sub eax,0 - does it do anything?

Question

I just opened a file in IDA Pro and I found some code that looks completely useless. However, I thought it might have some use. Doesn't the sub eax,0 just subtract 0 from eax?

The code:

hinstDLL= dword ptr  4  
fdwReason= dword ptr  8  
lpReserved= dword ptr  0Ch  

mov     eax, [esp+fdwReason]  
sub     eax, 0  
jz      short loc_10001038  

Solution 1

The sub instruction sets flags (OF, SF, ZF, AF, PF, and CF, according to the documentation) - the mov instruction does not. The jz will jump only if the zero flag (ZF) is set, so if you want to jump based on the value in eax that flag has to be set appropriately.

Solution 2

The sub instruction will set the zero flag if its result is zero. In this case this means that the zero flag will be set if eax is zero.

So these three instructions check if [esp+fdwReason] is zero and jump to loc_10001038 in that case.

Comments

[Filip Haglund]

I thought you needed to use the TEST instruction for that. Thanks!

[Carl Norum]

test does do an implicit bitwise AND, but doesn't affect all of the same flags. From the test docs: "The OF and CF flags are set to 0. The SF, ZF, and PF flags are set according to the result... The state of the AF flag is undefined."