I think there is a world of wild and crazy C++ (like, boost::spirit-grade, or std::allocator-using) that you're imagining, that is not what I am thinking of. If you took C, and added vector<T> and hash_table<K, V>, added constructors and destructors so you don't have to call cleanup functions, you'd get a language which most sensible non-embedded C programs would map to, which then maps upward to C++.
Maybe some templated functions like std::min<T> and std::max<T> and add_checking_overflow<T> would be nice to have too.
An example of where I think properly written C diverges from portable assembler is things like here: https://github.com/Tarsnap/tarsnap/blob/master/libcperciva/d...
It depends on whether you think ELASTICARRAY_DECL is within the scope of portable assembler. (It gives you type safety!) (And I don't know what advanced assembly languages can offer in terms of that -- maybe they do too.)