On Tue, Oct 9, 2012 at 10:55 AM, Tijl Coosemans <[email protected]> wrote: > 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
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. ;) Jeffrey _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
