Re: [boost] Notice: Boost.Atomic (atomic operations library)

Subject: Re: [boost] Notice: Boost.Atomic (atomic operations library)
From: Helge Bahmann (hcb_at_[hidden])
Date: 2009-12-04 12:27:31

Am Friday 04 December 2009 18:06:31 schrieb Phil Endecott:
> Hi Helge,
>
> In your x86 code you have:
>
> bool compare_exchange_strong(T &e, T d, memory_order
> order=memory_order_seq_cst) volatile
> {
>      T prev=e;
>      __asm__ __volatile__("lock cmpxchgb %1, %2\n" : "=a" (prev) : "q"
> (d), "m" (i), "a" (e) : "memory");
>      bool success=(prev==e);
>      e=prev;
>      return success;
> }
>
>
> Can you explain why 'e' is a reference and why you assign back to it?

the standard requires the "found" value to be passed back, so you don't need 
to perform another "load" on retrying the operation

> Maybe it would help if you could write out what that asm does in
> pseudo-code.  The kernel_cmpxchg that I have does:

there is a pseudo-code explanation in the boost::atomic class documentation

Helge