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116 lines
5.1 KiB
HTML
116 lines
5.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Blocks (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Blocks (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Blocks (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Statements.html#Statements" rel="up" title="Statements">
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<link href="Statement-Sequences.html#Statement-Sequences" rel="next" title="Statement Sequences">
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<link href="Basic-Statements.html#Basic-Statements" rel="prev" title="Basic Statements">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Blocks"></a>
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<div class="header">
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<p>
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Next: <a href="Statement-Sequences.html#Statement-Sequences" accesskey="n" rel="next">Statement Sequences</a>, Previous: <a href="Basic-Statements.html#Basic-Statements" accesskey="p" rel="prev">Basic Statements</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</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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</div>
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<hr>
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<a name="Blocks-1"></a>
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<h4 class="subsection">11.7.2 Blocks</h4>
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<a name="index-Blocks"></a>
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<p>Block scopes and the variables they declare in GENERIC are
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expressed using the <code>BIND_EXPR</code> code, which in previous
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versions of GCC was primarily used for the C statement-expression
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extension.
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</p>
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<p>Variables in a block are collected into <code>BIND_EXPR_VARS</code> in
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declaration order through their <code>TREE_CHAIN</code> field. Any runtime
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initialization is moved out of <code>DECL_INITIAL</code> and into a
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statement in the controlled block. When gimplifying from C or C++,
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this initialization replaces the <code>DECL_STMT</code>. These variables
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will never require cleanups. The scope of these variables is just the
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body
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</p>
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<p>Variable-length arrays (VLAs) complicate this process, as their size
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often refers to variables initialized earlier in the block and their
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initialization involves an explicit stack allocation. To handle this,
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we add an indirection and replace them with a pointer to stack space
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allocated by means of <code>alloca</code>. In most cases, we also arrange
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for this space to be reclaimed when the enclosing <code>BIND_EXPR</code> is
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exited, the exception to this being when there is an explicit call to
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<code>alloca</code> in the source code, in which case the stack is left
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depressed on exit of the <code>BIND_EXPR</code>.
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</p>
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<p>A C++ program will usually contain more <code>BIND_EXPR</code>s than
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there are syntactic blocks in the source code, since several C++
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constructs have implicit scopes associated with them. On the
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other hand, although the C++ front end uses pseudo-scopes to
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handle cleanups for objects with destructors, these don’t
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translate into the GIMPLE form; multiple declarations at the same
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level use the same <code>BIND_EXPR</code>.
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</p>
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</body>
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</html>
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