Re: Question on volatile functions and GCC

[Tobias Burnus <burnus at net-b dot de>]

Jonathan Wakely wrote:
> On 5 March 2013 09:27, David Paterson wrote:
> > In addition to Jonathan's answer on the use of "volatile", it's worth adding
> > that it's not only used for memory mapped hardware. There are many other
> > uses, such as inter-thread communication, or indeed the example you
> > show below.
>
> Only in broken code.
>
> volatile is not for multithreading, you need proper synchronization
> for interthread communication.

Maybe he meant something like using volatile to prevent code moves with 
asynchronous communication (be it I/O or communication with libraries 
like MPI, or also call to some functions, which internally invoke some 
kind of synchronization.)

However, if I understand it correctly, GCC doesn't do such code moves* 
and C/C++ also does not allow them – even not without "volatile". That's 
different to Fortran, which allows for some code moves across procedure 
calls; hence, Fortran has – additionally to "volatile" – the 
"asynchronous" attribute to prevent those.

Tobias

* Side note: See PR49733 which tracks a case, where for Fortran code the 
lack of the asynchronous attribute is not used for optimizations.