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145 lines
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145 lines
7.0 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Tree overview - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="GENERIC.html#GENERIC" title="GENERIC">
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<link rel="prev" href="Deficiencies.html#Deficiencies" title="Deficiencies">
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<link rel="next" href="Types.html#Types" title="Types">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Tree-overview"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Types.html#Types">Types</a>,
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Previous: <a rel="previous" accesskey="p" href="Deficiencies.html#Deficiencies">Deficiencies</a>,
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Up: <a rel="up" accesskey="u" href="GENERIC.html#GENERIC">GENERIC</a>
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<hr>
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</div>
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<h3 class="section">10.2 Overview</h3>
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<p><a name="index-tree-1713"></a><a name="index-TREE_005fCODE-1714"></a>
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The central data structure used by the internal representation is the
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<code>tree</code>. These nodes, while all of the C type <code>tree</code>, are of
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many varieties. A <code>tree</code> is a pointer type, but the object to
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which it points may be of a variety of types. From this point forward,
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we will refer to trees in ordinary type, rather than in <code>this
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font</code>, except when talking about the actual C type <code>tree</code>.
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<p>You can tell what kind of node a particular tree is by using the
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<code>TREE_CODE</code> macro. Many, many macros take trees as input and
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return trees as output. However, most macros require a certain kind of
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tree node as input. In other words, there is a type-system for trees,
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but it is not reflected in the C type-system.
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<p>For safety, it is useful to configure GCC with <samp><span class="option">--enable-checking</span></samp>.
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Although this results in a significant performance penalty (since all
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tree types are checked at run-time), and is therefore inappropriate in a
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release version, it is extremely helpful during the development process.
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<p>Many macros behave as predicates. Many, although not all, of these
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predicates end in ‘<samp><span class="samp">_P</span></samp>’. Do not rely on the result type of these
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macros being of any particular type. You may, however, rely on the fact
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that the type can be compared to <code>0</code>, so that statements like
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<pre class="smallexample"> if (TEST_P (t) && !TEST_P (y))
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x = 1;
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</pre>
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<p class="noindent">and
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<pre class="smallexample"> int i = (TEST_P (t) != 0);
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</pre>
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<p class="noindent">are legal. Macros that return <code>int</code> values now may be changed to
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return <code>tree</code> values, or other pointers in the future. Even those
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that continue to return <code>int</code> may return multiple nonzero codes
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where previously they returned only zero and one. Therefore, you should
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not write code like
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<pre class="smallexample"> if (TEST_P (t) == 1)
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</pre>
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<p class="noindent">as this code is not guaranteed to work correctly in the future.
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<p>You should not take the address of values returned by the macros or
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functions described here. In particular, no guarantee is given that the
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values are lvalues.
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<p>In general, the names of macros are all in uppercase, while the names of
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functions are entirely in lowercase. There are rare exceptions to this
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rule. You should assume that any macro or function whose name is made
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up entirely of uppercase letters may evaluate its arguments more than
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once. You may assume that a macro or function whose name is made up
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entirely of lowercase letters will evaluate its arguments only once.
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<p>The <code>error_mark_node</code> is a special tree. Its tree code is
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<code>ERROR_MARK</code>, but since there is only ever one node with that code,
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the usual practice is to compare the tree against
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<code>error_mark_node</code>. (This test is just a test for pointer
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equality.) If an error has occurred during front-end processing the
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flag <code>errorcount</code> will be set. If the front end has encountered
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code it cannot handle, it will issue a message to the user and set
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<code>sorrycount</code>. When these flags are set, any macro or function
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which normally returns a tree of a particular kind may instead return
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the <code>error_mark_node</code>. Thus, if you intend to do any processing of
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erroneous code, you must be prepared to deal with the
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<code>error_mark_node</code>.
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<p>Occasionally, a particular tree slot (like an operand to an expression,
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or a particular field in a declaration) will be referred to as
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“reserved for the back end”. These slots are used to store RTL when
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the tree is converted to RTL for use by the GCC back end. However, if
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that process is not taking place (e.g., if the front end is being hooked
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up to an intelligent editor), then those slots may be used by the
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back end presently in use.
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<p>If you encounter situations that do not match this documentation, such
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as tree nodes of types not mentioned here, or macros documented to
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return entities of a particular kind that instead return entities of
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some different kind, you have found a bug, either in the front end or in
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the documentation. Please report these bugs as you would any other
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bug.
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<ul class="menu">
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<li><a accesskey="1" href="Macros-and-Functions.html#Macros-and-Functions">Macros and Functions</a>: Macros and functions that can be used with all trees.
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<li><a accesskey="2" href="Identifiers.html#Identifiers">Identifiers</a>: The names of things.
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<li><a accesskey="3" href="Containers.html#Containers">Containers</a>: Lists and vectors.
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</ul>
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<!-- -->
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<!-- Trees -->
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<!-- -->
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</body></html>
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