How fast *is* 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 **REPNZ SCASB,** 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'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 **REPNZ
SCASB** clearly indicates that assembly language is in order at this
point.

* * * * *

"g;"

"Yogi"

"igoY"

"Adrian"

"Conclusion"

"You don't know what you know"

* * * * *

Searching approach

(16K)

(16K)

(16K)

(\<1K)

(16K)

(16K)

REPNZ SCASB on first char a(Listing 9.1)

8.2

7.5

9.7

0.4

7.4

8.1

REPNZ SCASB on least common char (Listing 9.2)

7.6

7.5

7.5

0.5

7.5

7.5

Boyer-Moore in C (Listing 14.1)

71.0

38.4

37.7

1.8

18.2

9.2

Standard Boyer-Moore in ASM(code not shown)

38.5

21.0

20.5

0.8

9.4

4.8

Quick handling of first mismatch Boyer-Moore in ASM(Listing 14.3)

14.1

8.9

7.7

0.4

4.0

2.0

\<=255 pattern length + sentinelBoyer-Moore in ASM(Listing 14.4)

8.1

5.2

4.6

0.3

2.6

1.2

Search pattern (approximate distance searched before match is shown in
parentheses).\
 Times are in milliseconds; shorter is better.

* * * * *

Table 14.1 Comparison of searching techniques.

* * * * *

The entry "Standard Boyer-Moore in ASM" 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 **REPNZ SCASB.** 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't it?