Subj : Re: posix and lock-free algorithms
To   : comp.programming.threads
From : David Schwartz
Date : Thu Aug 11 2005 02:48 pm


"Alexander Terekhov" <terekhov@web.de> wrote in message 
news:42FB7410.5FD8C256@web.de...

> To begin with, even if compiler won't optimize-out your per-thread
> locks (which is perfectly legal), compiler and/or hardware can
> transform your
>
> :         Singleton* tmp = new Singleton;
> :         local.lock(); local.unlock(); // just a memory barrier
> :         pInstance = tmp;
>
> to (in effect):
>
> < pseudo-code; assume everything is throw()-nothing >
>
>          local.lock();
>          pInstance = Singleton::operator new();
>          ::new(pInstance) Singleton();
>          local.unlock();

    No, it can't. The POSIX lock/unlock functions are defined as 
synchronizing memory. If this means anything at all, it means loads/stores 
can't be reordered across those functions.

    DS

.