inline assembly question (memory side-effects)

[Till Straumann]

Hi.

What is the proper way to tell gcc that a
inline assembly statement either modifies
a particular area of memory or needs it
to be updated/in-sync because the assembly
reads from it.

E.g., assume I have a

struct blah {
    int sum;
  ...
};

which is accessed by my assembly code.

int my_chksum(struct blah *blah_p)
{
   blah_p->sum = 0;
   /* assembly code computes checksum, writes to blah_p->sum */
   asm volatile("..."::"b"(blah_p));
   return blah_p->sum;
}

How can I tell gcc that the object *blah_p
is accessed (read and/or written to) by the
asm (without declaring the object volatile or
using a general "memory" clobber).
(If I don't take any measures then gcc won't know
that blah_p->sum is modified by the assembly and
will optimize the load operation away and return
always 0)

A while ago (see this thread
http://gcc.gnu.org/ml/gcc/2008-03/msg00976.html)
I was told that simply adding the affected
memory area in question as memory ('m') output-
or input-operands is not appropriate -- note that
this is in contradiction to what the gcc info page
suggests (section about 'extended asm':

  "If you know how large the accessed memory
   is, you can add it as input or output..."

along with an example).

Could somebody please clarify (please don't suggest how
this particular example could be improved but try to
answer the fundamental question) ?

Thanks
-- Till

PS: please CC me as I'm not subscribed to the gcc list