146 lines
No EOL
2.2 KiB
Markdown
146 lines
No EOL
2.2 KiB
Markdown
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? |