[cfe-commits] C11 <stdatomic.h>

Tijl Coosemans tijl at coosemans.org
Tue Oct 9 10:55:22 PDT 2012

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));
>>>> 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

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

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