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(gcc-300.info)Tree overview

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Overview
========

   The central data structure used by the internal representation is the
`tree'.  These nodes, while all of the C type `tree', are of many
varieties.  A `tree' is a pointer type, but the object to which it
points may be of a variety of types.  From this point forward, we will
refer to trees in ordinary type, rather than in `this font', except
when talking about the actual C type `tree'.

   You can tell what kind of node a particular tree is by using the
`TREE_CODE' macro.  Many, many macros take a trees as input and return
trees as output.  However, most macros require a certain kinds of tree
node as input.  In other words, there is a type-system for trees, but
it is not reflected in the C type-system.

   For safety, it is useful to configure GCC with `--enable-checking'.
Although this results in a significant performance penalty (since all
tree types are checked at run-time), and is therefore inappropriate in a
release version, it is extremely helpful during the development process.

   Many macros behave as predicates.  Many, although not all, of these
predicates end in `_P'.  Do not rely on the result type of these macros
being of any particular type.  You may, however, rely on the fact that
the type can be compared to `0', so that statements like
     if (TEST_P (t) && !TEST_P (y))
       x = 1;

and
     int i = (TEST_P (t) != 0);

are legal.  Macros that return `int' values now may be changed to
return `tree' values, or other pointers in the future.  Even those that
continue to return `int' may return multiple nonzero codes where
previously they returned only zero and one.  Therefore, you should not
write code like
     if (TEST_P (t) == 1)

as this code is not guaranteed to work correctly in the future.

   You should not take the address of values returned by the macros or
functions described here.  In particular, no guarantee is given that the
values are lvalues.

   In general, the names of macros are all in uppercase, while the
names of functions are entirely in lower case.  There are rare
exceptions to this rule.  You should assume that any macro or function
whose name is made up entirely of uppercase letters may evaluate its
arguments more than once.  You may assume that a macro or function
whose name is made up entirely of lowercase letters will evaluate its
arguments only once.

   The `error_mark_node' is a special tree.  Its tree code is
`ERROR_MARK', but since there is only ever one node with that code, the
usual practice is to compare the tree against `error_mark_node'.  (This
test is just a test for pointer equality.)  If an error has occurred
during front-end processing the flag `errorcount' will be set.  If the
front end has encountered code it cannot handle, it will issue a
message to the user and set `sorrycount'.  When these flags are set,
any macro or function which normally returns a tree of a particular
kind may instead return the `error_mark_node'.  Thus, if you intend to
do any processing of erroneous code, you must be prepared to deal with
the `error_mark_node'.

   Occasionally, a particular tree slot (like an operand to an
expression, or a particular field in a declaration) will be referred to
as "reserved for the back end."  These slots are used to store RTL when
the tree is converted to RTL for use by the GCC back end.  However, if
that process is not taking place (e.g., if the front end is being hooked
up to an intelligent editor), then those slots may be used by the back
end presently in use.

   If you encounter situations that do not match this documentation,
such as tree nodes of types not mentioned here, or macros documented to
return entities of a particular kind that instead return entities of
some different kind, you have found a bug, either in the front end or in
the documentation.  Please report these bugs as you would any other bug.

Macros and Functions

Macros and functions that can be used with all trees.

Identifiers

The names of things.

Containers

Lists and vectors.