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.

2004-09-09:
  In an effort to decrease execution time, single char tree code
  classes were changed to enumerated values.

  Old way:

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

  New way:

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

2001-02-26:
  A DECL_INITIAL of NULL_TREE or error_mark_node in a VAR_DECL is no longer
  taken to signify a tentative definition which should not be emitted until
  end-of-file.  Frontends which want that behavior should set
  DECL_DEFER_OUTPUT before calling rest_of_decl_compilation.

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.

?? 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
  affect 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)