[cfe-commits] C11 <stdatomic.h>

[Richard Smith]

Ping.

On Tue, Sep 18, 2012 at 6:47 PM, Richard Smith <richard at metafoo.co.uk>wrote:

> 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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