Reverse Engineering for Beginners

CHAPTER 14. LOOPS CHAPTER 14. LOOPS

$LN2@main:
mov eax, DWORD PTR _i$[ebp] ; here is what we do after each iteration:
add eax, 1 ; add 1 to (i) value
mov DWORD PTR _i$[ebp], eax
$LN3@main:
cmp DWORD PTR _i$[ebp], 10 ; this condition is checked before each iteration
jge SHORT $LN1@main ; if (i) is biggest or equals to 10, lets finish loop'
mov ecx, DWORD PTR _i$[ebp] ; loop body: call printing_function(i)
push ecx
call _printing_function
add esp, 4
jmp SHORT $LN2@main ; jump to loop begin
$LN1@main: ; loop end
xor eax, eax
mov esp, ebp
pop ebp
ret 0
_main ENDP

As we see, nothing special.

GCC 4.4.1 emits almost the same code, with one subtle difference:

Listing 14.2: GCC 4.4.1

main proc near

var_20 = dword ptr -20h
var_4 = dword ptr -4

push ebp

mov ebp, esp

and esp, 0FFFFFFF0h

sub esp, 20h

mov [esp+20h+var_4], 2 ; (i) initializing

jmp short loc_8048476

loc_8048465:
mov eax, [esp+20h+var_4]
mov [esp+20h+var_20], eax
call printing_function
add [esp+20h+var_4], 1 ; (i) increment

loc_8048476:
cmp [esp+20h+var_4], 9
jle short loc_8048465 ; if i<=9, continue loop
mov eax, 0
leave
retn
main endp

Now let’s see what we get with optimization turned on (/Ox):

Listing 14.3: Optimizing MSVC

_main PROC
push esi
mov esi, 2
$LL3@main:
push esi
call _printing_function
inc esi
add esp, 4
cmp esi, 10 ; 0000000aH
jl SHORT $LL3@main
xor eax, eax
pop esi
ret 0
_main ENDP