(** Decomposing 64-bit ints as pairs of 32-bit ints *)
Definition loword (n: int) : Int.int := Int.repr (unsigned n).
Definition hiword (n: int) : Int.int := Int.repr (unsigned (shru n (repr Int.zwordsize))).
Definition ofwords (hi lo: Int.int) : int :=
or (shl (repr (Int.unsigned hi)) (repr Int.zwordsize)) (repr (Int.unsigned lo)).

Lemma bits_ofwords:
forall hi lo i, 0 <= i < zwordsize ->
testbit (ofwords hi lo) i =
if zlt i Int.zwordsize then Int.testbit lo i else Int.testbit hi (i - Int.zwordsize).