[eru]

> Unaligned access being fine in one architecture, but not in others would create separate dialects, regardless of being blessed by ISO C.

That doesn't mean unaligned access would need to be UB. It could be implementation defined. Or could just be defined to result in an error on all machines.

[leni536]

Implementation-defined without further constraining the behavior is not much better than undefined.

Defining it to always error would add overhead even for proper aligned access on x86, as the generated code would need to explicitly check in many cases.

[eru]

> Implementation-defined without further constraining the behavior is not much better than undefined.

No, it's a lot better. Because implementations have to define it.