Re: [PATCH] Destructor support for C++11 thread_local variables

[Jason Merrill <jason at redhat dot com>]

On 10/11/2012 02:38 AM, Siddhesh Poyarekar wrote:
> On Wed, 10 Oct 2012 16:20:19 -0700 (PDT), Roland wrote:
> > I really hope there will be a way to use this from C, e.g.
> >
> > __thread type x;
> > static void __attribute__((thread_destructor (x))) foo (type *x_ptr)
> > { ... }

I don't think we want implicit destructor calls like this from C; in G++ 
__thread and thread_local behave differently because supporting dynamic 
initialization and destruction causes run-time overhead for all uses.

I think it would make sense to have a user-accessible thread_atexit 
entry point, however, so that people can explicitly register the 
destructor when they explicitly initialize the variable.

> The destructor does take an object pointer.  I could keep a static
> process-wide list of destructors instead of the current thread-local
> one and use locking to do this.  It won't work if the destructor
> depends on some context within its current thread.

Right, that's the problem.  In general we can't assume that reaching 
into another thread's local data will work sensibly.

> C++11x does not seem to say anything about thread_local behaviour when
> DSOs are involved.

Nope, C++11 doesn't say anything about DSOs at all.  So far it's been 
too hard to resolve the differences in semantics between ELF and PE into 
something useful.

Jason