Opaque sizeof.

Lemma split2_data_at__Tarray_tuchar
: forall {cs} (sh : Share.t)  (n n1 : Z) (p : val),
0 <= n1 <= n -> isptr p ->field_compatible (Tarray tuchar n noattr) [] p ->
@data_at_ cs sh (Tarray tuchar n noattr) p =
@data_at_ cs sh (Tarray tuchar n1 noattr) p *
@data_at_ cs sh (Tarray tuchar (n - n1) noattr)
(field_address0 (Tarray tuchar n noattr) [ArraySubsc n1] p).