Not to contradict others messages here, but to add to them: the argument (made elsewhere) by RISC-V peeps is that Intel etc regularly introduce rather junky ins

Not to contradict others messages here, but to add to them: the argument (made elsewhere) by RISC-V peeps is that Intel etc regularly introduce rather junky instructions that they can and do patent (the mechanism of), merely in order to continue to patent encumber the collective ISA, and so keep the network effect as their purely proprietary possession, long after the truly significant patents have lapsed; since compatibility would now require the new instructions, too. I'm not making that argument, although it may well be correct, I'm pointing to it.