[cfe-commits] C11 <stdatomic.h>

[Richard Smith]

On Mon, Sep 17, 2012 at 4:11 PM, Eli Friedman <eli.friedman at gmail.com>wrote:

> On Sat, Sep 15, 2012 at 1:05 AM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >
> >
> > On Fri, Sep 14, 2012 at 8:17 PM, Eli Friedman <eli.friedman at gmail.com>
> > wrote:
> >>
> >> On Fri, Sep 14, 2012 at 7:48 PM, Richard Smith <richard at metafoo.co.uk>
> >> wrote:
> >> > Hi,
> >> >
> >> > The attached patch adds an implementation of <stdatomic.h> to the set
> of
> >> > headers provided by Clang. Since this header is so compiler-dependent,
> >> > it
> >> > seems that we are the most rational component to be providing this
> >> > header
> >> > (even though, for instance, some flavors of BSD already provide their
> >> > own).
> >> > Please review!
> >>
> >> +// Clang allows memory_order_consume ordering for __c11_atomic_store,
> >> +// even though C11 doesn't allow it for atomic_store.
> >>
> >> That looks like a bug...
> >
> >
> > Possibly it's a bug in the specification for atomic_flag_clear?
> > memory_order_consume doesn't seem to have any meaning for a store
> operation.
> >
> >>
> >> Please put the new warning in a separate commit.
> >
> >
> > r163964.
> >
> >> It looks like standard requires that we expose functions named
> >> atomic_thread_fence, atomic_signal_fence, atomic_flag_test_and_set,
> >> atomic_flag_test_and_set_explicit, and atomic_flag_clear; your version
> >> of stdatomic.h doesn't include declarations for these functions (which
> >> is required by C11 7.1.4p1).
> >
> >
> > Ugh. And C11 7.1.2/6 requires them to have external linkage. I don't want
> > these functions to require linking to a library. We could emit them weak
> and
> > inline, but then we'll get a weak copy in every TU which includes this
> > header, which seems fairly egregious. Is there currently any way to emit
> a
> > function as linkonce_odr from C? Do you have any suggestions as to how to
> > proceed?
>
> There isn't any way to get linkonce_odr from C at the moment; patches
> welcome.  I don't see any issues with that from the standpoint of the
> standard; I'm a little worried about ABI-compat issues, though.
> (Specifically, if the system provides the header, having our own
> linkonce_odr version could cause strange linker errors.)
>
> We could put it into compiler-rt, and say that if someone tries to use
> the function instead of the macro without linking in compiler-rt,
> that's an error.  Not particularly satisfying either, but somewhat
> simpler.


After some discussion with Chandler, we think the best approach is to say
that the definition of these functions belongs in libc, and to provide only
declarations of them. A patch for that approach is attached.
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