Reverse Engineering for Beginners

CHAPTER 22. UNIONS CHAPTER 22. UNIONS

call _printf
add esp, 12
dec esi
jne SHORT $LL3@main
xor eax, eax
pop esi
ret 0
_main ENDP

Function names are so strange here because this example was compiled as C++ and this is name mangling in C++, we will
talk about it later:51.1.1 on page 522.

If we compile this in MSVC 2012, it uses the SIMD instructions for the FPU, read more about it here:27.5 on page 422.

22.1.2 MIPS.

Listing 22.2: Optimizing GCC 4.4.5

float_rand:

var_10 = -0x10
var_4 = -4

lui $gp, (gnu_local_gp >> 16)
addiu $sp, -0x20
la $gp, (gnu_local_gp & 0xFFFF)
sw $ra, 0x20+var_4($sp)
sw $gp, 0x20+var_10($sp)
; call my_rand():
jal my_rand
or $at, $zero ; branch delay slot, NOP
; $v0=32-bit pseudorandom value
li $v1, 0x7FFFFF
; $v1=0x7FFFFF
and $v1, $v0, $v1
; $v1=pseudorandom value & 0x7FFFFF
lui $a0, 0x3F80
; $a0=0x3F800000
or $v1, $a0
; $v1=pseudorandom value & 0x7FFFFF | 0x3F800000
; matter of the following instruction is still hard to get:
lui $v0, ($LC0 >> 16)
; load 1.0 into $f0:
lwc1 $f0, $LC0
; move value from $v1 to coprocessor 1 (into register $f2)
; it behaves like bitwise copy, no conversion done:
mtc1 $v1, $f2
lw $ra, 0x20+var_4($sp)
; subtract 1.0. leave result in $f0:
sub.s $f0, $f2, $f0
jr $ra
addiu $sp, 0x20 ; branch delay slot

main:

var_18 = -0x18
var_10 = -0x10
var_C = -0xC
var_8 = -8
var_4 = -4

lui $gp, (__gnu_local_gp >> 16)

addiu $sp, -0x28

la $gp, (__gnu_local_gp & 0xFFFF)

sw $ra, 0x28+var_4($sp)

sw $s2, 0x28+var_8($sp)

sw $s1, 0x28+var_C($sp)

sw $s0, 0x28+var_10($sp)

sw $gp, 0x28+var_18($sp)