If you want really to understand how the GNU Pascal language front-end works internally and perhaps want to improve the compiler, it is important that you understand GPC's internal data structures.
The data structure used by the language front-end to hold all information about your Pascal program are the so-called “tree nodes”. (Well, it needn't be Pascal source – tree nodes are language independent.) The tree nodes are kind of objects, connected to each other via pointers. Since the GNU compiler is written in C and was created at a time where nobody really thought about object-oriented programming languages yet, a lot of effort has been taken to create these “objects” in C.
Here is an extract from the “object hierarchy”. Omissions are marked with “...”; nodes in parentheses are “abstract”: They are never instantiated and aren't really defined. They only appear here to clarify the structure of the tree node hierarchy. The complete list is in ../tree.def; additional information can be found in ../tree.h.
(tree_node) | |--- ERROR_MARK { enables GPC to continue after an error } | |--- (identifier) | | | |--- IDENTIFIER_NODE | | | \--- OP_IDENTIFIER | |--- TREE_LIST { a list of nodes, also used as a | general-purpose "container object" } | |--- TREE_VEC | |--- BLOCK | |--- (type) { information about types } | | | |--- VOID_TYPE | | | |--- INTEGER_TYPE | ... | | | |--- RECORD_TYPE | | | |--- FUNCTION_TYPE | | | \--- LANG_TYPE { for language-specific extensions } | |--- INTEGER_CST { an integer constant } | |--- REAL_CST | |--- STRING_CST | |--- COMPLEX_CST | |--- (declaration) | | | |--- FUNCTION_DECL | ... | | | |--- TYPE_DECL | | | \--- VAR_DECL | |--- (reference) | | | |--- COMPONENT_REF | ... | | | \--- ARRAY_REF | |--- CONSTRUCTOR | \--- (expression) | |--- MODIFY_EXPR { assignment } | |--- PLUS_EXPR { addition } ... | |--- CALL_EXPR { procedure/function call } | |--- GOTO_EXPR | \--- LOOP_EXPR { for all loops }
Most of these tree nodes – struct variables in fact – contain pointers to other tree nodes. A TREE_LIST for instance has a TREE_VALUE and a TREE_PURPOSE slot which can contain arbitrary data; a third pointer TREE_CHAIN points to the next TREE_LIST node and thus allows us to create linked lists of tree nodes.
One example: When GPC reads the list of identifiers in a variable declaration
var Foo, Bar, Baz: Integer;
the parser creates a chain of TREE_LISTs whose TREE_VALUEs hold IDENTIFIER_NODEs for the identifiers Foo, Bar, and Baz. The function declare_variables() (declared in declarations.c) gets this tree list as a parameter, does some magic, and finally passes a chain of VAR_DECL nodes to the back-end.
The VAR_DECL nodes in turn have a pointer TREE_TYPE which holds a _TYPE node – an INTEGER_TYPE node in the example above. Having this, GPC can do type-checking when a variable is referenced.
For another example, let's look at the following statement:
Baz := Foo + Bar;
Here the parser creates a MODIFY_EXPR tree node. This node has two pointers, TREE_OPERAND[0] which holds a representation of Baz, a VAR_DECL node, and TREE_OPERAND[1] which holds a representation of the sum Foo + Bar. The sum in turn is represented as a PLUS_EXPR tree node whose TREE_OPERAND[0] is the VAR_DECL node Foo, and whose TREE_OPERAND[1] is the VAR_DECL node Bar. Passing this (the MODIFY_EXPR node) to the back-end results in assembler code for the assignment.
If you want to have a closer look at these tree nodes, write a line {$debug-tree FooBar} into your program with FooBar being some identifier in your program. This tells GPC to output the contents of the IDENTIFIER_NODE to the standard error file handle in human-readable form.
While hacking and debugging GPC, you will also wish to have a look at these tree nodes in other cases. Use the debug_tree() function to do so. (In fact {$debug-tree FooBar} does nothing else than to debug_tree() the IDENTIFIER_NODE of the Foobar identifier node – note the capitalization of the first character in the internal representation.)