Reverse Engineering for Beginners

CHAPTER 21. STRUCTURES CHAPTER 21. STRUCTURES

mov ecx, DWORD PTR _t$[ebp]

and ecx, -2139095041 ; 807fffffH - drop exponent

; add original value without exponent with new calculated exponent
or ecx, eax
mov DWORD PTR _t$[ebp], ecx

push 4

lea edx, DWORD PTR _t$[ebp]

push edx

lea eax, DWORD PTR _f$[ebp]

push eax

call _memcpy

add esp, 12

fld DWORD PTR _f$[ebp]

mov esp, ebp
pop ebp
ret 0
?f@@YAMM@Z ENDP ; f

A bit redundant. If it was compiled with/Oxflag there would be nomemcpy()call, thefvariable is used directly. But it is
easier to understand by looking at the unoptimized version.

What would GCC 4.4.1 with-O3do?

Listing 21.28: Optimizing GCC 4.4.1

; f(float)
public _Z1ff
_Z1ff proc near

var_4 = dword ptr -4
arg_0 = dword ptr 8

push ebp
mov ebp, esp
sub esp, 4
mov eax, [ebp+arg_0]
or eax, 80000000h ; set minus sign
mov edx, eax
and eax, 807FFFFFh ; leave only significand and exponent in EAX
shr edx, 23 ; prepare exponent
add edx, 2 ; add 2
movzx edx, dl ; clear all bits except 7:0 in EAX
shl edx, 23 ; shift new calculated exponent to its place
or eax, edx ; add new exponent and original value without exponent
mov [ebp+var_4], eax
fld [ebp+var_4]
leave
retn
_Z1ff endp

public main
main proc near
push ebp
mov ebp, esp
and esp, 0FFFFFFF0h
sub esp, 10h
fld ds:dword_8048614 ; -4.936
fstp qword ptr [esp+8]
mov dword ptr [esp+4], offset asc_8048610 ; "%f\n"
mov dword ptr [esp], 1
call ___printf_chk
xor eax, eax
leave
retn
main endp