http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- The code generated appears fully in accordance with the semantics of C11. You refer to 5.1.2.4#14, the definition of "carries a dependency". The term "carries a dependency" is used outside its own definition only in the definition of "dependency-ordered before". This testcase contains no instances of "dependency-ordered before", because any such instance must directly or indirectly involve a case of "A performs a release operation on an atomic object M, and, in another thread, B performs a consume operation on M and reads a value written by any side effect in the release sequence headed by A", and the test involves no atomic objects. "dependency-ordered before", in turn, is only used in the definition of "inter-thread happens before", which is only used in the definition of "happens before". Please provide a complete testcase, using _Atomic or <stdatomic.h> (and so using GCC 4.9, of course) that demonstrates any bug: that is, that does not contain a data race according to the C11 definition, but where GCC has reordered code in a way that introduces one (and so one can be demonstrated by enough iterations of the threads in the testcase) - or where the code generated fails some other semantics associated with "happens before", such as those for "visible side effect" and "visible sequence of side effects". I expect any such bug, and fix, would probably not be a front-end issue but an issue with the atomic built-in functions failing to ensure appropriate ordering in the presence of dependencies involving atomic operations (dependencies not involving such operations having no effects on C11 semantics).
