complex IF statement in assembly

Question

How should I write such an if statement in assembly?

if ((a == b AND a > c) OR c == b) { ...

Platform: Intel 32-bit machine

Answer 1

You will need to break the if statement into a series of comparisons and jumps. In the same way that in C you could write:

int test = 0;

if (a == b) {
  if (a > c) {
    test = 1;
  }
}

// assuming lazy evaluation of or:
if (!test) {
  if (c == b) {
    test = 1;
  }
}

if (test) {
  // whole condition checked out
}

Which breaks the complex expression into constituent parts your asm will do likewise, although you can write it more cleanly in asm by jumping to the parts that are still relevant.

Assuming a, b and c are being passed to you on the stack (if they're not load them from elsewhere obviously)

        mov     eax, DWORD PTR [ebp+8] 
        cmp     eax, DWORD PTR [ebp+12] ; a == b?
        jne     .SECOND                 ; if it's not then no point trying a > c 
        mov     eax, DWORD PTR [ebp+8]
        cmp     eax, DWORD PTR [ebp+16] ; a > c?
        jg      .BODY                   ; if it is then it's sufficient to pass the
.SECOND:
        mov     eax, DWORD PTR [ebp+16]
        cmp     eax, DWORD PTR [ebp+12] ; second part of condition: c == b?
        jne     .SKIP
.BODY:
        ; .... do stuff here
        jmp     .DONE
.SKIP:
        ; this is your else if you have one
.DONE: