| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Duffs Device (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Duffs Device (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Duffs Device (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="Statements.html" rel="up" title="Statements">
|
|---|
| 33 | <link href="Case-Ranges.html" rel="next" title="Case Ranges">
|
|---|
| 34 | <link href="switch-Example.html" rel="prev" title="switch Example">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Duffs-Device"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Case-Ranges.html" accesskey="n" rel="next">Case Ranges</a>, Previous: <a href="switch-Example.html" accesskey="p" rel="prev">switch Example</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</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>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="Duff_0027s-Device"></span><h3 class="section">19.9 Duff’s Device</h3>
|
|---|
| 64 | <span id="index-Duff_0027s-device"></span>
|
|---|
| 65 |
|
|---|
| 66 | <p>The cases in a <code>switch</code> statement can be inside other control
|
|---|
| 67 | constructs. For instance, we can use a technique known as <em>Duff’s
|
|---|
| 68 | device</em> to optimize this simple function,
|
|---|
| 69 | </p>
|
|---|
| 70 | <div class="example">
|
|---|
| 71 | <pre class="example">void
|
|---|
| 72 | copy (char *to, char *from, int count)
|
|---|
| 73 | {
|
|---|
| 74 | while (count > 0)
|
|---|
| 75 | *to++ = *from++, count--;
|
|---|
| 76 | }
|
|---|
| 77 | </pre></div>
|
|---|
| 78 |
|
|---|
| 79 | <p>which copies memory starting at <var>from</var> to memory starting at
|
|---|
| 80 | <var>to</var>.
|
|---|
| 81 | </p>
|
|---|
| 82 | <p>Duff’s device involves unrolling the loop so that it copies
|
|---|
| 83 | several characters each time around, and using a <code>switch</code> statement
|
|---|
| 84 | to enter the loop body at the proper point:
|
|---|
| 85 | </p>
|
|---|
| 86 | <div class="example">
|
|---|
| 87 | <pre class="example">void
|
|---|
| 88 | copy (char *to, char *from, int count)
|
|---|
| 89 | {
|
|---|
| 90 | if (count <= 0)
|
|---|
| 91 | return;
|
|---|
| 92 | int n = (count + 7) / 8;
|
|---|
| 93 | switch (count % 8)
|
|---|
| 94 | {
|
|---|
| 95 | do {
|
|---|
| 96 | case 0: *to++ = *from++;
|
|---|
| 97 | case 7: *to++ = *from++;
|
|---|
| 98 | case 6: *to++ = *from++;
|
|---|
| 99 | case 5: *to++ = *from++;
|
|---|
| 100 | case 4: *to++ = *from++;
|
|---|
| 101 | case 3: *to++ = *from++;
|
|---|
| 102 | case 2: *to++ = *from++;
|
|---|
| 103 | case 1: *to++ = *from++;
|
|---|
| 104 | } while (--n > 0);
|
|---|
| 105 | }
|
|---|
| 106 | }
|
|---|
| 107 | </pre></div>
|
|---|
| 108 |
|
|---|
| 109 |
|
|---|
| 110 |
|
|---|
| 111 |
|
|---|
| 112 | </body>
|
|---|
| 113 | </html>
|
|---|
