CODEWITHSUNDEEP
cassemblyx86
DriverBoy
DriverBoy

Reputation: 997

How do you interpret this assembly?

Extracted from nginx:

static ngx_inline ngx_atomic_uint_t
ngx_atomic_cmp_set(ngx_atomic_t *lock, ngx_atomic_uint_t old,
ngx_atomic_uint_t set)
{
    u_char res;
    __asm__ volatile (
    NGX_SMP_LOCK
    " cmpxchgl %3, %1; "
    " sete %0; "
    : "=a" (res) : "m" (*lock), "a" (old), "r" (set) : "cc", "memory");
    return res;
}

I don't understand the syntax the assembly instruction are combined(it's using a different syntax than printf uses),what is it doing at all?

Upvotes: 2

Views: 196

Answers (1)

elder_george
elder_george

Reputation: 7879

Given this and ignoring operations atomicity, function is equivalent to:

 static ngx_inline ngx_atomic_uint_t
 ngx_atomic_cmp_set(ngx_atomic_t *lock, ngx_atomic_uint_t old,
 ngx_atomic_uint_t set)
 {
     u_char res;
     if (*lock == old){
         *lock = set;
         res = 1;
     } else{
         res = *lock
     }
     return res;
  }

Upvotes: 2

Related Questions