Re: [Boost-users] [thread] variables in thread examples require 'volatile'?

Subject: Re: [Boost-users] [thread] variables in thread examples require 'volatile'?
From: Stonewall Ballard (sb.list_at_[hidden])
Date: 2009-12-08 08:44:51

On Dec 8, 2009, at 1:21 AM, Gottlob Frege wrote:
> ...
> The reason that data_ready is NOT 'left' in a register is because a
> function with unknown side-effects (cond.wait) is being called.  You
> could just as easily have:
> 
> while (!data_ready)
>   some_library_function();
> 
> and the compiler will ensure that data_ready is re-read (ie not left
> in a register) because it is a global, and the compiler is unsure
> whether or not some_library_function() modifies data_ready or not.  In
> theory the compiler could go look into some_library_function() and
> figure that out, but in practice it doesn't.
> 
> As for memory barriers, cond.wait() locks and unlocks the mutex, which
> puts in the necessary memory barriers.  Note that the order of
> lock/unlock/relock is such that data_ready is only read while holding
> the lock.  Of course, nothing here says whether the other thread
> *wrote* data_ready inside a lock or with the necessary
> release-barrier, but let's assume that it did.
> 
> So recap:
> 
>>>> volatile is not needed in that example because access is protected by a mutex.
> 
> No, I'd say volatile is not needed because functions with unknown
> side-effects are being called (and/or if the mutex code was somehow
> magically inlined, then we can assume the compiler recognizes the
> memory-barrier intrinsics and forces memory re-reads because of that).

I woke up in the middle of the night last night with that realization, so I completely agree with this analysis.

Don't you hate it when that happens? :-)

> ...

 - Stoney

-- 
Stonewall Ballard 
stoney_at_[hidden]           http://stoney.sb.org/