From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-03-22 20:33:06
Anthony Williams wrote:
> "Peter Dimov" <pdimov_at_[hidden]> writes:
>> I don't see call_once in jss_thread.zip, by the way; maybe you
>> forgot to put it into the archive?
> Oops. Thanks for spotting that. I've added it to the archive, and
> updated it to take multiple arguments in passing.
Some comments on that:
void call_once(once_flag& flag,Function f)
// Try for a quick win: if the proceedure has already been called
// just skip through:
long const function_complete_flag_value=0xc15730e2;
detail::win32::handle_holder const closer(mutex_handle);
detail::win32_mutex_scoped_lock const lock(mutex_handle);
The first load needs to be a load_acquire; the second can be ordinary since
it's done under a lock. The store needs to be store_release.
An interlocked_read is stronger ('ordered') and more expensive than needed
on a hardware level, but is 'relaxed' on a compiler level under MSVC 7.1
(the optimizer moves code around it). It's 'ordered' for the compiler as
well under 8.0; the intrinsics have been changed to be compiler barriers as
well. InterlockedExchange is similar.
A load_acquire can be implemented as a volatile read under 8.0, and a
volatile read followed by _ReadWriteBarrier under 7.1.
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