Well, I would clarify - In reference to the memory models I was talking about, it only applies to code written as part of those kernels, userspace code does not "inherit" that memory model. And I don't think portability of kernel code is much of a concern, the memory model is only one of a large variety of problems.

That said, for userspace, pthreads already gives enough guarantees on ordering via mutex's that there aren't really any problems unless you're trying to do atomics in userspace, which before C11 would have been hard to do portability. And the rest of the things I was talking about like the aliasing model are defined as part of the C standard (C89 and C99), so it's always the same regardless of the compiler (ignoring possible bugs).