The more complicated the instruction is, the less likely it was implemented to spec on all the various products and mask steppings you might execute on ... and

The more complicated the instruction is, the less likely it was implemented to spec on all the various products and mask steppings you might execute on ... and the less likely its published definition exactly matches C or C++ Standard and platform C ABI specs. And, the less likely that ABI spec nails down all the details. Compiler implementors don't like to guess, but don't get a choice. If the instruction provided doesn't match the Standard, which do they implement? Both choices are wrong.