gem5 VS riscv-perf-model

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.

I 100% agree on verilator. It’s by far the most efficient tool for many simulation and verification needs. Hands down. Even more so if you are building CPUs

UVM definitely has its place still in verification flows. It for something as complex and flexible as a CPU for sure, it’s hard to beat the amount of cycles and coverage you get from verilator. (EDA license fees are a big part of that, but also the fact that verilator generates just C++ code you can integrate with the rest of your s/w flow is invaluable).

But: What is also important for CPU is good validation and compliance test, (random or replayed or directed) instruction stream generators, ISS to validate agains etc. There are so much more scenarios to check because a CPU needs to handle any sequence of any instructions flawlessly.

Those tools are also crucial. And they are being built or sold for RISC-V now too. And also: I much prefer combining those with verilator than with commercial eda tools. And don’t forget: before you start building a lot of RTL, you’ll typically will also build C based higher level performance models of the micro architecture, eg https://github.com/riscv-software-src/riscv-perf-model