Reversing : The Hacker's Guide to Reverse Engineering

Here is the same code with a slight modification:

mov eax, DWORD PTR [c]

mov ecx, DWORD PTR [array]

LoopStart:

cmp DWORD PTR [ecx+eax*4], 0

jne NextCycle

mov DWORD PTR [ecx+eax*4], eax

NextCycle:

add eax, 1

cmp eax, 1000

jl SHORT LoopStart

The only difference here is that NextCycleis now placed earlier, before the

counter-incrementing code. This means that unlike before, the continue

statement will increment the counter andrun the loop’s logic. This indicates

that the loop was probably implemented using the forkeyword. Another

way of implementing this type of sequence without using a forloop is by

using a whileor do...whileloop and incrementing the counter insidethe

conditional statement, using the ++operator. In this case, the logical statement

would look like this:

do { ... } while (++c < 1000);

Loop Unrolling

Loop unrolling is a code-shaping level optimization that is not CPU- or

instruction-set-specific, which means that it is essentially a restructuring of the

high-level code aimed at producing more efficient machine code. The follow-

ing is an assembly language example of a partially unrolled loop:

xor ecx,ecx

pop ebx

lea ecx,[ecx]

LoopStart:

mov edx,dword ptr [esp+ecx*4+8]

add edx,dword ptr [esp+ecx*4+4]

add ecx,3

add edx,dword ptr [esp+ecx*4-0Ch]

add eax,edx

cmp ecx,3E7h

jl LoopStart

This loop is clearly a partially unrolled loop, and the best indicator that this

is the case is the fact that the counter is incremented by three in each iteration.

Essentially what the compiler has done is it duplicated the loop’s body three

508 Appendix A

21_574817 appa.qxd 3/16/05 8:54 PM Page 508