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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/50364] Volatile miscompilation
Date: Thu, 13 Oct 2011 08:43:00 -0000
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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."