158 lines
2.7 KiB
Markdown
158 lines
2.7 KiB
Markdown
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[Previous](14-02.html) [Table of Contents](index.html) [Next](14-04.html)
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How fast *is* Boyer-Moore? Listing 14.1 is a C implementation of
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Boyer-Moore searching; Listing 14.2 is a test-bed program that searches
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up to the first 32K of a file for a pattern. Table 14.1 (all times
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measured with Turbo Profiler on a 20 MHz cached 386, searching a
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modified version of the text of this chapter) shows that this
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implementation is generally much slower than **REPNZ SCASB,** although
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it does come close when searching for long patterns. Listing 14.1 is
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designed primarily to make later assembly implemenmore comprehensible,
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rather than faster; Sedge’s implementation uses arrays rather than
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pointers, is a great deal more compact and very clever, and may be
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somewhat faster. Regardless, the far superior performance of **REPNZ
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SCASB** clearly indicates that assembly language is in order at this
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point.
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* * * * *
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“g;”
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“Yogi”
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“igoY”
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“Adrian”
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“Conclusion”
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“You don’t know what you know”
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* * * * *
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Searching approach
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(16K)
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(16K)
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(16K)
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(\<1K)
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(16K)
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(16K)
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REPNZ SCASB on first char a(Listing 9.1)
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8.2
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7.5
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9.7
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0.4
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7.4
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8.1
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REPNZ SCASB on least common char (Listing 9.2)
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7.6
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7.5
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7.5
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0.5
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7.5
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7.5
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Boyer-Moore in C (Listing 14.1)
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71.0
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38.4
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37.7
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1.8
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18.2
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9.2
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Standard Boyer-Moore in ASM(code not shown)
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38.5
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21.0
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20.5
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0.8
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9.4
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4.8
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Quick handling of first mismatch Boyer-Moore in ASM(Listing 14.3)
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14.1
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8.9
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7.7
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0.4
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4.0
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2.0
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\<=255 pattern length + sentinelBoyer-Moore in ASM(Listing 14.4)
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8.1
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5.2
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4.6
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0.3
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2.6
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1.2
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Search pattern (approximate distance searched before match is shown in
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parentheses).\
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Times are in milliseconds; shorter is better.
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* * * * *
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Table 14.1 Comparison of searching techniques.
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* * * * *
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The entry “Standard Boyer-Moore in ASM” in Table 14.1 refers to
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straight-forward hand optimization of Listing 14.1, code that is not
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included in this chapter for the perfectly good reason that it is slower
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in most cases than **REPNZ SCASB.** I say this casually now, but not so
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yesterday, when I had all but concluded that Boyer-Moore was simply
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inferior on the x86, due to two architectural quirks: the string
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instructions and slow branch. I had even coined a neat phrase for it:
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Architecture is destiny. Has a nice ring, doesn’t it?
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------------------------ --------------------------------- --------------------
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[Previous](14-02.html) [Table of Contents](index.html) [Next](14-04.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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