nginx/src/os/unix/ngx_gcc_atomic_x86.h

/*
 * Copyright (C) Igor Sysoev
 */


#if (NGX_SMP)
#define NGX_SMP_LOCK  "lock;"
#else
#define NGX_SMP_LOCK
#endif


/*
 * "cmpxchgl  r, [m]":
 *
 *     if (eax == [m]) {
 *         zf = 1;
 *         [m] = r;
 *     } else {
 *         zf = 0;
 *         eax = [m];
 *     }
 *
 * 
 * The "q" is any of the %eax, %ebx, %ecx, or %edx registers.
 * The "=a" and "a" are the %eax register.  Although we can return result
 * in any register, we use %eax because it is used in cmpxchgl anyway.
 * The "cc" means that flags were changed.
 */

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)
{
    ngx_atomic_uint_t  res;

    __asm__ volatile (

         NGX_SMP_LOCK
    "    cmpxchgl  %3, %1;   "
    "    setz      %b0;      "
    "    movzbl    %b0, %0;  "

    : "=a" (res) : "m" (*lock), "a" (old), "q" (set) : "cc", "memory");

    return res;
}


/*
 * "xaddl  r, [m]":
 *
 *     temp = [m];
 *     [m] += r;
 *     r = temp;
 *
 *
 * The "+q" is any of the %eax, %ebx, %ecx, or %edx registers.
 * The "cc" means that flags were changed.
 */


#if !(__GNUC__ == 2 && __GNUC_MINOR__ <= 7)

static ngx_inline ngx_atomic_int_t
ngx_atomic_fetch_add(ngx_atomic_t *value, ngx_atomic_int_t add)
{
    __asm__ volatile (

         NGX_SMP_LOCK
    "    xaddl  %0, %1;   "

    : "+q" (add) : "m" (*value) : "cc", "memory");

    return add;
}


#else /* (__GNUC__ == 2 && __GNUC_MINOR__ <= 7) */

/*
 * gcc 2.7 does not support "+q", so we have to use the fixed %eax ("=a" and
 * "a") and this adds two superfluous instructions in the end of code,
 * something like this: "mov %eax, %edx / mov %edx, %eax".
 */

static ngx_inline ngx_atomic_int_t
ngx_atomic_fetch_add(ngx_atomic_t *value, ngx_atomic_int_t add)
{
    ngx_atomic_uint_t  old;

    __asm__ volatile (

         NGX_SMP_LOCK
    "    xaddl  %2, %1;   "

    : "=a" (old) : "m" (*value), "a" (add) : "cc", "memory");

    return old;
}

#endif