[cfe-commits] C11 <stdatomic.h>

Jeffrey Yasskin jyasskin at googlers.com
Tue Oct 9 11:17:13 PDT 2012

On Tue, Oct 9, 2012 at 10:55 AM, Tijl Coosemans <tijl at coosemans.org> wrote:
> On 09-10-2012 19:27, Jeffrey Yasskin wrote:
>> On Tue, Oct 9, 2012 at 9:55 AM, Tijl Coosemans <tijl at coosemans.org> wrote:
>>> On 08-10-2012 01:34, Jeffrey Yasskin wrote:
>>>> On Sun, Oct 7, 2012 at 3:42 PM, Tijl Coosemans <tijl at coosemans.org> wrote:
>>>>> On 07-10-2012 20:53, Richard Smith wrote:
>>>>>> On Sun, Oct 7, 2012 at 10:53 AM, Tijl Coosemans <tijl at coosemans.org> wrote:
>>>>>>> On 05-10-2012 20:36, Jeffrey Yasskin wrote:
>>>>>>>> On Fri, Oct 5, 2012 at 11:27 AM, Tijl Coosemans <tijl at coosemans.org>
>>>>>>>> wrote:
>>>>>>>>> On 04-10-2012 23:04, Richard Smith wrote:
>>>>>>>>>> On Thu, Oct 4, 2012 at 12:23 PM, Richard Smith <richard at metafoo.co.uk>
>>>>>>>>>> wrote:
>>>>>>>>>>> On Thu, Oct 4, 2012 at 5:18 AM, Tijl Coosemans <tijl at coosemans.org>
>>>>>>>>>>> wrote:
>>>>>>>>>>>>>> The patch implements atomic_flag on top of
>>>>>>>>>>>>>> atomic_bool, but that means atomic_flag f =
>>>>>>>>>>>>>> ATOMIC_FLAG_INIT is an atomic store.
>>>>>>>>>>>>> Not true. There is no need for the initialization of an
>>>>>>>>>>>>> _Atomic variable to use an atomic write, and the code Clang
>>>>>>>>>>>>> emits does not perform one.
>>>>>>>>>>>> Ok, but reinitialisation like f = ATOMIC_FLAG_INIT then.
>>>>>>>>>>> As far as I can see, that is not a valid use of ATOMIC_FLAG_INIT.
>>>>>>>>> I think it's valid, because the other atomic types can be
>>>>>>>>> reinitialised using atomic_init and there's no such function
>>>>>>>>> for atomic_flag.
>>>>>>>> That's a feature request for the C or C++ standard, not something
>>>>>>>> clang should implement on its own. Remember that Richard is
>>>>>>>> implementing a spec that's already written, not trying to invent what
>>>>>>>> might be useful.
>>>>>>> Maybe I shouldn't have used the word reinitialisation. It isn't
>>>>>>> something special. It's what you do when you need to reset some
>>>>>>> state to recover from an error, e.g. in a device driver if the
>>>>>>> device crashes you reset the device and reinitialise any state
>>>>>>> kept by the driver. For normal types you use simple assignment
>>>>>>> for that, for _Atomic types you can use atomic_init and for
>>>>>>> atomic_flag (which is not an atomic type) you should be able to
>>>>>>> assign ATOMIC_FLAG_INIT.
>>>>>> 'should' here sounds like your own opinion. Can you point to somewhere in
>>>>>> the C11 standard which justifies this? Why not just use atomic_clear with
>>>>>> memory_order_relaxed?
>>>>> Well you should be able to do it because there's no alternative.
>>>>> atomic_clear performs an atomic operation and initialisation
>>>>> shouldn't require atomicity.
>>>>> Perhaps a better example is:
>>>>> atomic_flag *f = malloc(sizeof(*f));
>>>>> *f = ATOMIC_FLAG_INIT;
>>>>> If assigning ATOMIC_FLAG_INIT isn't valid you cannot initialise this
>>>>> flag at all.
>>>> 7.17.8 (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf)
>>>> says, "An atomic_flag that is not explicitly initialized with
>>>> ATOMIC_FLAG_INIT is initially in an indeterminate state." That is,
>>>> it's either set or clear, not undefined, so you can put it into a
>>>> known state by calling atomic_flag_clear().
>>>> That does mean that atomic_flag needs to be known to the compiler
>>>> since it's the only type (or one of very few) that doesn't cause
>>>> undefined behavior when it's uninitialized.
>>> Indeterminate means set, clear or trap representation.
>>> 3.19.2 indeterminate value: either unspecified or a trap representation
>>> 3.19.3 unspecified value: any valid value may be chosen in every instance
>>> 3.19.4 trap representation: a value that need not be valid for this type
>>> 3.19.5 perform a trap: interrupt execution of program such that no further
>>> operations are performed. Note that fetching a trap representation might
>>> perform a trap but is not required to.
>>> So the question is if atomic_flag_clear is guaranteed to work with a
>>> flag in an invalid state. I think hardware support for this type is
>>> allowed to assume the flag is in a valid state before any atomic
>>> operations are used on it. But even if it does work, initialisation
>>> doesn't require atomicity and shouldn't for performance reasons.
>> Oops. C is different from C++ here, and I didn't double-check before
>> posting. C++ says, "The macro ATOMIC_FLAG_INIT shall be defined in
>> such a way that it can be used to initialize an object of type
>> atomic_flag to the clear state. For a static-duration object, that
>> initialization shall be static. It is unspecified whether an
>> unitialized atomic_flag object has an initial state of set or clear."
>> I think you have found a C11 defect here, but again, you should bring
>> that up with the C committee, not just clang.
>> Note that "performance reasons" are really unconvincing unless you
>> come with a benchmark.
> It seems more like a defect in C++11. C11 had the same wording but
> they changed it into indeterminate, which makes sense because an
> uninitialised byte has more values than set and clear. It looks like
> the C++ committee wanted to adopt this but forgot about it?
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1421.pdf item 2.2
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1379.htm

So file it as a C++ defect. In neither case is clang the right place
to make a decision about it.

> About the performance reason, I think it's safe to assume that on
> most if not all architectures non-atomic is faster than atomic.

It is not safe to assume that. On x86 and ARM, a relaxed atomic store
is exactly the same instruction as a non-atomic store. The difference
is in the allowed compiler optimizations, which may or may not apply
to atomic_flag initialization in actual use. (In general, it is never
safe to assume anything about performance. It is subtle and thinks you
taste good with ketchup. ;)


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