Comment # 1 on bug 154 from art.08.09@gmail.com

>     asm {"
>       mov  %2, %0;
>       lock;
>       xadd %0, (%1);
> 
>       "
>       : "=r"(retVal)
>       : "r"(counter), "r"(addition)
>       : "memory"; }

> when compiling with -O2 the exchangeAndAdd function assembly is the same but
> it is also inlined into main like so (invalid assembly):
> 
>   406614:       ba 02 00 00 00          mov    $0x2,%edx
>   406619:       48 c7 44 24 08 0a 00    movq   $0xa,0x8(%rsp)
>   406620:       00 00 
>   406622:       48 8d 44 24 08          lea    0x8(%rsp),%rax
>   406627:       48 89 d0                mov    %rdx,%rax
>   40662a:       f0 48 0f c1 00          lock xadd %rax,(%rax)
> 
> This obviously segfaults as rax is garbaged.

That happens because you're modifying the output before using the inputs.
Not a bug; you need to mark the output as an earlyclobber:

ulong /*RAX*/ exchangeAndAdd(ulong * counter /*RSI*/, ulong addition /*RDI*/)
{
    ulong retVal = void;
    asm {"
      mov  %2, %0;
      lock;
      xadd %0, (%1);

      "
      : "=&r"(retVal)
      : "r"(counter), "r"(addition)
      : "memory"; }
    return retVal;
}