Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : Gianni Mariani
Date : Tue Feb 15 2005 09:38 pm

Alexander Terekhov wrote:
....
> 
> And don't forget that compilers also reorder.

In the case I gave above, the compiler is not allowed to re-order if 
it's conforming.

  In the case of
> overly constrained hardware, "per-thread" synchronization is
> needed to preclude "bad things" on *compiler's* part (hardware
> aside for a moment).

In the example I gave, hardware is the issue.

> 
> [...]
> 
>>describe here.  Each thread has different memory mapped to the same
>>address (don't try to take the address of an __thread variable though).
> 
> 
> Why so?

 From a disassemble of :

     static __thread Type * x;
     if ( ! x )

mov    %gs:0x0,%eax
test   %eax,%eax
jne    xxxx

You'll notice the %gs segement specifier.  I suspect that the gs: points 
to a different segment for each thread.  If you try to take the address, 
  you're not pointing to the same place.

I liked the SGI Irix way where the same address was mapped to different 
pysical memory for each thread.

.