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Retidy, removing align=left from headings and empty paragraphs

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James Gregory 2013-12-30 15:37:13 +11:00
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@ -1,5 +1,4 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
@ -19,17 +18,21 @@
<center>
<table border="1">
<tr>
<td><a href="08-01.html">Previous</a></td>
<td>
<a href="08-01.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
<td>
<a href="index.html">Table of Contents</a>
</td>
<td><a href="08-03.html">Next</a></td>
<td>
<a href="08-03.html">Next</a>
</td>
</tr>
</table>
</center>
<p><br /></p>
<h3 id="Heading5">Stack Frames Slow So Much</h3>
<p>C compilers work within the stack frame model, whereby variables reside in a block of stack memory and are accessed via offsets from BP. Compilers may store a couple of variables in registers and may briefly keep other variables in registers when they&rsquo;re used repeatedly, but the stack frame is the underlying architecture. It&rsquo;s a nice architecture; it&rsquo;s flexible, convenient, easy to program, and makes for fairly compact code. However, stack frames have a few drawbacks. They must be constructed and destroyed, which takes both time and code. They are so easy to use that they tend to bias the assembly language programmer in favor of accessing memory variables more often than might be necessary. Finally, you cannot use BP as a general-purpose register if you intend to access a stack frame, and having that seventh register available is sometimes useful indeed.</p>
@ -50,7 +53,7 @@
<p>In assembly language you have full control over segments. Use it, and, if necessary, reorganize your code to minimize segment loading.</p>
<h4 align="left" id="Heading7">Why Speeding Up Is Hard to Do</h4>
<h4 id="Heading7">Why Speeding Up Is Hard to Do</h4>
<p>You might think that the most obvious advantage assembly language has over C is that it allows the use of all forms of instructions and all registers in all ways, whereas C compilers tend to use a subset of registers and instructions in a limited number of ways. Yes and no. It&rsquo;s true that C compilers typically don&rsquo;t generate instructions such as <b>XLAT,</b> rotates, or the string instructions. On the other hand, <b>XLAT</b> and rotates are useful in a limited set of circumstances, and string instructions <i>are</i> used in the C library functions. In fact, C library code is likely to be carefully optimized by experts, and may be much better than equivalent code you&rsquo;d produce yourself.</p>
@ -113,20 +116,24 @@ jz Match
<p>That said, let me show some of these precepts in action.</p>
<h4 align="left" id="Heading9">A C-to-Assembly Case Study</h4>
<h4 id="Heading9">A C-to-Assembly Case Study</h4>
<p>Listing 8.1 is the sample C application I&rsquo;m going to use to examine optimization in action. Listing 8.1 isn&rsquo;t really complete&mdash;it doesn&rsquo;t handle the &ldquo;no-matches&rdquo; case well, and it assumes that the sum of all matches will fit into an <b>int&mdash;</b>but it will do just fine as an optimization example.</p>
<p><br /></p>
<center>
<table border="1">
<tr>
<td><a href="08-01.html">Previous</a></td>
<td>
<a href="08-01.html">Previous</a>
</td>
<td><a href="index.html">Table of Contents</a></td>
<td>
<a href="index.html">Table of Contents</a>
</td>
<td><a href="08-03.html">Next</a></td>
<td>
<a href="08-03.html">Next</a>
</td>
</tr>
</table>
</center>