How they live inside of a structure is far into implementation-defined land. No ABI (afaik) says where bits live if they do not fill an octet. In practice, the compilers agree with themselves over time, but that is "code as spec" which is everyone's favorite thing for interop.
It's well defined in the system V ABI, isn't it? Compilers do follow that. Otherwise no layout is defined by the C and C++ standards, even outside of bitfields.
edit: It's under "Bit-Fields" in [1]. I'm not entirely happy with the wording, but it's there.
