[cfe-commits] C11 <stdatomic.h>

[Eli Friedman]

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.

-Eli