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Index > Main > lea with if statement?

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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
Code:
cmp eax,ebx
mov  eax,ecx
jb   @f
dec       eax
sub      edx,eax
@@:    


What is the equivalent using an if statement? I can't figure it out. Embarassed


The closest I have come up with generates
Code:
cmp     eax,ebx
jb   @f
mov       eax,ecx
dec  eax
sub      edx,eax
jmp  blahblah
@@:
mov      eax,ecx
blahblah:    



Please help.. I think if statements could make my code more readable but I don't want it to be bloated..




P.S. these are just examples


Last edited by Azu on 18 Mar 2009, 04:46; edited 1 time in total
Post 18 Mar 2009, 03:59
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
mov eax,ecx also works, it also doesn't change any flags. Plus, t is one byte shorter and has better scheduling than lea
Post 18 Mar 2009, 04:41
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
Thanks, updated the example. Still have the same question, though. Any answers would be appreciated.
Post 18 Mar 2009, 04:46
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
Do you mean how to use the macro ".if"? Or the assembler "if"?

Basically the code you show above looks fine to me. jcc is really all that is available on x86 unless you want to delve into cmovcc.
Post 18 Mar 2009, 04:49
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
What I meant was;


Please post the code using if statements instead of cmp and jb, that generates the code in the first example. I can not figure it out.

For FASM, not jcc.
Post 18 Mar 2009, 04:51
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
Okay, so I think you want to use the ".if" macro set.
Code:
include "if.inc" ;comes with the windows package download
.if eax,b,ebx
  ;do something
.else
  ;do something else
.endif    
I think that is how they work. I've never used them before.
Post 18 Mar 2009, 04:55
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
Azu wrote:
What is the equivalent using an if statement? I can't figure it out. Embarassed


The closest I have come up with generates
Code:
cmp      eax,ebx
jb   @f
mov       eax,ecx
dec  eax
sub      edx,eax
jmp  blahblah
@@:
mov      eax,ecx
blahblah:    


Thought there was some kind of trick to it and I was just doing something wrong. Guess it's just impossible afterall?
Post 18 Mar 2009, 05:00
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
Well, I presume it works like this:
Code:
include "if.inc" ;comes with the windows package download
.if eax,nb,ebx
  mov     eax,ecx
  dec     eax
  sub     edx,eax
.else
  mov     eax,ecx
.endif    
Is that what you are asking?
Post 18 Mar 2009, 05:03
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
BTW: The .if macros just generate what you wrote in the first post. There is no magic, unfortunately.
Post 18 Mar 2009, 05:04
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
I was just wondering if code like
Code:
cmp     eax,ebx
mov     eax,ecx
jb      @f
dec     eax
sub     edx,eax
@@:    
can be generated with if statements (and if so how), or only bloated code like
Code:
cmp     eax,ebx
jb      @f
mov     eax,ecx
dec     eax
sub     edx,eax
jmp     blahblah
@@:
mov     eax,ecx
blahblah:    
Post 18 Mar 2009, 05:05
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17621
Location: In your JS exploiting you and your system
revolution
I see, I am not expert with these macro but as far as I can tell you will only get the bloated version.

I would suggest that if you are interested in making efficient code for things like the above that using .if/.else/.endif structures is not the best way. Manual coding is always going to give at least as good and sometimes better.
Post 18 Mar 2009, 05:11
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
Code:
cmp eax, ebx
mov eax, ecx

.if ~CARRY?
  dec eax
  sub edx, eax
.endif    


Question
Post 18 Mar 2009, 06:20
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Azu



Joined: 16 Dec 2008
Posts: 1159
Azu
Thanks guys Smile
Post 18 Mar 2009, 06:33
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