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<meta name="vspubdate" content="07/01/97" />
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
<title>Michael Abrash's Graphics Programming Black Book Special Edition: The Game of Life</title><!-- HEADER -->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: The Game of Life</title>
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<meta name="pages" content="338-340" />
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<li>Cells change state relatively infrequently.</li>
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<h3><a id="Heading8"></a>Bringing In the Right Brain</h3>
<h3 id="Heading8">Bringing In the Right Brain</h3>
<p>In the previous section, we saw how a C<small>++</small> program could be sped up about eight times simply by rearranging the data and code in straightforward ways. Now we&rsquo;re going to see how right-brain non-linear optimization can speed things up by another four times&mdash;and make the code <i>simpler.</i></p>
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<p>I have two objectives to achieve in the remainder of this chapter. First, I want to show that optimization consists of many levels, from assembly language up to conceptual design, and that assembly language kicks in pretty late in the optimization process. Second, I want to encourage you to saturate your brain with everything you know about any particular optimization problem, then make space for your right brain to solve the problem.</p>
<h4 align="left"><a id="Heading9"></a>Re-Examining the Task</h4>
<h4 align="left" id="Heading9">Re-Examining the Task</h4>
<p>Earlier in this chapter, we looked at a straightforward Game of Life implementation, then increased performance considerably by making the implementation a little less abstract and a little less general. We made a small change to the cellmap format, adding padding bytes off the edges so that pointer arithmetic would always work, but the major optimizations were moving the critical code into a single loop and using pointers rather than member functions whenever possible. In other words, we took what we already knew and made it more efficient.</p>
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<p><i>Know your data.</i></p>
<p><a id="Fig3"><!-- </A><A HREF="javascript:displayWindow('images/17-03.jpg',405,113 )"> --><img src="images/17-03.jpg" /><br />
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<BR><A HREF="javascript:displayWindow('images/17-03.jpg',405,113)"> --><b>Figure 17.3</b></a>&nbsp;&nbsp;<i>New cell format.</i></p>
<p><a id="Fig3"><img src="images/17-03.jpg" /><br />
<b>Figure 17.3</b></a>&nbsp;&nbsp;<i>New cell format.</i></p>
<h4 align="left"><a id="Heading10"></a>Acting on What We Know</h4>
<h4 align="left" id="Heading10">Acting on What We Know</h4>
<p>Once we&rsquo;ve changed the cellmap format to store neighbor counts as well as states, with a byte for each cell, we can get another performance boost by again examining what we know about our data. I said earlier that most cells are off during any given generation. This means that most cells have no neighbors that are on. Since the cell map representation for an off-cell that has no neighbors is a zero byte, we can skip over scads of unchanged cells at a pop simply by scanning for non-zero bytes. This is much faster than explicitly testing cell states and neighbor counts, and lends itself beautifully to assembly language implementation as <b>REPZ SCASB</b> or (with a little cleverness) <b>REPZ SCASW.</b> (Unfortunately, there&rsquo;s no C library function that can scan memory for the next byte that&rsquo;s non-zero.)</p>
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Graphics Programming Black Book &copy; 2001 Michael Abrash
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