https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86631
--- Comment #3 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- The relevant point is after do_compile calls lang_dependent_init. Since PTRDIFF_TYPE is a string, it's a pain to do anything with it until after the front end has set up tree nodes (preferably those target macros would become hooks returning enum values, but that still wouldn't help get the information earlier, because converting itk_int conveniently to a max value requires integer_type_node to have been set up).
