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James Gregory 2013-12-30 18:34:31 +11:00
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09-03.md
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@ -5,19 +5,19 @@
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’s revelation make? Plenty. Even when the entire C function call to
Rob's revelation make? Plenty. Even when the entire C function call to
**FindString** is timed—**strlen** calls, parameter pushing, calling,
setup, and all—the version of **FindString** in Listing 9.2, which is
directed by Listing 9.3 to scan for the infrequently-occurring “Q,” is
directed by Listing 9.3 to scan for the infrequently-occurring "Q," is
about 40 percent faster on a 20 MHz cached 386 for the test search of
Listing 9.3 than is the version of **FindString** in Listing 9.1, which
always scans for the first character, in this case “E.” However, when
always scans for the first character, in this case "E." However, when
only the search loops (the code that actually does the searching) in the
two versions of **FindString** are compared, Listing 9.2 is more than
*twice* as fast as Listing 9.1—a remarkable improvement over code that
already uses **REPNZ SCASB** and **REPZ CMPS**.
What I like so much about Rob’s approach is that it demonstrates that
What I like so much about Rob'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 **REPNZ SCASB** and
@ -59,9 +59,9 @@ using **REPZ CMPS** to check scanning matches.
.code
public _FindString
_FindStringprocnear
push bp ;preserve caller’s stack frame
push bp ;preserve caller's stack frame
mov bp,sp ;point to our stack frame
push si ;preserve caller’s register variables
push si ;preserve caller's register variables
push di
cld ;make string instructions increment pointers
mov si,[bp+SearchString] ;pointer to string to search for
@ -80,12 +80,12 @@ using **REPZ CMPS** to check scanning matches.
mov di,[bp+Buffer] ;point ES:DI to buffer to search thru
lodsb ;put the first byte of the search string in AL
mov bp,si ;set aside pointer to the second search byte
dec bx ;don’t need to compare the first byte of the
; string with CMPS; we’ll do it with SCAS
dec bx ;don't need to compare the first byte of the
; string with CMPS; we'll do it with SCAS
FindStringLoop:
mov cx,dx ;put remaining buffer search length in CX
repnz scasb ;scan for the first byte of the string
jnz FindStringNotFound ;not found, so there’s no match
jnz FindStringNotFound ;not found, so there's no match
;found, so we have a potential match-check the
; rest of this candidate location
push di ;remember the address of the next byte to scan
@ -96,14 +96,14 @@ using **REPZ CMPS** to check scanning matches.
shr cx,1 ;convert to word for faster search
jnc FindStringWord ;do word search if no odd byte
cmpsb ;compare the odd byte
jnz FindStringNoMatch ;odd byte doesn’t match, so we
; haven’t found the search string here
jnz FindStringNoMatch ;odd byte doesn't match, so we
; haven't found the search string here
FindStringWord:
jcxz FindStringFound ;test whether we’ve already checked
jcxz FindStringFound ;test whether we've already checked
; the whole string; if so, this is a match
; bytes long; if so, we’ve found a match
; bytes long; if so, we've found a match
repz cmpsw ;check the rest of the string a word at a time
jz FindStringFound ;it’s a match
jz FindStringFound ;it's a match
FindStringNoMatch:
pop di ;get back pointer to the next byte to scan
and dx,dx ;is there anything left to check?
@ -117,9 +117,9 @@ using **REPZ CMPS** to check scanning matches.
; address of the byte after the start of the
; potential match)
FindStringDone:
pop di ;restore caller’s register variables
pop di ;restore caller's register variables
pop si
pop bp ;restore caller’s stack frame
pop bp ;restore caller's stack frame
ret
_FindStringendp
end