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| <title>GNU Compiler Collection (GCC) Internals: Tree overview</title> |
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| Next: <a href="Types.html#Types" accesskey="n" rel="next">Types</a>, Previous: <a href="Deficiencies.html#Deficiencies" accesskey="p" rel="prev">Deficiencies</a>, Up: <a href="GENERIC.html#GENERIC" accesskey="u" rel="up">GENERIC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Overview-1"></a> |
| <h3 class="section">10.2 Overview</h3> |
| <a name="index-tree"></a> |
| <a name="index-TREE_005fCODE"></a> |
| |
| <p>The central data structure used by the internal representation is the |
| <code>tree</code>. These nodes, while all of the C type <code>tree</code>, are of |
| many varieties. A <code>tree</code> 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 <code>this |
| font</code>, except when talking about the actual C type <code>tree</code>. |
| </p> |
| <p>You can tell what kind of node a particular tree is by using the |
| <code>TREE_CODE</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. |
| </p> |
| <p>For safety, it is useful to configure GCC with <samp>--enable-checking</samp>. |
| 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. |
| </p> |
| <p>Many macros behave as predicates. Many, although not all, of these |
| predicates end in ‘<samp>_P</samp>’. 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 <code>0</code>, so that statements like |
| </p><div class="smallexample"> |
| <pre class="smallexample">if (TEST_P (t) && !TEST_P (y)) |
| x = 1; |
| </pre></div> |
| <p>and |
| </p><div class="smallexample"> |
| <pre class="smallexample">int i = (TEST_P (t) != 0); |
| </pre></div> |
| <p>are legal. Macros that return <code>int</code> values now may be changed to |
| return <code>tree</code> values, or other pointers in the future. Even those |
| that continue to return <code>int</code> may return multiple nonzero codes |
| where previously they returned only zero and one. Therefore, you should |
| not write code like |
| </p><div class="smallexample"> |
| <pre class="smallexample">if (TEST_P (t) == 1) |
| </pre></div> |
| <p>as this code is not guaranteed to work correctly in the future. |
| </p> |
| <p>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. |
| </p> |
| <p>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. |
| </p> |
| <p>The <code>error_mark_node</code> is a special tree. Its tree code is |
| <code>ERROR_MARK</code>, but since there is only ever one node with that code, |
| the usual practice is to compare the tree against |
| <code>error_mark_node</code>. (This test is just a test for pointer |
| equality.) If an error has occurred during front-end processing the |
| flag <code>errorcount</code> will be set. If the front end has encountered |
| code it cannot handle, it will issue a message to the user and set |
| <code>sorrycount</code>. When these flags are set, any macro or function |
| which normally returns a tree of a particular kind may instead return |
| the <code>error_mark_node</code>. Thus, if you intend to do any processing of |
| erroneous code, you must be prepared to deal with the |
| <code>error_mark_node</code>. |
| </p> |
| <p>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. |
| </p> |
| <p>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. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <tr><td align="left" valign="top">• <a href="Macros-and-Functions.html#Macros-and-Functions" accesskey="1">Macros and Functions</a>:</td><td> </td><td align="left" valign="top">Macros and functions that can be used with all trees. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Identifiers.html#Identifiers" accesskey="2">Identifiers</a>:</td><td> </td><td align="left" valign="top">The names of things. |
| </td></tr> |
| <tr><td align="left" valign="top">• <a href="Containers.html#Containers" accesskey="3">Containers</a>:</td><td> </td><td align="left" valign="top">Lists and vectors. |
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