Well I can't speak for all CPUs but yes, at arm at least, lower-level RTL is used for the CPU design. Combined with that will be modeling at various levels. C+

Well I can't speak for all CPUs but yes, at arm at least, lower-level RTL is used for the CPU design. Combined with that will be modeling at various levels. C++ models can be used for performance exploration for instance. Cycle accurate (or close to cycle accurate) models for the CPU are useful but also hard to work with. You may well be developing the accurate model in combination with the CPU itself. In early design work you'll hack features into an older model just to see how they might work in isolation or even just stare at some spreadsheets with performance counters and make an educated guess as to how some new feature or structure will effect them and use that to calculate potential performance increases.