As far as I can tell, SiFive refuse to say what open source code (if any) their commercial cores such as the E31 are based on, or what changes they've made to it if it is based on open source code. See https://forums.sifive.com/t/more-details-for-e31-core/1165 Basically, it's a proprietary blob which you don't know the internals of just like ARM. They've confirmed that U54 and some of their FPGA-only cores are based on Rocket, but those aren't the cores they're offering commercially.
