2 adjacent pointers swap with CMPXCHG8B instruction

atomicswap.cpp

#include <atomic>

// pointer pack
typedef struct {
  int* p[2];
} pack_t;

void atomic_swap(std::atomic<pack_t>& pack)
{
  pack_t actual{{nullptr, nullptr}};
  pack_t expect = actual;
  while (!pack.compare_exchange_weak(actual, expect)) {
    expect.p[0] = actual.p[1];
    expect.p[1] = actual.p[0];
  }
}

int a, b;
// swap target
std::atomic<pack_t> pack({&a, &b});
atomic_swap(pack);

out_x86.s

; $ gcc -std=c++11 -O2 atomicswap.cpp -S
__Z11atomic_swapRSt6atomicI6pack_tE:
        pushl   %edi
        xorl    %ecx, %ecx
        pushl   %esi
        xorl    %esi, %esi
        pushl   %ebx
        subl    $16, %esp
        movl    32(%esp), %edi
        movl    $0, 8(%esp)
        movl    $0, 12(%esp)
        jmp     L2
        .p2align 4,,7
L3:
        movl    12(%esp), %esi
        movl    8(%esp), %ecx
L2:
        movl    8(%esp), %eax
        movl    %esi, %ebx
        movl    12(%esp), %edx
        lock cmpxchg8b  (%edi)
        movl    %eax, 8(%esp)
        movl    %edx, 12(%esp)
        jne     L3
        addl    $16, %esp
        popl    %ebx
        popl    %esi
        popl    %edi
        ret

CMPXCHG8B--Compare and Exchange 8 Bytes

IF (EDX:EAX == DEST)
ZF = 1
DEST = ECX:EBX
ELSE
ZF = 0
EDX:EAX = DEST