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| <title>Using the GNU Compiler Collection (GCC): Label Attributes</title> |
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| <a name="Label-Attributes"></a> |
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| Next: <a href="Attribute-Syntax.html#Attribute-Syntax" accesskey="n" rel="next">Attribute Syntax</a>, Previous: <a href="Function-Attributes.html#Function-Attributes" accesskey="p" rel="prev">Function 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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| <a name="Label-Attributes-1"></a> |
| <h3 class="section">6.32 Label Attributes</h3> |
| <a name="index-Label-Attributes"></a> |
| |
| <p>GCC allows attributes to be set on C labels. See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>, for |
| details of the exact syntax for using attributes. Other attributes are |
| available for functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), variables |
| (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>) and for types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>). |
| </p> |
| <p>This example uses the <code>cold</code> label attribute to indicate the |
| <code>ErrorHandling</code> branch is unlikely to be taken and that the |
| <code>ErrorHandling</code> label is unused: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| asm goto ("some asm" : : : : NoError); |
| |
| /* This branch (the fall-through from the asm) is less commonly used */ |
| ErrorHandling: |
| __attribute__((cold, unused)); /* Semi-colon is required here */ |
| printf("error\n"); |
| return 0; |
| |
| NoError: |
| printf("no error\n"); |
| return 1; |
| </pre></div> |
| |
| <dl compact="compact"> |
| <dt><code>unused</code></dt> |
| <dd><a name="index-unused-label-attribute"></a> |
| <p>This feature is intended for program-generated code that may contain |
| unused labels, but which is compiled with <samp>-Wall</samp>. It is |
| not normally appropriate to use in it human-written code, though it |
| could be useful in cases where the code that jumps to the label is |
| contained within an <code>#ifdef</code> conditional. |
| </p> |
| </dd> |
| <dt><code>hot</code></dt> |
| <dd><a name="index-hot-label-attribute"></a> |
| <p>The <code>hot</code> attribute on a label is used to inform the compiler that |
| the path following the label is more likely than paths that are not so |
| annotated. This attribute is used in cases where <code>__builtin_expect</code> |
| cannot be used, for instance with computed goto or <code>asm goto</code>. |
| </p> |
| </dd> |
| <dt><code>cold</code></dt> |
| <dd><a name="index-cold-label-attribute"></a> |
| <p>The <code>cold</code> attribute on labels is used to inform the compiler that |
| the path following the label is unlikely to be executed. This attribute |
| is used in cases where <code>__builtin_expect</code> cannot be used, for instance |
| with computed goto or <code>asm goto</code>. |
| </p> |
| </dd> |
| </dl> |
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