GNU Compiler Collection (GCC) Internals: Adding new DECL node types

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10.4.2.2 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

Reading them from the generated all-tree.def file (which in turn includes all the tree.def files), gencheck.c is used during GCC’s build to generate the *_CHECK macros for all tree codes.

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