bcraig added a comment. In https://reviews.llvm.org/D23719#520969, @rsmith wrote:
> In https://reviews.llvm.org/D23719#520954, @bcraig wrote: > > > In https://reviews.llvm.org/D23719#520952, @rsmith wrote: > > > > > Are we really guaranteed that the C and C++ compiler behave the same way > > > here? I don't see why that would necessarily be the case. > > > > > > For libc++, std::atomic is implemented in terms of _Atomic. So as long as > > the C++ compiler doesn't butcher _Atomic, it seems that the behavior would > > be the same. > > > I don't see any good reason to assume that's the case. GCC, for instance, > does not define _Atomic *at all* in C++ mode; the implementation used by > libc++ in that case is completely different. Also, as far as I know, libc++ > did not previously require the host to have a C11 compiler. And there's no > reason to assume that the C compiler picked up by cmake is in any way related > to the C++ compiler. > > Bottom line: if you want to know how the C++ compiler behaves, you need to > test the C++ compiler not the C compiler. I managed to completely miss "#define _Atomic(x) __gcc_atomic::__gcc_atomic_t<x>" in the atomic header. On a different note, the general class of checks that LLVM and LIBCXX have in place to detect atomics generally don't work for cross-compilation. The set(CMAKE_REQUIRED_FLAGS ...) calls overwrite important settings. I end up setting HAVE_CXX_ATOMICS_WITHOUT_LIB=ON, HAVE_CXX_ATOMICS64_WITHOUT_LIB=ON, and LIBCXX_HAVE_CXX_ATOMICS_WITHOUT_LIB=ON explicitly, because the automatic check just doesn't work. So I look forward to the day when updates to these checks stop breaking my build. Given Richard's comments, I don't think this check is the one. https://reviews.llvm.org/D23719 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
