Cythoning and then trying to compile the attached test causes gcc-4.9 to error out with: physics.c:1840:165: error: invalid entry to OpenMP structured block if (unlikely(!__pyx_v_XY.memview)) { __Pyx_RaiseUnboundMemoryviewSliceNogil("XY"); {__pyx_filename = __pyx_f[0]; __pyx_lineno = 21; __pyx_clineno = __LINE__; goto __pyx_L5_error;} } clang version 3.5 (with openmp) gives the less helpful "use of undeclared label '__pyx_L5_error'". Ray Jones