Re: private static object construction bug?

[Fergus Henderson <fjh at cs dot mu dot OZ dot AU>]

On 05-Aug-2002, Mark Mitchell <mark@codesourcery.com> wrote:
> 
> 
> --On Monday, August 05, 2002 09:48:49 AM -0700 Joe Buck 
> <Joe.Buck@synopsys.com> wrote:
> 
> >
> >> I entered a bug report on this (7458) that was promptly closed by
> >> a Cygwin developer, but I believe incorrectly so, so I am posting here.
> >
> > I've squashed the example a bit, but here it is:
> >
> > struct the_struct {
> > 	int i;
> > 	the_struct() { i = 0;}
> > 	~the_struct() {	i = 1;}
> > };
> >
> > void function() {
> > 	static the_struct temporary;
> > }
> >
> > The decision to close the bug report was correct.
> 
> Actually, I think it was only sort-of correct.
> 
> The ABI specifies functions a compiler can use to get thread safe
> construction of local temporaries.

Did you mean to say "local statics"?

I don't see what would be non-thread-safe about constructing
local temporaries, since temporaries have automatic storage
duration and get constructed on the thread's stack.

> The compiler can choose not to
> use them.  G++ makes that choice, but partly due to not getting
> around to calling the functions.  The intended code sequence is
> not really any slower after the object has been constructed.
> 
> Right now, we do this approximately (there are exceptions to worry
> about!):
> 
>   if (!guard) {
>     guard = 1;
>     // Construct variable.
>   }
> 
> The new sequence would be:
> 
>   if (!guard) {
>     __cxa_guard_acquire (&guard); // I forget the exact name.
>     // Construct variable.
>     __cxa_guard_release (&guard):
>   }

Hmm. I don't understand how this would solve the problem.
It would prevent simultaneous construction, but the
variable could still get constructed twice, couldn't it?
That could still lead to problems.

A slightly different code sequence could solve the double-construction
problem.

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.