Re: Compiler ordering barriers in C++0x

James Kanze <>
Mon, 5 May 2008 08:32:47 CST
On May 5, 3:09 pm, "Chris Thomasson" <> wrote:

"Szabolcs Ferenczi" <> wrote in message

On May 3, 2:13 pm, Anthony Williams <> wrote:

Szabolcs Ferenczi <> writes:

On May 2, 12:43 pm, Anthony Williams <> wrote:

All accesses to shared data MUST be synchronized with atomics: [...]

Can you elaborate this point please. How can you
generally synchronise N processes with help of atomics?
Do you mean only two processes under certain

If any thread modifies shared data that is not of type
atomic_xxx, the developer must ensure appropriate
synchronization with any other thread that accesses that
shared data in order to avoid a data race (and the
undefined behaviour that comes with that).

It is clear that you must synchronise access to shared
variable. Normally you must use a Critical Region for that.

It would be interesting to know what you mean by a "Critical
Region". My understanding of it is simply that it is a section
of code which is protected by some synchronization mechanism, so
that 1) only one thread can enter it at a time, and 2) memory
synchronization occurs on entering and leaving. In which case,
it's neither necessary nor sufficient to ensure "appropriate
synchronization". But I think Microsoft has redefined it to
mean something else.

I was curious how do you synchronise access to shared data
with atomics.

Really? How do you think some mutexs, semaphores, non-blocking
algorithms ect, ect, are actually implemented?

They synchronize access. On my usual platform (Sun Sparc), this
means inserting the appropriate membar instructions before
and/or after the access. Something which, of course, you can't
do in current C++; you need some inline assembler. (But you
know that.)

James Kanze (GABI Software)
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