computedGoto

computedGoto pragma

The computedGoto pragma can be used to tell the compiler how to compile a Nim case in a while true statement. Syntactically it has to be used as a statement inside the loop:

type
  MyEnum = enum
    enumA, enumB, enumC, enumD, enumE

proc vm() =
  var instructions: array[0..100, MyEnum]
  instructions[2] = enumC
  instructions[3] = enumD
  instructions[4] = enumA
  instructions[5] = enumD
  instructions[6] = enumC
  instructions[7] = enumA
  instructions[8] = enumB

  instructions[12] = enumE
  var pc = 0
  while true:
    {.computedGoto.}
    let instr = instructions[pc]
    case instr
    of enumA:
      echo "yeah A"
    of enumC, enumD:
      echo "yeah CD"
    of enumB:
      echo "yeah B"
    of enumE:
      break
    inc(pc)

vm()

As the example shows, computedGoto is mostly useful for interpreters. If the underlying backend (C compiler) does not support the computed goto extension the pragma is simply ignored.