I think we have a conflict of definition? I would call instruction fetch and decode behind an unresolved branch speculation. If you have a machine with a sing

I think we have a conflict of definition? I would call instruction fetch and decode behind an unresolved branch speculation. If you have a machine with a single execution stage which can't issue instructions in parallel with the branch then of course you can't have these speculated instructions execute because whether they are valid will always be resolved before they get to the execution or writeback stages. But the mechanics of putting instructions into the pipeline on the basis of speculation and quashing them if the branch resolves differently than predicted is the same and re-use the same hardware mechanisms that handle exceptions, making the speculative issuing of instructions a clear win from a design perspective. It's the details of the pipeline that determine whether it's possible to trigger the Spectre vulnerability before the branch resolves. Register renaming isn't required for speculation and you can even design out of order micro architectures without it. On an in-order processor you just have to make sure that writeback occurs after the branch is resolved and just flush the pipeline above the branch if the branch resolves incorrectly. No need to mess about with checkpoints because the register state is always valid. Well, there are some considerations with store commands too if you're using superscalar execution but it's still pretty easy. But again, these are all mechanisms you need in any event because your processor has to deal with interrupts that might be thrown at any moment by external triggers or memory faults. EDIT: I suppose it's possible in theory that you could look at an upcoming branch, predict how it will resolve, and then fetch the relevant instruction data into your instruction cache. But I'd tend to call that an instruction pre-fetcher by analogy to the data pre-fetcher eveyone uses rather than a branch predictor. And I'm not aware of anyone every having done that.