Reverse Engineering for Beginners

CHAPTER 12. CONDITIONAL JUMPS CHAPTER 12. CONDITIONAL JUMPS

my_max:
mov edx, DWORD PTR [esp+4]
mov eax, DWORD PTR [esp+8]
; EDX=A
; EAX=B
; compare A and B:
cmp edx, eax
; if A>=B, load A value into EAX
; the instruction idle if otherwise (if A<B)
cmovge eax, edx
ret

my_min:
mov edx, DWORD PTR [esp+4]
mov eax, DWORD PTR [esp+8]
; EDX=A
; EAX=B
; compare A and B:
cmp edx, eax
; if A<=B, load A value into EAX
; the instruction idle if otherwise (if A>B)
cmovle eax, edx
ret

12.4.2 64-bit.

#include <stdint.h>

int64_t my_max(int64_t a, int64_t b)
{
if (a>b)
return a;
else
return b;
};

int64_t my_min(int64_t a, int64_t b)
{
if (a<b)
return a;
else
return b;
};

There is some unneeded value shuffling, but the code is comprehensible:

Listing 12.30: Non-optimizing GCC 4.9.1 ARM64

my_max:
sub sp, sp, #16
str x0, [sp,8]
str x1, [sp]
ldr x1, [sp,8]
ldr x0, [sp]
cmp x1, x0
ble .L2
ldr x0, [sp,8]
b .L3
.L2:
ldr x0, [sp]
.L3:
add sp, sp, 16
ret

my_min:
sub sp, sp, #16
str x0, [sp,8]
str x1, [sp]