[regression, performance] iterator makes deep copy for openArray, can cause major slowdowns

[timotheecour]

the recently merged #13712 causes a performance regression. Inline iterators are intended for speed, we shouldn't introduce un-necessary copies. /cc @krux02

Example

block:
  let n = 10_000_0000
  var s = newSeq[int](n)
  proc p2(): var seq[int] = s

  iterator ip1(v: openArray[int]): auto =
    yield v[0]
    yield v[^1]

  import times
  let t = cpuTime()
  for i in 0..<100:
    for x in ip1(p2()):
      doAssert x != -1
  echo cpuTime() - t

Current Output

12.171379 # seconds

Expected Output

7.000000000034756e-06 # seconds, before that PR, eg with nim 1.0.6

Possible Solution

• revert fix #13417 #13712, and fix Iterator with openArray parameter make the argument evaluated many times #13417 differently, by making openArray first class, see make openArray first-class RFCs#88

EDIT: the revert has been done in #13728

Additional Information

• regression introduced in fix #13417 #13712

[krux02]

# arrays will deep copy here (pretty bad).

Well I even put a comment about this performance problem in the code. So this is nothing new. I would have been nicer if this would have been complained ahead of time. But I guess performance is ranked higher than correctness in this particular case.