Optional modules

[Timmmm]

See the discussions on riscv/sail-riscv#910 - for the RISC-V model we want to be able to build the SV/Formal outputs with a subset of the model (e.g. without Smstateen). In order to make that work we can use scattered functions all over the place, but it feels a bit hacky. It would be nice if there was a built-in way to have optional modules. E.g. in the main reset() function you could write

  // "On reset, all writable mstateen bits are initialized by the hardware to zeros."
  if $module(stateen) then reset_stateen();

It would eliminate that code if the module is not being built. Similarly:

function clause currentlyEnabled(Ext_Zfinx) = hartSupports(Ext_Zfinx) & currentlyEnabled(Ext_Zicsr) &
   (not($module(stateen)) | is_zfinx_enabled_by_stateen())

Ideally with full type checking as if you did have all the modules enabled. That's probably super hard though.

Anyway something better than scattered functions would be nice.

[Alasdair]

Yes, this is something I've considered. We should be able to re-use the existing flow typing machinery to make the if $module(...) then work.