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| 22 | <title>Optimization Pragmas (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Pragmas.html" rel="up" title="Pragmas">
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| 33 | <link href="Static-Assertions.html" rel="next" title="Static Assertions">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Optimization-Pragmas"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Severity-Pragmas.html" accesskey="p" rel="prev">Severity Pragmas</a>, Up: <a href="Pragmas.html" accesskey="u" rel="up">Pragmas</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="Optimization-Pragmas-1"></span><h4 class="subsection">30.1.3 Optimization Pragmas</h4>
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| 64 |
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| 65 | <p>These pragmas enable a particular optimization for specific function
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| 66 | definitions. The settings take effect at the end of a function
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| 67 | definition, so the clean place to use these pragmas is between
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| 68 | function definitions.
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| 69 | </p>
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| 70 | <dl compact="compact">
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| 71 | <dt><code>#pragma GCC optimize <var>optimization</var></code></dt>
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| 72 | <dt><code>_Pragma ("GCC optimize <var>optimization</var>")</code></dt>
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| 73 | <dd><p>These pragmas enable the optimization <var>optimization</var> for the
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| 74 | following functions. For example,
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| 75 | </p>
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| 76 | <div class="example">
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| 77 | <pre class="example">_Pragma ("GCC optimize -fforward-propagate")
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>says to apply the ‘<samp>forward-propagate</samp>’ optimization to all
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| 81 | following function definitions. Specifying optimizations for
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| 82 | individual functions, rather than for the entire program, is rare but
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| 83 | can be useful for getting around a bug in the compiler.
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| 84 | </p>
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| 85 | <p>If <var>optimization</var> does not correspond to a defined optimization
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| 86 | option, the pragma is erroneous. To turn off an optimization, use the
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| 87 | corresponding ‘<samp>-fno-</samp>’ option, such as
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| 88 | ‘<samp>-fno-forward-propagate</samp>’.
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| 89 | </p>
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| 90 | </dd>
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| 91 | <dt><code>#pragma GCC target <var>optimizations</var></code></dt>
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| 92 | <dt><code>_Pragma ("GCC target <var>optimizations</var>")</code></dt>
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| 93 | <dd><p>The pragma ‘<samp>GCC target</samp>’ is similar to ‘<samp>GCC optimize</samp>’ but is
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| 94 | used for platform-specific optimizations. Thus,
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| 95 | </p>
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| 96 | <div class="example">
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| 97 | <pre class="example">_Pragma ("GCC target popcnt")
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| 98 | </pre></div>
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| 99 |
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| 100 | <p>activates the optimization ‘<samp>popcnt</samp>’ for all
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| 101 | following function definitions. This optimization is supported
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| 102 | on a few common targets but not on others.
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| 103 | </p>
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| 104 | </dd>
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| 105 | <dt><code>#pragma GCC push_options</code></dt>
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| 106 | <dt><code>_Pragma ("GCC push_options")</code></dt>
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| 107 | <dd><p>The ‘<samp>push_options</samp>’ pragma saves on a stack the current settings
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| 108 | specified with the ‘<samp>target</samp>’ and ‘<samp>optimize</samp>’ pragmas.
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| 109 | </p>
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| 110 | </dd>
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| 111 | <dt><code>#pragma GCC pop_options</code></dt>
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| 112 | <dt><code>_Pragma ("GCC pop_options")</code></dt>
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| 113 | <dd><p>The ‘<samp>pop_options</samp>’ pragma pops saved settings from that stack.
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| 114 | </p>
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| 115 | <p>Here’s an example of using this stack.
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| 116 | </p>
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| 117 | <div class="example">
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| 118 | <pre class="example">_Pragma ("GCC push_options")
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| 119 | _Pragma ("GCC optimize forward-propagate")
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| 120 |
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| 121 | /* <span class="roman">Functions to compile</span>
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| 122 | <span class="roman">with the <code>forward-propagate</code> optimization.</span> */
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| 123 |
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| 124 | _Pragma ("GCC pop_options")
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| 125 | /* <span class="roman">Ends enablement of <code>forward-propagate</code>.</span> */
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| 126 | </pre></div>
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| 127 |
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| 128 | </dd>
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| 129 | <dt><code>#pragma GCC reset_options</code></dt>
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| 130 | <dt><code>_Pragma ("GCC reset_options")</code></dt>
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| 131 | <dd><p>Clears all pragma-defined ‘<samp>target</samp>’ and ‘<samp>optimize</samp>’
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| 132 | optimization settings.
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| 133 | </p></dd>
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| 134 | </dl>
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| 135 |
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| 136 | <hr>
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| 137 | <div class="header">
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| 138 | <p>
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| 139 | Previous: <a href="Severity-Pragmas.html" accesskey="p" rel="prev">Severity Pragmas</a>, Up: <a href="Pragmas.html" accesskey="u" rel="up">Pragmas</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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