to the mailing list only. Don't send another copy to me.
Timur Tabi wrote:
>
> ** Reply to message from George Anzinger <[email protected]> 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.
>
I know the feeling. I am currently strugling with "inconsistant
constraints". Still, I must assume that form 1 was used instead of 2
for some reason....
George