To be clear, when I (and most people) say OoO, I don't mean just the act of executing instructions out-of-order. I mean the whole modern paradigm of "complex br

To be clear, when I (and most people) say OoO, I don't mean just the act of executing instructions out-of-order. I mean the whole modern paradigm of "complex branch predictors, controlling wide front-ends, feeding schedulers with wide back-ends and hundreds or even thousands of instructions in flight". It's a little annoying that OoO is overloaded in this way. I have seen some people suggesting we should be calling these designs "Massively-Out-of-Order" or "Great-Big-Out-of-Order" in order to be more specific, but that terminology isn't in common use. And yes, there are some designs out there which are technically out-of-order, but don't count as MOoO/GBOoO. The early PowerPC cores come to mind. It's not that executing instructions out-of-order benefits from complex branch prediction and wide execution units, OoO is what made it viable to start using wide execution units and complex branch prediction in the first place. A simple in-order core simply can't extract that much parallelism, the benefits drop off quickly after two-wide super scalar. And accurate branch prediction is of limited usefulness when the pipeline is that short. There are really only two ways to extract more parallelism. You either do complex out-of-order scheduling (aka dynamic scheduling), or you take the VLIW approach and try to solve it with static scheduling, like the Itanium. They really are just two sides of the same "I want a wide core" coin. And we all know how badly the Itanium failed.