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Comment by Taniwha | Hacker News Reader

Even with out of order scheduling there's still use from instruction scheduling - move those loads as far forward in the instruction stream as you can - a CPU can't execute instructions out of order if it hasn't decoded them yet

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These days CPUs have instruction windows that are hundreds of instructions deep. That typically allows for way more load hoisting than a compiler can realistically do (and that's pretty much the reason why all in-order or VLIW designs have fallen out of favor for general purpose computation).

They can't do a load before actually reaching the load instruction, and they can't fill the instruction buffer all that fast, maybe 5 instructions per clock, depending on loads of factors. They don't know what instructions are on the critical path, so it is perfectly possible to pick wrong ones in this regard. Having a big instruction buffer does not necessarily mean that the CPU can always pick a perfect selection of instruction from it, that is already a very hard issue that only becomes harder when you give it over a hundred to pick from.

In short, the out of order instruction buffer can do some amazing stuff for code that would otherwise run much slower, that doesn't mean you can't gain or lose performance by reordering instructions. For non-trivial code the best composition is almost certainly different between CPUs.

OoO windows do not need to do a perfect job. They just need to do a better job than the compiler at exposing memory parallelism, and that's usually the case.

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