Right now there is no documentation for the GCC tree -> rtl interfaces
(or more generally the interfaces for adding new languages).

Such documentation would be of great benefit to the project.  Until such
time as we can formally start documenting the interface this file will
serve as a repository for information on these interface and any incompatable
changes we've made.

Apr 1, 1998:
  Front-ends must now define lang_print_xnode.  It is safe for this
  function to do nothing.  See c-lang.c.

Feb 1, 1998:

  GCC used to store structure sizes & offsets to elements as bitsize
  quantities.  This causes problems because a structure can only be
  (target memsize / 8) bytes long (this may effect arrays too).  This
  is particularly problematical on machines with small address spaces.

  So:

    All trees that represent sizes in bits should have a TREE_TYPE of
    bitsizetype (rather than sizetype).

    Accordingly, when such values are computed / initialized, care has to
    be takes to use / compute the proper type.

    When a size in bits is converted into a size in bytes, which is expressed
    in trees, care should be taken to change the tree's type again to sizetype.

    We've updated C, C++, Fortran & Objective-C to work with the new
    scheme.  Other languages will need to be updated accordingly.
    Contact amylaar@cygnus.com for additional information.

?? 1997:

  In an effort to decrease cache thrashing and useless loads we've changed the
  third argument to the DEFTREECODE macro to be a single char.  This will
  effect languages that defined their own tree codes (usually in a .def file).

  Old way:

    DEFTREECODE (CLASS_METHOD_DECL, "class_method_decl", "d", 0)

  New way:

    DEFTREECODE (CLASS_METHOD_DECL, "class_method_decl", 'd', 0)