l4re-core VS gem5

The idea is to write a C++ model that that produces cycle accurate outputs of the branch predictor, core pipeline, queues, memory latency, cache hierarchy, prefetch behaviour, etc. Transistor level accuracy isn't needed as long as the resulting cycle timings are identical or near identical. The improvement in workload runtime compared to a Verilog simulation is precisely because they aren't trying to model every transistor, but just the important parameters which effect performance.

Let's take a simple example: Instead of modeling a 64-bit adder in all its gory transistor level detail, you can just have the model return the correct data after 1 "cycle" or whatever your ALU latency is. As long as that cycle latency is the same as the real hardware, you'll get an accurate performance number.

What's particularly useful about these models is they enable much easier and faster state space exploration to see how a circuit would perform, well before going ahead with the Verilog implementation, which relatively speaking can take circuit designers ages. "How much faster would my CPU be if it had a 20% larger register file" can be answered in a day or two before getting a circuit designer to go try and implement such a thing.

If you want an open source example, take a look at the gem5 project (https://www.gem5.org). It's not quite as sophisticated as the proprietary models used in industry, but it's a used widely in academia and open source hardware design and is a great place to start.