Intel Skylake/Kaby Lake processors: broken hyper-threading

Yes, and I've just verified that clang (at least) will generate movb %dh, 0(%rax) in at least one test case. Put it in a loop though and it'll happily vectorise it into shifts and shuffles.

That use doesn't feel like one that would upset a modern (threaded) pipeline though, because there is no chance of a write-after-read style problem. I think that the issue is much more likely to involve writing to one of the [abcd]h registers, which would need to be unpacked into a read-modify-update sequence of micro-ops. For "make an int from left-shifted bytes" style code, I've only seen the compiler generate byte loads followed by left-shifts, rather than loading byte values directly into high reg bytes.