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Comment by phire | Hacker News Reader

From what I understand (as I didn't start writing code until the mid-2000s), loop unrolling was a very highly regarded optimisation of the early 90s.

And it was a very long time before programmers (and compilers) realised just how bad of an idea loop unrolling was once we entered the era of superscalar processors with instruction caches and half-decent branch prediction.

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It never occurred to me that loop unrolling could end up being a de-optimization due to compiler optimizations!

The things the parent comment referred to (instruction caches and branch prediction) are in the processor hardware rather than other compiler optimisations.

To be fair early 90s branch prediction was garbage. Back then it seemed like every new processor release was accompanied by a note on how branch prediction was improved by 80-90% compared to its predecessor.

To be even more fair, counted constant sized loops never required particularly good branch prediction, especially if the code is well laid out so that your "miss" does literally one more cheap step.

Misprediction there means you computed one more vector or value, not a huge loss compared to a cache miss. Branch misprediction hurts a lot when it forces a cache miss mostly, whether it's for data or for instructions, or ties up a deep instruction pipeline completely with lousy abort, which most CPUs do not have. (See: Nehalem.)

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