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145 lines
6.5 KiB
HTML
145 lines
6.5 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>Label Attributes (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Label Attributes (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Label Attributes (Using the GNU Compiler Collection (GCC))">
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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="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Enumerator-Attributes.html#Enumerator-Attributes" rel="next" title="Enumerator Attributes">
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<link href="x86-Type-Attributes.html#x86-Type-Attributes" rel="prev" title="x86 Type Attributes">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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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="Label-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="Enumerator-Attributes.html#Enumerator-Attributes" accesskey="n" rel="next">Enumerator Attributes</a>, Previous: <a href="Type-Attributes.html#Type-Attributes" accesskey="p" rel="prev">Type Attributes</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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="Label-Attributes-1"></a>
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<h3 class="section">6.34 Label Attributes</h3>
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<a name="index-Label-Attributes"></a>
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<p>GCC allows attributes to be set on C labels. See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for
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details of the exact syntax for using attributes. Other attributes are
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available for functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), variables
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(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>), enumerators (see <a href="Enumerator-Attributes.html#Enumerator-Attributes">Enumerator Attributes</a>),
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statements (see <a href="Statement-Attributes.html#Statement-Attributes">Statement Attributes</a>), and for types
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(see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>).
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</p>
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<p>This example uses the <code>cold</code> label attribute to indicate the
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<code>ErrorHandling</code> branch is unlikely to be taken and that the
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<code>ErrorHandling</code> label is unused:
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</p>
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<div class="smallexample">
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<pre class="smallexample">
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asm goto ("some asm" : : : : NoError);
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/* This branch (the fall-through from the asm) is less commonly used */
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ErrorHandling:
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__attribute__((cold, unused)); /* Semi-colon is required here */
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printf("error\n");
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return 0;
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NoError:
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printf("no error\n");
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return 1;
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</pre></div>
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<dl compact="compact">
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<dt><code>unused</code></dt>
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<dd><a name="index-unused-label-attribute"></a>
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<p>This feature is intended for program-generated code that may contain
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unused labels, but which is compiled with <samp>-Wall</samp>. It is
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not normally appropriate to use in it human-written code, though it
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could be useful in cases where the code that jumps to the label is
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contained within an <code>#ifdef</code> conditional.
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</p>
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</dd>
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<dt><code>hot</code></dt>
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<dd><a name="index-hot-label-attribute"></a>
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<p>The <code>hot</code> attribute on a label is used to inform the compiler that
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the path following the label is more likely than paths that are not so
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annotated. This attribute is used in cases where <code>__builtin_expect</code>
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cannot be used, for instance with computed goto or <code>asm goto</code>.
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</p>
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</dd>
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<dt><code>cold</code></dt>
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<dd><a name="index-cold-label-attribute"></a>
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<p>The <code>cold</code> attribute on labels is used to inform the compiler that
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the path following the label is unlikely to be executed. This attribute
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is used in cases where <code>__builtin_expect</code> cannot be used, for instance
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with computed goto or <code>asm goto</code>.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Enumerator-Attributes.html#Enumerator-Attributes" accesskey="n" rel="next">Enumerator Attributes</a>, Previous: <a href="Type-Attributes.html#Type-Attributes" accesskey="p" rel="prev">Type Attributes</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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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</body>
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</html>
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