RE: code optomization:any way to do this better?

  • From: "Sina Bahram" <sbahram@xxxxxxxxx>
  • To: <programmingblind@xxxxxxxxxxxxx>
  • Date: Mon, 17 Jan 2011 11:32:40 -0500

Also, I'd advise you to take a look at the following

Note: all the following examples assume that es and ds are pointing at the 
proper segments containing the destination and source
strings.

Comparing Str1 to Str2:

                lea     si, Str1
                lea     di, Str2

; Get the minimum length of the two strings.

                mov     al, Str1
                mov     cl, al
                cmp     al, Str2
                jb      CmpStrs
                mov     cl, Str2

; Compare the two strings.

CmpStrs:        mov     ch, 0
                cld
        repe    cmpsb
                jne     StrsNotEqual

; If CMPS thinks they're equal, compare their lengths 
; just to be sure.

                cmp     al, Str2
StrsNotEqual:
At label StrsNotEqual, the flags will contain all the pertinent information 
about the ranking of these two strings. You can use the
conditional jump instructions to test the result of this comparison.


Art of assembly:
http://webster.cs.ucr.edu/AoA/DOS/ch15/CH15-3.html

take care,
Sina


-----Original Message-----
From: programmingblind-bounce@xxxxxxxxxxxxx 
[mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Littlefield, Tyler
Sent: Monday, January 17, 2011 11:06 AM
To: programmingblind@xxxxxxxxxxxxx
Subject: Re: code optomization:any way to do this better?

On a side note, here's my code cleaned up, for anyone who cares.
section .text
global _strcmp
_strcmp:
enter 0,0
;we copy our arguments to EBX and ECX
mov EBX, [EBP+8]
mov ECX, [EBP+12]
.loop:
;we need one value in a register
mov EDX, [ECX]
;check for null termination
cmp byte [EBX], 0
jne .notnull
;we have a null termination.
;if the other string is null terminated, we jump to success. otherwise 
it fails because they obviously aren't equal.
.null:
cmp byte [ECX], 0
je .success
jne .fail
;byte wasn't null, now we check for null on the other byte.
;if one is null, it's a fail because again they aren't equal. If it is 
not null, we do another check.
.notnull:
cmp byte [ECX], 0
;not equal, we check for equalness between the two now.
je .fail
;we check for equalness between the two bytes here.
.check:
cmp [EBX], EDX
jne .fail
;here we increase pointers and jump back up to the top of the loop.
.next:
inc EBX
inc ECX
jmp .loop
;strings compared fully
.success:
mov EAX,1
jmp .finish
;strings did not compare fully.
.fail:
mov EAX, 0
;code cleanup.
.finish:
leave
ret

On 1/17/2011 9:03 AM, Littlefield, Tyler wrote:
> OK, a couple questions from your message. I can see letting things 
> fall through, and I actually got rid of a jmp before this. My question 
> is how I should handle this without those paths.
> For example, I need to check the two strings, to make sure that there 
> is something there, or that one of the bytes is not null.
> Last, what do you mean by promote the code up one level?
> Thanks,
> On 1/17/2011 8:48 AM, Sina Bahram wrote:
>> You have to do the same thing that I suggested in my last mail with 
>> your fail and success jumps.
>>
>> Here is a rule. Never, ever, ever, never, ever have two orthogonal 
>> jumps beside one another. You will never have a situation where
>> you need more than one jump, because mathematically, this is 
>> equivalent to this:
>>
>> If(true)
>> Do stuff
>> Else if(false)
>> Do stuff
>>
>> That's redundant, right? Because of course the else is false, there's 
>> no need to check it.
>>
>>
>> Also, you need to promote code up a level, if you'd like less code 
>> space bloat.
>>
>> You compare the byte stored at ECX to 0 in both branches of your 
>> jump, then you jump based on that result. Get rid of it from both
>> branches. In fact, just get rid of those branches all together. There 
>> is no point ot them, since  they contain the same code, before
>> they each individually jump to fail, success, notnull, and null, or 
>> whatever the heck the four paths are.
>>
>> So just collapse all these jumps down.
>>
>>
>> Take care,
>> Sina
>>
>> -----Original Message-----
>> From: programmingblind-bounce@xxxxxxxxxxxxx 
>> [mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of 
>> Littlefield, Tyler
>> Sent: Sunday, January 16, 2011 9:56 PM
>> To: programmingblind@xxxxxxxxxxxxx
>> Subject: code optomization:any way to do this better?
>>
>> So I've been playing with assembly a lot lately, and was curious if
>> there was a better way to do this. most importantly, the whole three
>> branched if check (null, not null).
>> section .text
>> global _strcmp
>> _strcmp:
>> enter 0,0
>> ;we copy our arguments to EBX and ECX
>> mov EBX, [EBP+8]
>> mov ECX, [EBP+12]
>> .loop:
>> ;we need one value in a register
>> mov EDX, [ECX]
>> ;check for null termination
>> cmp byte [EBX], 0
>> je .null
>> jne .notnull
>> ;we have a null termination.
>> ;if the other string is null terminated, we jump to success. otherwise
>> it fails because they obviously aren't equal.
>> .null:
>> cmp byte [ECX], 0
>> je .success
>> jne .fail
>> ;byte wasn't null, now we check for null on the other byte.
>> ;if one is null, it's a fail because again they aren't equal. If it is
>> not null, we do another check.
>> .notnull:
>> cmp byte [ECX], 0
>> ;not equal, we check for equalness between the two now.
>> jne .check
>> je .fail
>> ;we check for equalness between the two bytes here.
>> .check:
>> cmp [EBX], EDX
>> je .next
>> jne .fail
>> ;here we increase pointers and jump back up to the top of the loop.
>> .next:
>> inc EBX
>> inc ECX
>> jmp .loop
>> ;strings compared fully
>> .success:
>> mov EAX,1
>> jmp .finish
>> ;strings did not compare fully.
>> .fail:
>> mov EAX, 0
>> ;code cleanup.
>> ;no need for a jmp, it just falls through.
>> .finish:
>> leave
>> ret
>>
>
>


-- 

Thanks,
Ty

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