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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


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