The list of compilable functions (given by
Compile`CompilerFunctions // Sort) shows one of the more interesting- and core-sounding functions:
Compile`SetIterate. It does not appear in the Mathematica Documentation Center. Moreover, except for references to merely the list of compilable functions, a search for
Compile`SetIterate on the Mathematica Stack Exchange comes up with nothing and a Google search for
Compile`SetIterate reveals nothing I can see relating to Mathematica.