BITS 32
GLOBAL _gcdAsm
SECTION .text
_gcdAsm:
push ebp
mov ebp,esp
push ebx
push ecx
push edx
push edi
mov eax,[ebp + 8]   ; eax = a (0 <= a <= 2^31 - 1)
mov ebx,[ebp + 12]  ; ebx = b (0 <= b <= 2^31 - 1)
mov ecx,eax
or ecx,ebx
bsf ecx,ecx         ; greatest common power of 2 of a and b
jnz notBoth0
mov eax,1           ; if a = 0 and b = 0, return 1
jmp done
notBoth0:
mov edi,ecx
test eax,eax
jnz aNot0
mov eax,ebx         ; if a = 0, return b
jmp done
aNot0:
test ebx,ebx
jz done             ; if b = 0, return a
bsf ecx,eax         ; "simplify" a as much as possible
shr eax,cl
bsf ecx,ebx         ; "simplify" b as much as possible
shr ebx,cl
mainLoop:
mov ecx,ebx
sub ecx,eax         ; b - a
sbb edx,edx         ; edx = 0 if b >= a or -1 if a > b
and ecx,edx         ; ecx = 0 if b >= a or b - a if a > b
add eax,ecx         ; a-new = min(a,b)
sub ebx,ecx         ; b-new = max(a,b)
sub ebx,eax         ; the difference is >= 0
bsf ecx,eax         ; "simplify" as much as possible by 2
shr eax,cl
bsf ecx,ebx         ; "simplify" as much as possible by 2
shr ebx,cl
jnz mainLoop        ; keep looping until ebx = 0
mov ecx,edi         ; shift back with common power of 2
shl eax,cl
done:
pop edi
pop edx
pop ecx
pop ebx
mov esp,ebp
pop ebp
ret                 ; eax = gcd(a,b)