Reverse Engineering Classes in ARM

Question

I came across this part in the book Reverse Engineering For Beginners book by Denis Yurichev. It writes about reverse engineering Classes in C++, but it doesn't provide any examples in ARM.

Page 546

_this$ = -4
; size = 4
??0c@@QAE@XZ PROC ; c::c, COMDAT
; _this$ = ecx
push ebp
mov ebp, esp
push ecx
mov DWORD PTR _this$[ebp], ecx
mov eax, DWORD PTR _this$[ebp]
mov DWORD PTR [eax], 667
mov ecx, DWORD PTR _this$[ebp]
mov DWORD PTR [ecx+4], 999
mov eax, DWORD PTR _this$[ebp]
mov esp, ebp
pop ebp
ret 0
??0c@@QAE@XZ ENDP ; c::c
_this$ = -4 ; size = 4
_a$ = 8
; size = 4
_b$ = 12
; size = 4
??0c@@QAE@HH@Z PROC ; c::c, COMDAT
; _this$ = ecx
push ebp
mov ebp, esp
push ecx
mov DWORD PTR _this$[ebp], ecx
mov eax, DWORD PTR _this$[ebp]
mov ecx, DWORD PTR _a$[ebp]
mov DWORD PTR [eax], ecx
mov edx, DWORD PTR _this$[ebp]
mov eax, DWORD PTR _b$[ebp]
mov DWORD PTR [edx+4], eax
mov eax, DWORD PTR _this$[ebp]
mov esp, ebp
pop ebp
ret 8
??0c@@QAE@HH@Z ENDP ; c::c

As you can see ECX is used to hold the pointer of this for accessing the members of the class. How is this done in ARM assembly?

Answer 1

Because it is compiler specific , it is not required to be internally the same always. It seems though that CLANG and GCC use R0 to store this.

C++

class MyClass {
  int field_;
 public:
  void set_field() { this->field_ = 42; }
};

void test() {
  MyClass x;
  x.set_field();
}

ARM:

test():
        push    {r11, lr}
        mov     r11, sp
        sub     sp, sp, #8
        add     r0, sp, #4
        bl      MyClass::set_field()
        mov     sp, r11
        pop     {r11, lr}
        bx      lr
MyClass::set_field():
        sub     sp, sp, #4
        str     r0, [sp]
        ldr     r1, [sp]
        mov     r0, #42
        str     r0, [r1]
        add     sp, sp, #4
        bx      lr

Intel :

MyClass::set_field():
        push    rbp
        mov     rbp, rsp
        mov     QWORD PTR [rbp-8], rdi
        mov     rax, QWORD PTR [rbp-8]
        mov     DWORD PTR [rax], 42
        nop
        pop     rbp
        ret
test():
        push    rbp
        mov     rbp, rsp
        sub     rsp, 16
        lea     rax, [rbp-4]
        mov     rdi, rax
        call    MyClass::set_field()
        nop
        leave
        ret