Comment by brucehoult

> except if the source is a high-byte register

That's just one more instruction, to right-align the AH, BH etc src operand prior to exactly the same instructions as above.

And, yes, this being 64 bit code compilers won't be generating such instructions. In fact they started avoiding them as soon as OoO hit in the Pentium Pro, P II, P III etc in the mid 90s because of "partial register update stalls".