I want to use following function to implement atomic exchange
operation,

t <- *lock; //store *lock into temp variable t
a -> *lock; // store input param a into *lock
return t; // return t


after objdump the object file, I was confused by the disassembler
code.

source code and disassembler code are listed following:


//source code
static int atomic_lock(int a, int* lock)
{
int t;

__asm__ __volatile__(
"1: lwarx %0,0,%2 \n\
stwcx. %3,0,%2 \n\
bne- 1b"
: "=&r" (t), "=m" (*lock)
: "r" (lock),"r" (a), "m" (*lock)
: "cc", "memory");

return t;
}


//disassembler code by objdump

00003884 <_Z11atomic_lockiPi>:
_Z11atomic_lockiPi():
3884: 94 21 ff e0 stwu r1,-32(r1)
3888: 7c 00 20 28 lwarx r0,r0,r4
388c: 7c 60 21 2d stwcx. r3,r0,r4
3890: 40 a2 ff f8 bne- 3888 <_Z11atomic_lockiPi+0x4
3894: 7c 03 03 78 mr r3,r0
3898: 38 21 00 20 addi r1,r1,32
389c: 4e 80 00 20 blr

In line-388c of disassembler code,
stwcx. r3,r0,r4

r0 is not zero, because r0 was modified at line-3338 "lwarx
r0,r0,r4",
and value of *lock is stored into r0.

so in line-388c "stwcx. r3,r0,r4" , this statement will store r3
into (r0)+(r4),

it does not the expected address: 0+(r4)


Is that right?