On Mon, Sep 17, 2012 at 4:11 PM, Eli Friedman <[email protected]>wrote:
> On Sat, Sep 15, 2012 at 1:05 AM, Richard Smith <[email protected]> > wrote: > > > > > > On Fri, Sep 14, 2012 at 8:17 PM, Eli Friedman <[email protected]> > > wrote: > >> > >> On Fri, Sep 14, 2012 at 7:48 PM, Richard Smith <[email protected]> > >> 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.
stdatomic.diff
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