[cfe-dev] C++ and volatile lvalue-to-rvalue

[Johannes Schaub]

Christopher Jefferson wrote:

> 
> On 13 Oct 2011, at 12:43, Abramo Bagnara wrote:
> 
>> 
>> I want to put this gcc bug report to clang developer attention.
>> 
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50364
>> 
>> The gcc developers believe that according to the standard
>> 
>> volatile int v;
>> x = v = 3;
>> 
>> does not imply necessarily a read from v.
> 
> If you want to argue that this should both write to, and read from v, then
> I think you fall foul of lack of a sequence point, so the behaviour is
> undefined anyway.
> 

As clarified by Abramo, the assignment and the read from v are sequenced. 
However the following would be undefined behavior:

    volatile int v = 0;
    int x = v + v;

Note how "v + v" does two side effects on v, and these side effects are not 
sequenced.