On 09-10-2012 19:27, Jeffrey Yasskin wrote: > On Tue, Oct 9, 2012 at 9:55 AM, Tijl Coosemans <[email protected]> wrote: >> On 08-10-2012 01:34, Jeffrey Yasskin wrote: >>> On Sun, Oct 7, 2012 at 3:42 PM, Tijl Coosemans <[email protected]> wrote: >>>> On 07-10-2012 20:53, Richard Smith wrote: >>>>> On Sun, Oct 7, 2012 at 10:53 AM, Tijl Coosemans <[email protected]> >>>>> wrote: >>>>> >>>>>> On 05-10-2012 20:36, Jeffrey Yasskin wrote: >>>>>>> On Fri, Oct 5, 2012 at 11:27 AM, Tijl Coosemans <[email protected]> >>>>>>> wrote: >>>>>>>> On 04-10-2012 23:04, Richard Smith wrote: >>>>>>>>> On Thu, Oct 4, 2012 at 12:23 PM, Richard Smith <[email protected]> >>>>>>>>> wrote: >>>>>>>>>> On Thu, Oct 4, 2012 at 5:18 AM, Tijl Coosemans <[email protected]> >>>>>>>>>> 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 About the performance reason, I think it's safe to assume that on most if not all architectures non-atomic is faster than atomic. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
