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

From: "Sina Bahram" <sbahram@xxxxxxxxx>
To: <programmingblind@xxxxxxxxxxxxx>
Date: Mon, 17 Jan 2011 10:42:00 -0500

The most obvious optimization is to remove your je.null completely. Just remove 
that line.,

It doesn't do anything. The reason is because if it's not null, it'll jump to 
the .notnull, but if it is null, then it'll fall
through. Remember jne is a conditional jump.

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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References:
- code optomization:any way to do this better?
  - From: Littlefield, Tyler

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

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