Re: [boost] Fw: [atomic] review results

Subject: Re: [boost] Fw: [atomic] review results
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-11-08 07:57:05

Tim Blechmann wrote:

> this implies that one can only use atomic integral types in shared memory, 
> but
> not std::atomic<>, because the std::atomic<> template has a per-instance
> is_lock_free member.

29.4/2: "The function atomic_is_lock_free (29.6) indicates whether the 
object is lock-free. In any given program execution, the result of the 
lock-free query shall be consistent for all pointers of the same type."

The query is not per-instance. It can't be performed at compile time though 
(this is probably motivated by instruction set differences like the famous 
386-486 divide where the program can only know at run time whether general 
atomics are available).

> i cannot find any reference in the standard, that std::atomic<T> is 
> lockfree,
> if there is a lockfree integral atomic type of the same size, ...

As a practical matter, it will be. Pedantically speaking, it is not possible 
to give this guarantee in the general case, because the integral type may 
have padding bits and trap representations. But I don't think that this is 
true on any platform that provides atomic operations, so in practice, we 
should be fine (although it's possible for an implementation to do the wrong 
thing in principle, but I doubt that many will).