frontend: Infinite Recursion error where only a finite recursion exists

[AndreasKrall]

The following example from the tutorial:

enumeration CsrDef : Bits<12> =   // defined control and status register indices
  { ustatus                       //  0 0x000  User mode restricted view of mstatus
  , uie      =   4                //  4 0x004  User mode Interrupt Enable
  , utvec                         //  5 0x005  User mode Trap VECtor base address
  , uscratch =  64                // 64 0x040  User mode SCRATCH
  , uepc                          // 65 0x041  User mode Exception Program Counter
  , ucause   =  CsrDef::uepc + 1  // 66 0x042  User mode exception CAUSE
  , utval                         // 67 0x043  User mode Trap VALue
  }

gives an infinite recursion error:

error: Infinite Recursion
     ╭──[test.vadl:1:1]
     │
   1 │> enumeration CsrDef : Bits<12> =   // defined control and status register indices
   2 │>   { ustatus                       //  0 0x000  User mode restricted view of mstatus
   3 │>   , uie      =   4                //  4 0x004  User mode Interrupt Enable
    ⋮  3 lines omitted here...
   7 │>   , ucause   =  CsrDef::uepc + 1  // 66 0x042  User mode exception CAUSE
   8 │>   , utval                         // 67 0x043  User mode Trap VALue
   9 │>   }
     │    
     │
     Definition `CsrDef` is defined by itself.

This is a finite recursion and it would be nice if that could be handled. Then the example in the tutorial would be correct again. Tests should not only be done with a direct recursion like here, but also with indirect recursions with other enumerations / constants.

[flofriday]

Yes, I think the infinite recursion checker might be too agressive in some aspects (I assume that recursive functions are also not possible).
I think it should be easy to implement a special rule that the children of an enumeration can reference it's parent.

[AndreasKrall]

I think the check is simple and should not be restricted to the parent, also indirect references should be possible. It is just necessary to compare both the enumeration id and the enumeration element id together, in the example above not only CsrDef, but also CsrDef and uepc together.

Recursive functions will always be forbidden.