[Bug c/59785] New: atomic_store should load source operand atomically?

[Bug c/59785] New: atomic_store should load source operand atomically?

[ma.jiang at zte dot com.cn]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59785

            Bug ID: 59785
           Summary: atomic_store should load  source operand atomically?
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ma.jiang at zte dot com.cn

gcc  provide a atomic_store interface as below.
void __atomic_store (type *ptr, type *val, int memmodel)

In manual, There is only a simple description ---"This is the generic version
of an atomic store. It stores the value of *val into *ptr."

But it seemed that this interface could not do what the description said when
*val is a global memory which might be messed up by other threads. 

unsigned long long int ta=123;
unsigned long long int tb=321;
int test()
{
        __atomic_store(&tb, &ta, __ATOMIC_RELAXED);
        return 0;
}
On ARM,the codes above give out a assembly instruction sequence as below.
        movw    r3, #:lower16:ta
        movt    r3, #:upper16:ta
        ldrd    r0, [r3]
        movw    r3, #:lower16:tb
        movt    r3, #:upper16:tb
.L3:
        ldrexd  r4, r5, [r3]
        strexd  r2, r0, r1, [r3]
        cmp     r2, #0
        bne     .L3

the source operand ta was not fetched atomically, so we might get a totally
wrong num from tb when ta were modified by other threads. 

I guess the atomic_store interface only provide a function as--"fetch val
non-atomically ,and then store it into ptr atomically."
This is exactlly what the gimple tree shows.
test ()
{
  long long unsigned int ta.0;
  int D.4127;

  ta.0 = ta;
  __atomic_store_8 (&tb, ta.0, 0);
  D.4127 = 0;
  return D.4127;
}

I think it would be better to give a more detailed desctription in the manual.
Or make a atomical load in the atomic_store.

[Bug c/59785] atomic_store should load source operand atomically?

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59785

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is atomic store, not atomic load, what do you expect?
The only reason for taking val as a pointer rather than the value itself is
that some values really need to live in memory and the API is generic.  If you
are trying to pass an address of a var that is modified by other threads, it is
just a bug in your source code, you likely want to do an atomic load into a
temporary automatic variable etc. which won't be modified by other threads and
only then atomic_store it.

[Bug c/59785] atomic_store should load source operand atomically?

[ma.jiang at zte dot com.cn]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59785

--- Comment #2 from ma.jiang at zte dot com.cn ---
Ok, Thanks for the reply. I know what you mean. A atomic_store is not
responsible for loading the source operand atomically as it is just a store.