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| 22 | <title>Label Value Caveats (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Labels-as-Values.html" rel="up" title="Labels as Values">
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| 33 | <link href="Statement-Exprs.html" rel="next" title="Statement Exprs">
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| 34 | <link href="Label-Value-Uses.html" rel="prev" title="Label Value Uses">
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| 57 | <body lang="en">
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| 58 | <span id="Label-Value-Caveats"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Label-Value-Uses.html" accesskey="p" rel="prev">Label Value Uses</a>, Up: <a href="Labels-as-Values.html" accesskey="u" rel="up">Labels as Values</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Label-Value-Caveats-1"></span><h4 class="subsection">19.14.2 Label Value Caveats</h4>
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| 64 |
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| 65 | <p>Jumping to a label defined in another function does not work.
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| 66 | It can cause unpredictable results.
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| 67 | </p>
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| 68 | <p>The best way to avoid this is to store label values only in
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| 69 | automatic variables, or static variables whose names are declared
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| 70 | within the function. Never pass them as arguments.
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| 71 | </p>
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| 72 | <span id="index-cloning"></span>
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| 73 | <p>An optimization known as <em>cloning</em> generates multiple simplified
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| 74 | variants of a function’s code, for use with specific fixed arguments.
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| 75 | Using label values in certain ways, such as saving the address in one
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| 76 | call to the function and using it again in another call, would make cloning
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| 77 | give incorrect results. These functions must disable cloning.
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| 78 | </p>
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| 79 | <p>Inlining calls to the function would also result in multiple copies of
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| 80 | the code, each with its own value of the same label. Using the label
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| 81 | in a computed goto is no problem, because the computed goto inhibits
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| 82 | inlining. However, using the label value in some other way, such as
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| 83 | an indication of where an error occurred, would be optimized wrong.
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| 84 | These functions must disable inlining.
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| 85 | </p>
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| 86 | <p>To prevent inlining or cloning of a function, specify
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| 87 | <code>__attribute__((__noinline__,__noclone__))</code> in its definition.
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| 88 | See <a href="Attributes.html">Attributes</a>.
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| 89 | </p>
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| 90 | <p>When a function uses a label value in a static variable initializer,
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| 91 | that automatically prevents inlining or cloning the function.
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| 92 | </p>
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| 93 |
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| 94 |
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| 95 |
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