##### 9.5.2.2 Adding new DECL node types

Adding a new `DECL` tree consists of the following steps

Add a new tree code for the `DECL` node
  For language specific `DECL` nodes, there is a `.def` file in each frontend directory where the tree code should be added. For `DECL` nodes that are part of the middle-end, the code should be added to `tree.def`.
Create a new structure type for the `DECL` node
  These structures should inherit from one of the existing structures in the language hierarchy by using that structure as the first member.

                struct tree_foo_decl
                {
                   struct tree_decl_with_vis common;
                }

  Would create a structure name `tree_foo_decl` that inherits from `struct tree_decl_with_vis`.

  For language specific `DECL` nodes, this new structure type should go in the appropriate `.h` file. For `DECL` nodes that are part of the middle-end, the structure type should go in `tree.h`.

Add a member to the tree structure enumerator for the node
  For garbage collection and dynamic checking purposes, each `DECL` node structure type is required to have a unique enumerator value specified with it. For language specific `DECL` nodes, this new enumerator value should go in the appropriate `.def` file. For `DECL` nodes that are part of the middle-end, the enumerator values are specified in `treestruct.def`.
Update `union tree_node`
  In order to make your new structure type usable, it must be added to `union tree_node`. For language specific `DECL` nodes, a new entry should be added to the appropriate `.h` file of the form

                  struct tree_foo_decl GTY ((tag ("TS_VAR_DECL"))) foo_decl;

  For `DECL` nodes that are part of the middle-end, the additional member goes directly into `union tree_node` in `tree.h`.

Update dynamic checking info
  In order to be able to check whether accessing a named portion of `union tree_node` is legal, and whether a certain `DECL` node contains one of the enumerated `DECL` node structures in the hierarchy, a simple lookup table is used. This lookup table needs to be kept up to date with the tree structure hierarchy, or else checking and containment macros will fail inappropriately.

  For language specific `DECL` nodes, their is an `init_ts` function in an appropriate `.c` file, which initializes the lookup table. Code setting up the table for new `DECL` nodes should be added there. For each `DECL` tree code and enumerator value representing a member of the inheritance hierarchy, the table should contain 1 if that tree code inherits (directly or indirectly) from that member. Thus, a `FOO_DECL` node derived from `struct decl_with_rtl`, and enumerator value `TS_FOO_DECL`, would be set up as follows

                tree_contains_struct[FOO_DECL][TS_FOO_DECL] = 1;
                tree_contains_struct[FOO_DECL][TS_DECL_WRTL] = 1;
                tree_contains_struct[FOO_DECL][TS_DECL_COMMON] = 1;
                tree_contains_struct[FOO_DECL][TS_DECL_MINIMAL] = 1;

  For `DECL` nodes that are part of the middle-end, the setup code goes into `tree.c`.

Add macros to access any new fields and flags
  Each added field or flag should have a macro that is used to access it, that performs appropriate checking to ensure only the right type of `DECL` nodes access the field.

  These macros generally take the following form

                #define FOO_DECL_FIELDNAME(NODE) FOO_DECL_CHECK(NODE)->foo_decl.fieldname

  However, if the structure is simply a base class for further structures, something like the following should be used

                #define BASE_STRUCT_CHECK(T) CONTAINS_STRUCT_CHECK(T, TS_BASE_STRUCT)
                #define BASE_STRUCT_FIELDNAME(NODE) \
                   (BASE_STRUCT_CHECK(NODE)->base_struct.fieldname