lea with if statement?

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Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 03:59

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

18 Mar 2009, 03:59

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 04:41

mov eax,ecx also works, it also doesn't change any flags. Plus, t is one byte shorter and has better scheduling than lea

18 Mar 2009, 04:41

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 04:46

Thanks, updated the example. Still have the same question, though. Any answers would be appreciated.

18 Mar 2009, 04:46

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 04:49

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.

18 Mar 2009, 04:49

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 04:51

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.

18 Mar 2009, 04:51

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 04:55

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.

18 Mar 2009, 04:55

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 05:00

Azu wrote:

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

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?

18 Mar 2009, 05:00

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 05:03

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?

18 Mar 2009, 05:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 05:04

BTW: The .if macros just generate what you wrote in the first post. There is no magic, unfortunately.

18 Mar 2009, 05:04

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 05:05

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:

18 Mar 2009, 05:05

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 18 Mar 2009, 05:11

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.

18 Mar 2009, 05:11

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 18 Mar 2009, 06:20

Code:

cmp eax, ebx
mov eax, ecx

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

18 Mar 2009, 06:20

Azu

Joined: 16 Dec 2008
Posts: 1159

Azu 18 Mar 2009, 06:33

Thanks guys Smile

18 Mar 2009, 06:33

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