Excessive stack allocation with multiple return points

[Aatch]

This function uses ~1Kb of stack (with optimisations)

function test() -> [u8,...256] {
  let x : [u8, ..256];
  let val = get_val();
  if val == 0 {
    x = [1, ..256];
    return x;
  } else if val == 1 {
    x = [4, ..256];
    return x;
  } else {
    x = [9, ..256];
    return x;
  }
}

While this function uses ~256 bytes of stack:

function test() -> [u8,...256] {
  let x : [u8, ..256];
  let val = get_val();
  if val == 0 {
    x = [1, ..256];
  } else if val == 1 {
    x = [4, ..256];
  } else {
    x = [9, ..256];
  }

  return x;
}

As far as I can tell, each return statement gets given it's own slot which is copied into the return pointer at the end. This happens to also result in 4 memcpy calls in total.

/cc @gankro @cgaebel

[Aatch]

Ok, found the issue. It was actually due to the check for nested returns being too conservative. I've got a fix in the works, just needs some polishing first.

[sinistersnare]

ccme ( i wish there was a button i could push so i did not have to do this, sorry!)

[ftxqxd]

@sinistersnare There is the ‘Subscribe’ button on the sidebar on the right, which makes you receive notifications from the issue. (I’m not sure if that behaves in exactly the same way as commenting, but I think it’s pretty similar.)

[sinistersnare]

@P1start I already get emails from every thread (I subscribe by default), but by ccing I get the notification in the "participating" section, which is much easier to see than the torrent of other things going through this repo.