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| 22 | <title>Optimization and Ordering (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="Order-of-Execution.html" rel="up" title="Order of Execution">
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| 57 | <body lang="en">
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| 58 | <span id="Optimization-and-Ordering"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Ordering-of-Operands.html" accesskey="p" rel="prev">Ordering of Operands</a>, Up: <a href="Order-of-Execution.html" accesskey="u" rel="up">Order of Execution</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-and-Ordering-1"></span><h3 class="section">10.6 Optimization and Ordering</h3>
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| 64 | <span id="index-optimization-and-ordering"></span>
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| 65 | <span id="index-ordering-and-optimization"></span>
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| 66 |
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| 67 | <p>Sequence points limit the compiler’s freedom to reorder operations
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| 68 | arbitrarily, but optimizations can still reorder them if the compiler
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| 69 | concludes that this won’t alter the results. Thus, in this code,
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">x++;
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| 73 | y = z;
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| 74 | x++;
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>there is a sequence point after each statement, so the code is
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| 78 | supposed to increment <code>x</code> once before the assignment to <code>y</code>
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| 79 | and once after. However, incrementing <code>x</code> has no effect on
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| 80 | <code>y</code> or <code>z</code>, and setting <code>y</code> can’t affect <code>x</code>, so
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| 81 | the code could be optimized into this:
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| 82 | </p>
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| 83 | <div class="example">
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| 84 | <pre class="example">y = z;
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| 85 | x += 2;
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| 86 | </pre></div>
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| 87 |
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| 88 | <p>Normally that has no effect except to make the program faster. But
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| 89 | there are special situations where it can cause trouble due to things
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| 90 | that the compiler cannot know about, such as shared memory. To limit
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| 91 | optimization in those places, use the <code>volatile</code> type qualifier
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| 92 | (see <a href="volatile.html">volatile</a>).
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| 93 | </p>
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| 94 |
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| 95 |
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| 96 |
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| 97 | </body>
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| 98 | </html>
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