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

  • From: "Sina Bahram" <sbahram@xxxxxxxxx>
  • To: <programmingblind@xxxxxxxxxxxxx>
  • Date: Mon, 17 Jan 2011 11:18:04 -0500

This might be easier to do over skype or something man.

That way we can both have the code in front of us, and you can line through it 
as I'm talking?

I got some time today, if you want.

Take care,
Sina

-----Original Message-----
From: programmingblind-bounce@xxxxxxxxxxxxx 
[mailto:programmingblind-bounce@xxxxxxxxxxxxx] On Behalf Of Littlefield, Tyler
Sent: Monday, January 17, 2011 11:04 AM
To: programmingblind@xxxxxxxxxxxxx
Subject: Re: code optomization:any way to do this better?

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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