[cfe-commits] C11 <stdatomic.h>

[Hal Finkel]

Richard,

I'd like to re-open this issue; our last thread on this seemed to fizzle out with a discussion on how the ABI should be defined, Clang/LLVM bugs and probable C11 (or perhaps C++11) defects. This, however, still leaves our users in the somewhat-unfortunate situation of not having a stdatomic.h header when using Clang on Linux (and several other platforms) -- and to make matters worse, we also don't define __STDC_NO_ATOMICS__. I've received requests from several of my users to resolve this issue, and I think that we should move-forward with adding this header. As I'm sure you know, GCC 4.9 now ships a stdatomic.h, as does FreeBSD. Both seem to use _Bool as the underlying type for atomic_flag (at least when __GCC_ATOMIC_TEST_AND_SET_TRUEVAL == 1).

Regarding atomic_flag_test_and_set and friends, and whether these will be defined by libc, regardless of whether we provide proper definitions, we should follow GCC 4.9 and FreeBSD and define these functions as macros (this is allowed by 7.1.4). This provides users with a fully-functional header file regardless of whether the system libc provides the required external function implementations (I don't know of any that do).

Thanks again,
Hal

----- Original Message -----
> From: "Richard Smith" <richard at metafoo.co.uk>
> To: "Eli Friedman" <eli.friedman at gmail.com>
> Cc: "cfe commits" <cfe-commits at cs.uiuc.edu>
> Sent: Tuesday, September 18, 2012 8:47:37 PM
> Subject: Re: [cfe-commits] C11 <stdatomic.h>
> 
> 
> 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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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory