Misc/abrash-black-book
Watch
1
0
Fork
You've already forked abrash-black-book
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Remove colour attributes from body and strip most of the font tags out

Browse source
This commit is contained in:
James Gregory 2013-12-30 13:57:02 +11:00
commit c1f88ddb41
362 changed files with 1632 additions and 1706 deletions
Download patch file Download diff file Expand all files Collapse all files

11
07-01.html
View file

@ -24,7 +24,7 @@
<!--CHAPTER=07//-->
<!--PAGES=136-139//-->
<!--UNASSIGNED1//-->
<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
<!--UNASSIGNED2//--></HEAD><body>
<CENTER>
<TABLE BORDER>
@ -36,9 +36,8 @@
</TABLE>
</CENTER>
<P><BR></P>
<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 7<BR>Local Optimization
</FONT></H2>
<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Optimizing Halfway between Algorithms and Cycle Counting</FONT></H3>
<H2><A NAME="Heading1"></A>Chapter 7<BR>Local Optimization</H2>
<H3><A NAME="Heading2"></A>Optimizing Halfway between Algorithms and Cycle Counting</H3>
<P>You might not think it, but there&rsquo;s much to learn about performance programming from the Great Buffalo Sauna Fiasco. To wit:
</P>
<P>The scene is Buffalo, New York, in the dead of winter, with the snow piled several feet deep. Four college students, living in typical student housing, are frozen to the bone. The third floor of their house, uninsulated and so cold that it&rsquo;s uninhabitable, has an ancient bathroom. One fabulously cold day, inspiration strikes:</P>
@ -54,7 +53,7 @@
</TABLE>
<P>So, drawing fortitude from the knowledge that our quest is a pure and worthy one, let&rsquo;s resume our exploration of assembly language instructions with hidden talents and instructions with well-known talents that are less than they appear to be. In the process, we&rsquo;ll come to see that there is another, very important optimization level between the algorithm/design level and the cycle-counting/individual instruction level. I&rsquo;ll call this middle level <I>local optimization;</I> it involves focusing on optimizing sequences of instructions rather than individual instructions, all with an eye to implementing designs as efficiently as possible given the capabilities of the x86 family instruction set.</P>
<P>And yes, in case you&rsquo;re wondering, the above story is indeed true. Was I there? Let me put it this way: If I were, I&rsquo;d never admit it!</P>
<H4 ALIGN="LEFT"><A NAME="Heading3"></A><FONT COLOR="#000077">When LOOP Is a Bad Idea</FONT></H4>
<H4 ALIGN="LEFT"><A NAME="Heading3"></A>When LOOP Is a Bad Idea</H4>
<P>Let&rsquo;s examine first an instruction that is less than it appears to be: <B>LOOP</B>. There&rsquo;s no mystery about what <B>LOOP</B> does; it decrements CX and branches if CX doesn&rsquo;t decrement to zero. It&rsquo;s so beautifully suited to the task of counting down loops that any experienced x86 programmer instinctively stuffs the loop count in CX and reaches for <B>LOOP</B> when setting up a loop. That&rsquo;s fine&mdash;<B>LOOP</B> does, of course, work as advertised&mdash;but there is one problem:</P>
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>On half of the processors in the x86 family, <B>LOOP</B> is slower than <B>DEC CX</B> followed by <B>JNZ</B>. (Granted, <B>DEC CX/JNZ</B> isn&rsquo;t precisely equivalent to <B>LOOP,</B> because <B>DEC</B> alters the flags and LOOP doesn&rsquo;t, but in most situations they&rsquo;re comparable.)</I></SMALL>
</TABLE>
@ -74,7 +73,7 @@
<hr width="90%" size="1" noshade>
<div align="center">
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
</div>
<!-- all of the reference materials (books) have the footer and subfoot reveresed -->
<!-- reference_subfoot = footer -->