64-cycle Division in DefaultConfig?

[seldridge]

I was doing some benchmarking of different instructions and I was noticing that integer division takes a lot longer than integer multiplication (or floating point multiplication) when using DefaultConfig.

Unless I'm mistakenly understanding the configuration (or am missing where a parameter gets overriden), I think that the default divUnroll value of 1 is going to cause default builds of Rocket Chip to have a 64-cycle divider, i.e., this counter has to count to 64.

Is this intentional, should the default config's divUnroll be changed to 8 (like mulUnroll), or am I mistaken in what's going on?

[terpstra]

You understood it correctly. Bumping divUnroll too high will negatively impact your frequency. Div is a slow instruction on all micro-architectures; it's just especially slow on rocket.

[seldridge]

Thanks @terpstra.

Current value of divUnroll is correctly set at 1. If a user wants something else, then that should be defined in their own configuration (and potentially impacting achievable frequency).

As all of this is intended, I'll close this.