AND vs OR vs TEST

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salefrais

Joined: 19 Dec 2006
Posts: 4

salefrais 19 Dec 2006, 20:11

Hello! I've seen many uses of "and", "or", and "test" instructions before "JMP" instructions (sorry about that, I knew something seemed wrong here!).
Example:

Code:

invoke MessageBox, HWND_DESKTOP, lpSomeText, lpTitle, MB_YESNO
and eax, eax ; or eax, eax ; test eax, eax
jnz some_label

Which is better to use? Is there a better instruction?
Thanks in advance.

Last edited by salefrais on 21 Dec 2006, 02:33; edited 2 times in total

19 Dec 2006, 20:11

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 19 Dec 2006, 21:03

we should also take CMP ("proper" way...) into account

19 Dec 2006, 21:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 19 Dec 2006, 23:03

If your goal is optimisation: The most optimal instruction is TEST. This is because there is no write back register resource allocated. With AND and OR each require a register to be written back. CMP requires more bytes to encode and thus pollutes the cache.

If your goal is something else: Use whichever instruction you are most comfortable with.

Last edited by revolution on 19 Dec 2006, 23:09; edited 1 time in total

19 Dec 2006, 23:03

asmfan

Joined: 11 Aug 2006
Posts: 392
Location: Russian

asmfan 19 Dec 2006, 23:05

some values are simplier and better tested with bitwise-operations -> *test* is considered the best way to test on zero val, also if we need to know is there some submask in val we cannot just use cmp. Wast variety of methods there are -> test on (-1= 0F...Fh) =

Code:

inc val
jz @F

etc...

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19 Dec 2006, 23:05

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 19 Dec 2006, 23:14

wait... i missed "before cmp" part. In such case, AFAIK, the instruction does nothing.

19 Dec 2006, 23:14

salefrais

Joined: 19 Dec 2006
Posts: 4

salefrais 20 Dec 2006, 15:28

I normally use "test" anyway with optimization in mind, so it's good to know that I'm not wrong in doing so. Very Happy

Thanks a lot!

20 Dec 2006, 15:28

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