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

Subject: Re: [boost] Notice: Boost.Atomic (atomic operations library)
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2009-12-04 12:06:31

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

if (*ptr == oldval) {
   *ptr = newval;
   return 0;
} else {
   return !0;
}

I think I can just write

bool compare_exchange_strong(T &e, T d, memory_order 
order=memory_order_seq_cst) volatile
{
   return kernel_cmpxchg(e,d,&i) == 0;
}

but the extra stuff in your x86 version makes me suspect there is more 
to it.

Phil.