Reverse Engineering for Beginners

CHAPTER 19. MANIPULATING SPECIFIC BIT(S) CHAPTER 19. MANIPULATING SPECIFIC BIT(S)

pop ebp
retn
f endp

19.5.2 x64

Let’s modify the example slightly to extend it to 64-bit:

#include <stdio.h>
#include <stdint.h>

#define IS_SET(flag, bit) ((flag) & (bit))

int f(uint64_t a)
{
uint64_t i;
int rt=0;

for (i=0; i<64; i++)

if (IS_SET (a, 1ULL<<i))

rt++;

return rt;
};

Non-optimizing GCC 4.8.2

So far so easy.

Listing 19.29: Non-optimizing GCC 4.8.2

f:
push rbp
mov rbp, rsp
mov QWORD PTR [rbp-24], rdi ; a
mov DWORD PTR [rbp-12], 0 ; rt=0
mov QWORD PTR [rbp-8], 0 ; i=0
jmp .L2
.L4:
mov rax, QWORD PTR [rbp-8]
mov rdx, QWORD PTR [rbp-24]
; RAX = i, RDX = a
mov ecx, eax
; ECX = i
shr rdx, cl
; RDX = RDX>>CL = a>>i
mov rax, rdx
; RAX = RDX = a>>i
and eax, 1
; EAX = EAX&1 = (a>>i)&1
test rax, rax
; the last bit is zero?
; skip the next ADD instruction, if it was so.
je .L3
add DWORD PTR [rbp-12], 1 ; rt++
.L3:
add QWORD PTR [rbp-8], 1 ; i++
.L2:
cmp QWORD PTR [rbp-8], 63 ; i<63?
jbe .L4 ; jump to the loop body begin, if so
mov eax, DWORD PTR [rbp-12] ; return rt
pop rbp
ret