Support unroll factors

[schneiderfelipe]

This can be done without information on loop length.

proc searchChar(s: string, c: char): int =
  unroll(4): for i in 0 .. s.high:
    if s[i] == c: return i
  result = -1

should produce something like

proc searchChar(s: string, c: char): int =
  for i in countup(0, s.high, 4):  # unroll factor goes here
    if s[i] == c: return i
    if s[i+1] == c: return i+1
    if s[i+2] == c: return i+2
    if s[i+3] == c: return i+3

  let n = ((s.high - 0) div 4) * 4 + 0  # we can do better here
  for i in n..s.high:  # eventually left out elements
    if s[i] == c: return i
  result = -1

We can even employ a default value for when the loop length is not known in advance.

[schneiderfelipe]

Interesting reference (with a more useful example).