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James Gregory 2013-12-30 12:25:34 +11:00
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<P><BR></P>
<P>Listing 9.1 implements the scan-on-first-character approach. Listing 9.2 scans for whatever character the caller specifies. Listing 9.3 is a test program used to compare the two approaches. How much difference does Rob&#146;s revelation make? Plenty. Even when the entire C function call to <B>FindString</B> is timed&#151;<B>strlen</B> calls, parameter pushing, calling, setup, and all&#151;the version of <B>FindString</B> in Listing 9.2, which is directed by Listing 9.3 to scan for the infrequently-occurring &#147;Q,&#148; is about 40 percent faster on a 20 MHz cached 386 for the test search of Listing 9.3 than is the version of <B>FindString</B> in Listing 9.1, which always scans for the first character, in this case &#147;E.&#148; However, when only the search loops (the code that actually does the searching) in the two versions of <B>FindString</B> are compared, Listing 9.2 is more than <I>twice</I> as fast as Listing 9.1&#151;a remarkable improvement over code that already uses <B>REPNZ SCASB</B> and <B>REPZ CMPS</B>.</P>
<P>What I like so much about Rob&#146;s approach is that it demonstrates that optimization involves much more than instruction selection and cycle counting. Listings 9.1 and 9.2 use pretty much the same instructions, and even use the same approach of scanning with <B>REPNZ SCASB</B> and using <B>REPZ CMPS</B> to check scanning matches.</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/09-04i.jpg"><TD WIDTH="95%" VALIGN="TOP" ALIGN="LEFT"><SMALL><I>The difference between Listings 9.1 and 9.2 (which gives you more than a doubling of performance) is due entirely to understanding the nature of the data being handled, and biasing the code to reflect that knowledge.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/i.jpg"><TD WIDTH="95%" VALIGN="TOP" ALIGN="LEFT"><SMALL><I>The difference between Listings 9.1 and 9.2 (which gives you more than a doubling of performance) is due entirely to understanding the nature of the data being handled, and biasing the code to reflect that knowledge.</I></SMALL>
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<P><A NAME="Fig2"><!-- </A><A HREF="javascript:displayWindow('images/09-02.jpg',409,306 )"> --><IMG SRC="images/09-02.jpg"><BR><!-- </A>
<BR><A HREF="javascript:displayWindow('images/09-02.jpg',409,306)"> --><FONT COLOR="#000077"><B>Figure 9.2</B></FONT></A>&nbsp;&nbsp;<I>Faster searching method for locating a text string.</I>