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Subject: Re: [OMPI devel] RFC: add atomic compare-and-swap that returns old value
From: George Bosilca (bosilca_at_[hidden])
Date: 2014-07-30 15:18:44

Why do you want to add new versions? This will lead to having two, almost
identical, sets of atomics that are conceptually equivalent but different
in terms of code. And we will have to maintained both!

I did a similar change in a fork of OPAL in another project but instead of
adding another flavor of atomics, I completely replaced the available ones
with a set returning the old value. I can bring the code over.


On Tue, Jul 29, 2014 at 5:29 PM, Paul Hargrove <phhargrove_at_[hidden]> wrote:

> On Tue, Jul 29, 2014 at 2:10 PM, Nathan Hjelm <hjelmn_at_[hidden]> wrote:
>> Is there a reason why the
>> current implementations of opal atomics (add, cmpset) do not return the
>> old value?
> Because some CPUs don't implement such an atomic instruction?
> On any CPU one *can* certainly synthesize the desired operation with an
> added read before the compare-and-swap to return a value that was present
> at some time before a failed cmpset. That is almost certainly sufficient
> for your purposes. However, the added load makes it (marginally) more
> expensive on some CPUs that only have the native equivalent of gcc's
> __sync_bool_compare_and_swap().
> -Paul
> --
> Paul H. Hargrove PHHargrove_at_[hidden]
> Future Technologies Group
> Computer and Data Sciences Department Tel: +1-510-495-2352
> Lawrence Berkeley National Laboratory Fax: +1-510-486-6900
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