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9.2 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 trees as input and
return trees as output. However, most macros require a certain kind 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 lowercase. 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.