So in your 'machine model is the physical machine' flavour, should "I cast an unaligned pointer to a byte array to int32_t and deref" on SPARC (a) do a bunch of

So in your 'machine model is the physical machine' flavour, should "I cast an unaligned pointer to a byte array to int32_t and deref" on SPARC (a) do a bunch of byte-load-and-shift-and-OR or (b) emit a simple word load which segfaults? If the former, it's not what the physical machine does, and if the latter, then you still need to write the code as "some portable other thing". Which is to say that the spec's UB here is in service of "allow the compiler to just emit a word load when you write *(int32_t)p". What I think the language is missing is a way to clearly write "this might be unaligned and/or wrong endianness, handle that". (Sometimes compilers provide intrinsics for this sort of gap, as they do with popcount and count-leading-zeroes; sometimes they recognize common open-coded idioms. But proper standardised support would be nicer.)