[cfe-dev] RFC: __atomic_* support for gcc 4.7 compatibility

[Jean-Daniel Dupas]

Le 11 avr. 2012 à 02:08, Chandler Carruth a écrit :

> On Wed, Apr 11, 2012 at 1:03 AM, Howard Hinnant <hhinnant at apple.com> wrote:
> On Apr 10, 2012, at 7:59 PM, Howard Hinnant wrote:
> 
> > On Apr 10, 2012, at 7:24 PM, Chandler Carruth wrote:
> >
> >> I want to call out this point:
> >>
> >> On Wed, Apr 11, 2012 at 12:25 AM, Richard Smith <richard at metafoo.co.uk> wrote:
> >> This approach, including the need to interpose in front of <atomic> and <shared_ptr> (and any other user of the atomics builtins in libstdc++) and detect whether we're about to include a header from libstdc++, seems sufficiently fragile that I'm still in favour of introducing GNU-compatible builtins.
> >>
> >> I think this is setting ourselves up for an endless game of catchup. What is worse, our users will suffer.
> >>
> >> Version skew between libstdc++ and Clang is inevitable. As a consequence, the advantages of being maximally compatible with libstdc++ are only present when we can be largely *forward* compatible. This seems impossible if we have to intercept ever header to reference a GCC builtin and introduce our wrapper for it.
> >>
> >>
> >> At a fundamental level, I am not yet comfortable with Clang, after defining __GNUC__ and acting as a heavily compatible compiler with GCC, going on to define builtin functions which share the same name but incompatible semantics with GCC builtins. This seems like a recipe for a long string of puzzling compilation failures, user frustration, and impeded adoption of Clang.
> >>
> >> I think we should continue with the long standing policy of, when reasonable and tenable, supporting the GCC builtins with their names, and allowing users to not care. Then, under a *different* naming pattern, and guarded by proper __has_feature macros etc, we should provide Clang-specific extensions which have the semantics we would rather see.
> >>
> >> While a realize this isn't ideal, and it requires renaming currently supported intrinsics I see few options left to us:
> >> 1) We haven't released with these intrinsics
> >> 2) GCC has released with its intrinsics
> >> 3) We failed to discuss our intrinsics sufficiently with the GCC folks to get them to implement the same set
> >> 4) GCC folks failed to discuss theirs with us sufficiently to get us to implement the same set
> >> 5) We need something for Clang v3.1, or it is instantly un-usable with GCC 4.7 and later.
> >>
> >> I don't like any of this, and it isn't how I would wish for things to proceed if I had it to do again, but this is where we are. We need to realize that, today, I cannot compile any meaningful C++98 code with the default C++ standard library on Linux (after a distro picks up GCC 4.7) and Clang. I do not think that is an acceptable state to release Clang under.
> >
> > Perhaps we need to stake out another namespace for clang atomic intrinsics.  Our current prefix is:
> >
> > __atomic_
> >
> > Suggestions:
> >
> > __clang_atomic_
> > __clng_atmc_
> > _ClangAtomic_
> > _CLNGatomic_
> > _CLNG_ATM_
> > _ClngAtm_
> >
> > The final suggestion has the same length as our current prefix.
> 
> 
> Or even shorter: :-)
> 
> __atomx_
> 
> I am quite lazy when it comes to typing, but perhaps here is not the time to save characters. ;] I would vote:
> 
> __clang_atomic_
> 

The prefix length should not be an issue.
As David said, these builtins should be reserved to implement <stdatomic.h> and <atomic>, so should never have to call them explicitly.

> or, based on the discussion between David and Richard, perhaps:
> 
> __c11_atomic_
> 
> to call out their close correspondance with the c11 spec.


-- Jean-Daniel




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