unsafe indexing by unboxed nativeint (and bigger) explicitly truncates the index when it shouldn't

[jvanburen]

The below code should either segfault immediately, or it should print both values. I claim it should do the former but 5.2.0minus-9 does the latter. Even worse, 5.2.0minus-8 and before prints a single value and then segfaults on the second invocation of go.

We should probably fix this by making the indexing arguments take a Naked_nativeint rather than a Naked_immediate.

type bigstring = (char, Bigarray.int8_unsigned_elt, Bigarray.c_layout) Bigarray.Array1.t

external unsafe_get_int32_u_le
  :  local_ bigstring
  -> pos:int64#
  -> int32#
  = "%caml_bigstring_get32u#_indexed_by_int64#"

external opaque_i64 : int64# -> int64# = "%opaque"
external box_int32 : int32# -> int32 = "%box_int32"
external unbox_int64 : int64 -> int64# = "%unbox_int64"

let[@inline never] print_int32u i32 =
  Printf.printf "%04lx\n%!" (box_int32 i32)

let[@inline never] go bigstring =
  let pos = unbox_int64 Int64.min_int in
  print_int32u (unsafe_get_int32_u_le bigstring ~pos);
  print_int32u (unsafe_get_int32_u_le bigstring ~pos:(opaque_i64 pos))

let () = go (Bigarray.Array1.of_array Bigarray.char Bigarray.c_layout [| 'a' ;'b' ; 'c'; 'd'|])