https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78158
--- Comment #17 from Jakub Jelinek <jakub at gcc dot gnu.org> --- It is hard to predict. That said, I think libtsan uses mostly the old style __sync_* intrinsics or when it uses atomic_load/store it uses them with memory model not known at compile time, therefore it acts as seq_cst anyway (at least GCC doesn't emit for __atomic_* builtins a switch depending on the memory model with the various sequences depending on memory model; thus memory model for the inlined hw atomics is only useful if it is compile time known constant). So it matters mainly for the libtsan bookkeeping (what mmodel it records).
