On Apr 8, 2011, at 8:52 PM, Dave Zarzycki wrote:
> Eli,
>
> Thanks for the feedback. I've updated the patch:
>
> <__sync_swap-v2.patch>
>
> For whatever it may be worth, plain atomic swaps are invaluable to Apple's
> modern threading library ("Grand Central Dispatch" a.k.a. libdispatch).
> <http://www.apple.com/macosx/technology/#grandcentral>
Looks good to me with the doc change. I applied it in r129189, thanks Dave!
-Chris
>
> davez
>
>
> On Apr 8, 2011, at 7:38 PM, Eli Friedman wrote:
>
>> On Fri, Apr 8, 2011 at 6:09 PM, Dave Zarzycki <[email protected]> wrote:
>>> Background:
>>>
>>> The __sync_*() atomic intrinsics do not directly expose the fundamental
>>> atomic swap operation that many processors support. One can indirectly get
>>> at the instruction by using __sync_lock_test_and_set(), but the
>>> documentation for that intrinsic does not promise to be an atomic swap on
>>> all architectures or even be a full barrier like the other __sync_*()
>>> intrinsics. This patch fixes this oversight:
>>
>> It's not really an oversight; the __sync_ intrinsics were invented by
>> Intel for Itanium, and the Itanium xchg instruction isn't a full
>> barrier. I don't see any harm in adding this anyway, although I'm not
>> really an expert in lock-free algorithms.
>>
>>> This is my first clang patch, so please be understanding if I made any
>>> newbie mistakes.
>>>
>>> Thanks for considering this change,
>>
>> I don't see anything wrong with the patch itself, but you should add a
>> note in docs/LanguageExtensions.html.
>>
>> -Eli
>
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