Unboxed returns (multiple return values)

[Ekdohibs]

This is a rebase of #1170, together with an attribute [@unboxed] that can be added on the definition of a function to decide to have an unboxed result. It does not trigger automatically, as that would require thinking about a probably complex strategy to avoid reallocations (leading to two test failures).

[Gbury]

I've found a bug ! Compiling and executing the following code:

let[@inline never][@unboxed] f x =
  ( (* each line has 20 elements *)
    x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
    x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
    x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
    x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x,
    x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x, x
  )

let () =
  let (
    _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _,
    _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _,
    _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _,
    _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _,
    _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, y, _, _, _
  )
    = f 5
  in
  print_int y;
  print_newline ()

Results in the output 0 instead of 5.

The cause seems to be that after the call, the stack pointer is moved before moving the result into a register.

It might be useful to add a few tests exercising unboxed result somewhere.

[Gbury]

the bug above is fixed by f50cb88

[Ekdohibs]

Subsumed by #1271.