Divergence. On an OoO pipeline fetch might be ahead of retire by hundreds of instructions. That's dozen of branches, and if each causes execution to speculative

Divergence. On an OoO pipeline fetch might be ahead of retire by hundreds of instructions. That's dozen of branches, and if each causes execution to speculatively fork, it wouldn't scale. You could potentially do it only for predicted "unpredictable" branches. Now the tradeoff is wasted power and execution units for dead work and so far the tradeoff was just not worth it. Some form of this has been experimented with. In the late '90 SPRC experimented with scouting threads and as mentioned else thread the Efficient Intel cores can fetch (but not execute) across branches.