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James Gregory 2013-12-30 12:50:32 +11:00
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<P><BR></P>
<P>How fast <I>is</I> Boyer-Moore? Listing 14.1 is a C implementation of Boyer-Moore searching; Listing 14.2 is a test-bed program that searches up to the first 32K of a file for a pattern. Table 14.1 (all times measured with Turbo Profiler on a 20 MHz cached 386, searching a modified version of the text of this chapter) shows that this implementation is generally much slower than <B>REPNZ SCASB,</B> although it does come close when searching for long patterns. Listing 14.1 is designed primarily to make later assembly implemenmore comprehensible, rather than faster; Sedge&#146;s implementation uses arrays rather than pointers, is a great deal more compact and very clever, and may be somewhat faster. Regardless, the far superior performance of <B>REPNZ SCASB</B> clearly indicates that assembly language is in order at this point.</P>
<P>How fast <I>is</I> Boyer-Moore? Listing 14.1 is a C implementation of Boyer-Moore searching; Listing 14.2 is a test-bed program that searches up to the first 32K of a file for a pattern. Table 14.1 (all times measured with Turbo Profiler on a 20 MHz cached 386, searching a modified version of the text of this chapter) shows that this implementation is generally much slower than <B>REPNZ SCASB,</B> although it does come close when searching for long patterns. Listing 14.1 is designed primarily to make later assembly implemenmore comprehensible, rather than faster; Sedge&rsquo;s implementation uses arrays rather than pointers, is a great deal more compact and very clever, and may be somewhat faster. Regardless, the far superior performance of <B>REPNZ SCASB</B> clearly indicates that assembly language is in order at this point.</P>
<TABLE WIDTH="100%"><TR>
<TD COLSPAN="7"><HR>
<TR>
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="30%">
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&#147;g;&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&#147;Yogi&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&#147;igoY&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&#147;Adrian&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&#147;Conclusion&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="20%">&#147;You don&#146;t know what you know&#148;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&ldquo;g;&rdquo;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&ldquo;Yogi&rdquo;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&ldquo;igoY&rdquo;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&ldquo;Adrian&rdquo;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="10%">&ldquo;Conclusion&rdquo;
<TH VALIGN="BOTTOM" ALIGN="LEFT" WIDTH="20%">&ldquo;You don&rsquo;t know what you know&rdquo;
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<TD VALIGN="TOP" ALIGN="LEFT">4.0
<TD VALIGN="TOP" ALIGN="LEFT">2.0
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<TD VALIGN="TOP" ALIGN="LEFT">&lt=255 pattern length &#43; sentinelBoyer-Moore in ASM(Listing 14.4)
<TD VALIGN="TOP" ALIGN="LEFT">&lt=255 pattern length + sentinelBoyer-Moore in ASM(Listing 14.4)
<TD VALIGN="TOP" ALIGN="LEFT">8.1
<TD VALIGN="TOP" ALIGN="LEFT">5.2
<TD VALIGN="TOP" ALIGN="LEFT">4.6
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<P>The entry &#147;Standard Boyer-Moore in ASM&#148; in Table 14.1 refers to straight-forward hand optimization of Listing 14.1, code that is not included in this chapter for the perfectly good reason that it is slower in most cases than <B>REPNZ SCASB.</B> I say this casually now, but not so yesterday, when I had all but concluded that Boyer-Moore was simply inferior on the x86, due to two architectural quirks: the string instructions and slow branch. I had even coined a neat phrase for it: Architecture is destiny. Has a nice ring, doesn&#146;t it?</P><P><BR></P>
<P>The entry &ldquo;Standard Boyer-Moore in ASM&rdquo; in Table 14.1 refers to straight-forward hand optimization of Listing 14.1, code that is not included in this chapter for the perfectly good reason that it is slower in most cases than <B>REPNZ SCASB.</B> I say this casually now, but not so yesterday, when I had all but concluded that Boyer-Moore was simply inferior on the x86, due to two architectural quirks: the string instructions and slow branch. I had even coined a neat phrase for it: Architecture is destiny. Has a nice ring, doesn&rsquo;t it?</P><P><BR></P>
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