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<!DOCTYPE html>
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<title>Exercise 23: Meet Duff's Device</title>
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<h1 class="title">Exercise 23: Meet Duff's Device</h1>
<p>This exercise is a brain teaser where I introduce you to one of the
most famous hacks in C called "Duff's Device", named after Tom Duff
the "inventor". This little slice of awesome (evil?) has nearly everything
you've been learning wrapped in one tiny little package. Figuring out
how it works is also a good fun puzzle.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">Part of the fun of C is that you can come up with crazy hacks like this,
but this is also what makes C annoying to use. It's good to learn about
these tricks because it gives you a deeper understanding of
the language and your computer. But, you should never use this. Always
strive for easy to read code.</p>
</div>
<p>Duff's device was "discovered" (created?) by Tom Duff and is a trick
with the C compiler that actually shouldn't work. I won't tell you what
it does yet since this is meant to be a puzzle for you to ponder and
try to solve. You are to get this code running and then try to figure
out what it does, and <em>why</em> it does it this way.</p>
<div class="highlight"><pre><a name="code--ex23.c-pyg.html-1"></a><span class="cp">#include <stdio.h></span>
<a name="code--ex23.c-pyg.html-2"></a><span class="cp">#include <string.h></span>
<a name="code--ex23.c-pyg.html-3"></a><span class="cp">#include "dbg.h"</span>
<a name="code--ex23.c-pyg.html-4"></a>
<a name="code--ex23.c-pyg.html-5"></a>
<a name="code--ex23.c-pyg.html-6"></a><span class="kt">int</span> <span class="nf">normal_copy</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">from</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">to</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">)</span>
<a name="code--ex23.c-pyg.html-7"></a><span class="p">{</span>
<a name="code--ex23.c-pyg.html-8"></a> <span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-9"></a>
<a name="code--ex23.c-pyg.html-10"></a> <span class="k">for</span><span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">count</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-11"></a> <span class="n">to</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">from</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
<a name="code--ex23.c-pyg.html-12"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-13"></a>
<a name="code--ex23.c-pyg.html-14"></a> <span class="k">return</span> <span class="n">i</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-15"></a><span class="p">}</span>
<a name="code--ex23.c-pyg.html-16"></a>
<a name="code--ex23.c-pyg.html-17"></a><span class="kt">int</span> <span class="nf">duffs_device</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">from</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">to</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">)</span>
<a name="code--ex23.c-pyg.html-18"></a><span class="p">{</span>
<a name="code--ex23.c-pyg.html-19"></a> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-20"></a> <span class="kt">int</span> <span class="n">n</span> <span class="o">=</span> <span class="p">(</span><span class="n">count</span> <span class="o">+</span> <span class="mi">7</span><span class="p">)</span> <span class="o">/</span> <span class="mi">8</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-21"></a>
<a name="code--ex23.c-pyg.html-22"></a> <span class="k">switch</span><span class="p">(</span><span class="n">count</span> <span class="o">%</span> <span class="mi">8</span><span class="p">)</span> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-23"></a> <span class="k">case</span> <span class="mi">0</span>: <span class="k">do</span> <span class="p">{</span> <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-24"></a> <span class="k">case</span> <span class="mi">7</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-25"></a> <span class="k">case</span> <span class="mi">6</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-26"></a> <span class="k">case</span> <span class="mi">5</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-27"></a> <span class="k">case</span> <span class="mi">4</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-28"></a> <span class="k">case</span> <span class="mi">3</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-29"></a> <span class="k">case</span> <span class="mi">2</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-30"></a> <span class="k">case</span> <span class="mi">1</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-31"></a> <span class="p">}</span> <span class="k">while</span><span class="p">(</span><span class="o">--</span><span class="n">n</span> <span class="o">></span> <span class="mi">0</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-32"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-33"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-34"></a>
<a name="code--ex23.c-pyg.html-35"></a> <span class="k">return</span> <span class="n">count</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-36"></a><span class="p">}</span>
<a name="code--ex23.c-pyg.html-37"></a>
<a name="code--ex23.c-pyg.html-38"></a><span class="kt">int</span> <span class="nf">zeds_device</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">from</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">to</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">)</span>
<a name="code--ex23.c-pyg.html-39"></a><span class="p">{</span>
<a name="code--ex23.c-pyg.html-40"></a> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-41"></a> <span class="kt">int</span> <span class="n">n</span> <span class="o">=</span> <span class="p">(</span><span class="n">count</span> <span class="o">+</span> <span class="mi">7</span><span class="p">)</span> <span class="o">/</span> <span class="mi">8</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-42"></a>
<a name="code--ex23.c-pyg.html-43"></a> <span class="k">switch</span><span class="p">(</span><span class="n">count</span> <span class="o">%</span> <span class="mi">8</span><span class="p">)</span> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-44"></a> <span class="k">case</span> <span class="mi">0</span>:
<a name="code--ex23.c-pyg.html-45"></a> <span class="n">again</span><span class="o">:</span> <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-46"></a>
<a name="code--ex23.c-pyg.html-47"></a> <span class="k">case</span> <span class="mi">7</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-48"></a> <span class="k">case</span> <span class="mi">6</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-49"></a> <span class="k">case</span> <span class="mi">5</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-50"></a> <span class="k">case</span> <span class="mi">4</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-51"></a> <span class="k">case</span> <span class="mi">3</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-52"></a> <span class="k">case</span> <span class="mi">2</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-53"></a> <span class="k">case</span> <span class="mi">1</span>: <span class="o">*</span><span class="n">to</span><span class="o">++</span> <span class="o">=</span> <span class="o">*</span><span class="n">from</span><span class="o">++</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-54"></a> <span class="k">if</span><span class="p">(</span><span class="o">--</span><span class="n">n</span> <span class="o">></span> <span class="mi">0</span><span class="p">)</span> <span class="k">goto</span> <span class="n">again</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-55"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-56"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-57"></a>
<a name="code--ex23.c-pyg.html-58"></a> <span class="k">return</span> <span class="n">count</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-59"></a><span class="p">}</span>
<a name="code--ex23.c-pyg.html-60"></a>
<a name="code--ex23.c-pyg.html-61"></a><span class="kt">int</span> <span class="nf">valid_copy</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">data</span><span class="p">,</span> <span class="kt">int</span> <span class="n">count</span><span class="p">,</span> <span class="kt">char</span> <span class="n">expects</span><span class="p">)</span>
<a name="code--ex23.c-pyg.html-62"></a><span class="p">{</span>
<a name="code--ex23.c-pyg.html-63"></a> <span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-64"></a> <span class="k">for</span><span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">count</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-65"></a> <span class="k">if</span><span class="p">(</span><span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">!=</span> <span class="n">expects</span><span class="p">)</span> <span class="p">{</span>
<a name="code--ex23.c-pyg.html-66"></a> <span class="n">log_err</span><span class="p">(</span><span class="s">"[%d] %c != %c"</span><span class="p">,</span> <span class="n">i</span><span class="p">,</span> <span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">expects</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-67"></a> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-68"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-69"></a> <span class="p">}</span>
<a name="code--ex23.c-pyg.html-70"></a>
<a name="code--ex23.c-pyg.html-71"></a> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-72"></a><span class="p">}</span>
<a name="code--ex23.c-pyg.html-73"></a>
<a name="code--ex23.c-pyg.html-74"></a>
<a name="code--ex23.c-pyg.html-75"></a><span class="kt">int</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">argv</span><span class="p">[])</span>
<a name="code--ex23.c-pyg.html-76"></a><span class="p">{</span>
<a name="code--ex23.c-pyg.html-77"></a> <span class="kt">char</span> <span class="n">from</span><span class="p">[</span><span class="mi">1000</span><span class="p">]</span> <span class="o">=</span> <span class="p">{</span><span class="sc">'a'</span><span class="p">};</span>
<a name="code--ex23.c-pyg.html-78"></a> <span class="kt">char</span> <span class="n">to</span><span class="p">[</span><span class="mi">1000</span><span class="p">]</span> <span class="o">=</span> <span class="p">{</span><span class="sc">'c'</span><span class="p">};</span>
<a name="code--ex23.c-pyg.html-79"></a> <span class="kt">int</span> <span class="n">rc</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-80"></a>
<a name="code--ex23.c-pyg.html-81"></a> <span class="c1">// setup the from to have some stuff</span>
<a name="code--ex23.c-pyg.html-82"></a> <span class="n">memset</span><span class="p">(</span><span class="n">from</span><span class="p">,</span> <span class="sc">'x'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-83"></a> <span class="c1">// set it to a failure mode</span>
<a name="code--ex23.c-pyg.html-84"></a> <span class="n">memset</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="sc">'y'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-85"></a> <span class="n">check</span><span class="p">(</span><span class="n">valid_copy</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="sc">'y'</span><span class="p">),</span> <span class="s">"Not initialized right."</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-86"></a>
<a name="code--ex23.c-pyg.html-87"></a> <span class="c1">// use normal copy to </span>
<a name="code--ex23.c-pyg.html-88"></a> <span class="n">rc</span> <span class="o">=</span> <span class="n">normal_copy</span><span class="p">(</span><span class="n">from</span><span class="p">,</span> <span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-89"></a> <span class="n">check</span><span class="p">(</span><span class="n">rc</span> <span class="o">==</span> <span class="mi">1000</span><span class="p">,</span> <span class="s">"Normal copy failed: %d"</span><span class="p">,</span> <span class="n">rc</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-90"></a> <span class="n">check</span><span class="p">(</span><span class="n">valid_copy</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="sc">'x'</span><span class="p">),</span> <span class="s">"Normal copy failed."</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-91"></a>
<a name="code--ex23.c-pyg.html-92"></a> <span class="c1">// reset</span>
<a name="code--ex23.c-pyg.html-93"></a> <span class="n">memset</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="sc">'y'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-94"></a>
<a name="code--ex23.c-pyg.html-95"></a> <span class="c1">// duffs version</span>
<a name="code--ex23.c-pyg.html-96"></a> <span class="n">rc</span> <span class="o">=</span> <span class="n">duffs_device</span><span class="p">(</span><span class="n">from</span><span class="p">,</span> <span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-97"></a> <span class="n">check</span><span class="p">(</span><span class="n">rc</span> <span class="o">==</span> <span class="mi">1000</span><span class="p">,</span> <span class="s">"Duff's device failed: %d"</span><span class="p">,</span> <span class="n">rc</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-98"></a> <span class="n">check</span><span class="p">(</span><span class="n">valid_copy</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="sc">'x'</span><span class="p">),</span> <span class="s">"Duff's device failed copy."</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-99"></a>
<a name="code--ex23.c-pyg.html-100"></a> <span class="c1">// reset</span>
<a name="code--ex23.c-pyg.html-101"></a> <span class="n">memset</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="sc">'y'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-102"></a>
<a name="code--ex23.c-pyg.html-103"></a> <span class="c1">// my version</span>
<a name="code--ex23.c-pyg.html-104"></a> <span class="n">rc</span> <span class="o">=</span> <span class="n">zeds_device</span><span class="p">(</span><span class="n">from</span><span class="p">,</span> <span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-105"></a> <span class="n">check</span><span class="p">(</span><span class="n">rc</span> <span class="o">==</span> <span class="mi">1000</span><span class="p">,</span> <span class="s">"Zed's device failed: %d"</span><span class="p">,</span> <span class="n">rc</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-106"></a> <span class="n">check</span><span class="p">(</span><span class="n">valid_copy</span><span class="p">(</span><span class="n">to</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="sc">'x'</span><span class="p">),</span> <span class="s">"Zed's device failed copy."</span><span class="p">);</span>
<a name="code--ex23.c-pyg.html-107"></a>
<a name="code--ex23.c-pyg.html-108"></a> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-109"></a><span class="nl">error:</span>
<a name="code--ex23.c-pyg.html-110"></a> <span class="k">return</span> <span class="mi">1</span><span class="p">;</span>
<a name="code--ex23.c-pyg.html-111"></a><span class="p">}</span>
</pre></div><p>In this code I have three versions of a copy function:</p>
<dl class="docutils">
<dt><tt class="docutils literal">normal_copy</tt></dt>
<dd>Which is just a plain <tt class="docutils literal"><span class="pre">for-loop</span></tt> that copies
characters from one array to another.</dd>
<dt><tt class="docutils literal">duffs_device</tt></dt>
<dd>This is the brain teaser called "Duff's Device", named
after Tom Duff, the person to blame for this delicious evil.</dd>
<dt><tt class="docutils literal">zeds_device</tt></dt>
<dd>A version of "Duff's Device" that just uses a goto so
you can get a clue about what's happening with the weird <tt class="docutils literal"><span class="pre">do-while</span></tt>
placement in <tt class="docutils literal">duffs_device</tt>.</dd>
</dl>
<p>Study these three functions before continuing. Try to explain what's
going on to yourself before continuing.</p>
<div class="section" id="what-you-should-see">
<h1>What You Should See</h1>
<p>There's no output from this program, it just runs and exits. You should
run it under valgrind and make sure there are no errors.</p>
</div>
<div class="section" id="solving-the-puzzle">
<h1>Solving The Puzzle</h1>
<p>The first thing to understand is that C is rather loose regarding some
of its syntax. This is why you can put half of a <tt class="docutils literal"><span class="pre">do-while</span></tt> in
one part of a <tt class="docutils literal"><span class="pre">switch-statement</span></tt>, then the other half somewhere
else and it will still work. If you look at my version with the <tt class="docutils literal">goto again</tt>
it's actually more clear what's going on, but make sure you understand
how that part works.</p>
<p>The second thing is how the default fallthrough semantics of
<tt class="docutils literal"><span class="pre">switch-statements</span></tt> means you can jump to a particular case, and
then it will just keep running until the end of the switch.</p>
<p>The final clue is the <tt class="docutils literal">count % 8</tt> and the calculation of <tt class="docutils literal">n</tt> at
the top.</p>
<p>Now, to solve how these functions work, do the following:</p>
<ul class="simple">
<li>Print this code out so you can write on some paper.</li>
<li>On a piece of paper, write each of the variables in a table as they
are when they get initialized right before the <tt class="docutils literal"><span class="pre">switch-statement</span></tt>.</li>
<li>Follow the logic to the switch, then do the jump to the right case.</li>
<li>Update the variables, including the <tt class="docutils literal">to</tt>, <tt class="docutils literal">from</tt>, and the
arrays they point at.</li>
<li>When you get to the <tt class="docutils literal">while</tt> part or my <tt class="docutils literal">goto</tt> alternative,
check your variables and then follow the logic either back to the
top of the <tt class="docutils literal"><span class="pre">do-while</span></tt> or to where the <tt class="docutils literal">again</tt> label is
located.</li>
<li>Follow through this manual tracing, updating the variables, until
you are sure you see how this flows.</li>
</ul>
<div class="section" id="why-bother">
<h2>Why Bother?</h2>
<p>When you've figured out how it actually works, the final question is: Why would
you ever want to do this? The purpose of this trick is to manually do "loop
unrolling". Large long loops can be slow, so one way to speed them up is to
find some fixed chunk of the loop, and then just duplicate the code in the loop
out that many times sequentially. For example, if you know a loop runs a
minimum of 20 times, then you can put the contents of the loop 20 times in the
source code.</p>
<p>Duff's device is basically doing this automatically by chunking up the loop
into 8 iteration chunks. It's clever and actually works, but these days a good
compiler will do this for you. You shouldn't need this except in the rare case
where you have <em>proven</em> it would improve your speed.</p>
</div>
</div>
<div class="section" id="extra-credit">
<h1>Extra Credit</h1>
<ul class="simple">
<li>Never use this again.</li>
<li>Go look at the Wikipedia entry for "Duff's Device" and see if you can
spot the error. Compare it to the version I have here and read the article
carefully to try to understand why the Wikipedia code won't work for you
but worked for Tom Duff.</li>
<li>Create a set of macros that lets you create any length device like this.
For example, what if you wanted to have 32 case statements and didn't want
to write out all of them? Can you do a macro that lays down 8 at a time?</li>
<li>Change the <tt class="docutils literal">main</tt> to conduct some speed tests to see which one is
really the fastest.</li>
<li>Read about <tt class="docutils literal">memcpy</tt>, <tt class="docutils literal">memmove</tt>, <tt class="docutils literal">memset</tt>, and also compare
their speed.</li>
<li>Never use this again!</li>
</ul>
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