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| 22 | <title>restrict Pointer Example (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="Type-Qualifiers.html" rel="up" title="Type Qualifiers">
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| 33 | <link href="Functions.html" rel="next" title="Functions">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="restrict-Pointer-Example"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="restrict-Pointers.html" accesskey="p" rel="prev">restrict Pointers</a>, Up: <a href="Type-Qualifiers.html" accesskey="u" rel="up">Type Qualifiers</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="restrict-Pointer-Example-1"></span><h3 class="section">21.4 <code>restrict</code> Pointer Example</h3>
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| 64 |
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| 65 | <p>Here are examples where <code>restrict</code> enables real optimization.
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| 66 | </p>
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| 67 | <p>In this example, <code>restrict</code> assures GCC that the array <code>out</code>
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| 68 | points to does not overlap with the array <code>in</code> points to.
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| 69 | </p>
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| 70 | <div class="example">
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| 71 | <pre class="example">void
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| 72 | process_data (const char *in,
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| 73 | char * restrict out,
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| 74 | size_t size)
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| 75 | {
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| 76 | for (i = 0; i < size; i++)
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| 77 | out[i] = in[i] + in[i + 1];
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| 78 | }
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| 79 | </pre></div>
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| 80 |
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| 81 | <p>Here’s a simple tree structure, where each tree node holds data of
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| 82 | type <code>PAYLOAD</code> plus two subtrees.
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| 83 | </p>
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| 84 | <div class="example">
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| 85 | <pre class="example">struct foo
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| 86 | {
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| 87 | PAYLOAD payload;
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| 88 | struct foo *left;
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| 89 | struct foo *right;
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| 90 | };
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| 91 | </pre></div>
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| 92 |
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| 93 | <p>Now here’s a function to null out both pointers in the <code>left</code>
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| 94 | subtree.
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| 95 | </p>
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| 96 | <div class="example">
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| 97 | <pre class="example">void
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| 98 | null_left (struct foo *a)
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| 99 | {
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| 100 | a->left->left = NULL;
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| 101 | a->left->right = NULL;
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| 102 | }
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| 103 | </pre></div>
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| 104 |
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| 105 | <p>Since <code>*a</code> and <code>*a->left</code> have the same data type,
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| 106 | they could legitimately alias (see <a href="Aliasing.html">Aliasing</a>). Therefore,
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| 107 | the compiled code for <code>null_left</code> must read <code>a->left</code>
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| 108 | again from memory when executing the second assignment statement.
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| 109 | </p>
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| 110 | <p>We can enable optimization, so that it does not need to read
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| 111 | <code>a->left</code> again, by writing <code>null_left</code> this in a less
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| 112 | obvious way.
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| 113 | </p>
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| 114 | <div class="example">
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| 115 | <pre class="example">void
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| 116 | null_left (struct foo *a)
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| 117 | {
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| 118 | struct foo *b = a->left;
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| 119 | b->left = NULL;
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| 120 | b->right = NULL;
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| 121 | }
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| 122 | </pre></div>
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| 123 |
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| 124 | <p>A more elegant way to fix this is with <code>restrict</code>.
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| 125 | </p>
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| 126 | <div class="example">
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| 127 | <pre class="example">void
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| 128 | null_left (struct foo *restrict a)
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| 129 | {
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| 130 | a->left->left = NULL;
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| 131 | a->left->right = NULL;
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| 132 | }
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| 133 | </pre></div>
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| 134 |
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| 135 | <p>Declaring <code>a</code> as <code>restrict</code> asserts that other pointers such
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| 136 | as <code>a->left</code> will not point to the same memory space as <code>a</code>.
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| 137 | Therefore, the memory location <code>a->left->left</code> cannot be the same
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| 138 | memory as <code>a->left</code>. Knowing this, the compiled code may avoid
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| 139 | reloading <code>a->left</code> for the second statement.
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| 140 | </p>
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| 141 |
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| 142 |
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| 143 |
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| 144 | </body>
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| 145 | </html>
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