Re: Double-Checked Locking pattern issue

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 8 Jan 2008 10:46:25 -0600

Message-ID:

<uKvF2WhUIHA.6060@TK2MSFTNGP05.phx.gbl>

"George" <George@discussions.microsoft.com> wrote in message
news:B74DF779-2F06-4F47-8C4E-87C3E7349EA6@microsoft.com...

Thanks Tom,

Actually, I have not tried to read assembly code. I will try it.

But what makes me confused is why? Why reorder (assignment before real
construct object instance) will improve performance?

You just aren't listening. Whether it improves performance is a hardware
detail. If there is a CPU out there which runs faster that way, it is
allowed to reorder those instructions. If there isn't a CPU out there which
runs faster that way, then no one will be able to explain how performance is
improved... but such a CPU could be released next week and then your code
would break.

The C++ memory model only describes a single thread of execution. The
compiler can do anything that doesn't break that memory model, including
reordering operations between sequence points, and not using memory
barriers -- which lets the CPU reorder instructions.

You have a fundamental error in your thinking. You are asking people to
prove something is broken, where instead you need to prove it is correct.
Lack of a counterexample does not constitute a valid proof.

regards,
George