I'm slightly surprised the author didn't mention the core I work on Ibex: https://github.com/lowRISC/ibex https://github.com/lowRISC/ibex but maybe not too surp

I'm slightly surprised the author didn't mention the core I work on Ibex: https://github.com/lowRISC/ibex https://github.com/lowRISC/ibex but maybe not too surprised as it's primary target isn't FPGA softcore, though it does work fine for this purpose. It's got an extensive verification environment (though relies on UVM so you need closed tools to run it, we have a verilator setup for running RISC-V compliance and arbitrary binaries), good documentation and we're pushing the performance up (you can get 3.09 CoreMark/MHz with the current master and that will increase with some branch prediction work I'm currently doing). It's configurable so you can choose your own trade offs between size and performance. Plus we've got support for the B (bitmanip) extension, which our verification environment also tests. All cores have their pros and cons and Ibex is no exception but if you're looking for an open CPU core I think it's well worth a look.