[cfe-commits] C11 <stdatomic.h>

[Richard Smith]

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?

Thanks! Richard
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