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> but how is one to reason about performance when some instructions can slow the CPU.

The same way one should always do so on an advanced out-of-order CPU largely being programmed in modern high level languages: through thorough profiling and performance testing.

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Empiricism? In my software development? No way.

I appreciate the humor but that's more time consuming than comparing instruction cycle counts and data dependencies. It's also workload dependent when it wouldn't be if they didn't come up with this throttling thing even if out of order execution makes it complicated.

Most programmers are not regularly writing primarily assembly or reviewing the assembly generated by their compiler. These are the scenarios where instruction cycle counts become relevant - everything else is abstracted too far to rely on anything other than profiling and testing. If that's you, then I'm sorry for my response which didn't really take you into account.

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