[Bug c++/50364] New: Volatile miscompilation

[Bug c++/50364] New: Volatile miscompilation

[abramobagnara at tin dot it]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364

             Bug #: 50364
           Summary: Volatile miscompilation
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: abramobagnara@tin.it


$ cat p.cc
volatile int a, b;
void f() {
  b = 1 + (a = 0);
}
$ g++ -O2 -S p.cc
$ cat p.s
        .file   "p.cc"
        .text
        .p2align 4,,15
        .globl  _Z1fv
        .type   _Z1fv, @function
_Z1fv:
.LFB0:
        .cfi_startproc
        movl    $0, a
        movl    $1, b
        ret
        .cfi_endproc
...

As the typescript above shows, the conversion between lvalue (a = 0) and
relative rvalue does not generate a volatile read memory access unlike what I
think that standard mandates.

[Bug c++/50364] Volatile miscompilation

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-10-12 15:54:19 UTC ---
Doesn't look like a C++ front-end issue.

[Bug c++/50364] Volatile miscompilation

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-10-13 08:43:10 UTC ---
That works as designed.  See the recent discussion about this very topic
on the gcc mailinglist and

2010-08-19  Nathan Sidwell  <nathan@codesourcery.com>
            Richard Guenther  <richard.guenther@gmail.com>

        * gimplify.c (gimplify_modify_expr): When assigning to volatiles,
        copy the src value and return a copy.
        * doc/extend.texi (Volatiles): Move from C++ to C and expand.
        (C++ Volatiles): Adjust to describe C++ semantics only.

esp.

"Assignments are also expressions and have an rvalue.  However when
assigning to a scalar volatile, the volatile object is not reread,
regardless of whether the assignment expression's rvalue is used or
not.  If the assignment's rvalue is used, the value is that assigned
to the volatile object.  For instance, there is no read of @var{vobj}
in all the following cases:

@smallexample
int obj;
volatile int vobj;
vobj = @var{something};
obj = vobj = @var{something};
obj ? vobj = @var{onething} : vobj = @var{anotherthing};
obj = (@var{something}, vobj = @var{anotherthing});
@end smallexample

If you need to read the volatile object after an assignment has
occurred, you must use a separate expression with an intervening
sequence point."