> Actually, you can't infer the alignment from the headers. You can see in the > definition of __m128i in the GCC headers that it lacks the __aligned__ portion > too, so all those types should have had the alignment of "long long" (8) but > don't. It's implemented in the compiler itself. I did see that! Thanks for saving me the trouble of trying to track it down. But given the compiler-dependent alignment differences, it seems one cannot portably use this type in a structure because different compilers on the same platform will pad differently. I don't want to do this anyway, but where is it written I can't? If I understand correctly, these types are not part of the platform ABI. That's the answer, right? So, what's the plan for _BitInt?