Question

I would like to understand how cmp and je/jg work in assembly. I saw few examples on google but I am still little bit confused. Below I have shown a part of assembly code that I am trying to convert to C language and the corresponding C code. Is it implemented in the right way or do I have a wrong understanding of how cmp works?

cmp    $0x3,%eax
je     A
cmp    $0x3,%eax
jg     B
cmp    $0x1,%eax
je     C


 int func(int x){


  if(x == 3)
    goto A;

  if (x >3)
    goto B;


  if(x == 1)
     goto C;

    A:
    ......

    B:
    ......

    C:
    ......

Answer 1

You understand correctly how cmp and je/jg work, but you have an error in your C code. This line:

if (*x == 1)

should be

if (x == 1)

Here is a pretty good summary of the x86 control flow instructions.

Also, there’s no reason to repeat the cmp instruction for the same values. Once you’ve executed it, you can test the results multiple ways without repeating the comparison. So your assembly code should look like this:

cmp    $0x3,%eax
je     A
jg     B
cmp    $0x1,%eax
je     C