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Groups > comp.lang.c++ > #69720
| From | Bo Persson <bo@bo-persson.se> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: std::atomic<T>::is_lock_free() |
| Date | 2019-11-12 16:46 +0100 |
| Message-ID | <h302fhFs1t7U1@mid.individual.net> (permalink) |
| References | <qqdvha$nka$1@news.albasani.net> <qqe180$jhd$1@news.albasani.net> <qqeb20$asj$1@news.albasani.net> <qqecn5$o7a$1@news.albasani.net> |
On 2019-11-12 at 14:39, Bonita Montero wrote:
>>>> Can anyone tell me whether std::atomic<T>::is_lock_free() isn't static
>>>> as well as constexpr? Having it non-static as well as non-constexpr
>>>> doesn't make sense for me.
>
>>> Someone on Stack Oveflow told me this: an atomic value might be un-
>>> aligned so that access might be not atomic. So the compiler can't say
>>> at compile-time whether the access can be atomic. But there's a static
>>> constexpr bool called std::atomic<T> ::is_always_lock_free.
>
>> On the other side I can't believe this because the language says that
>> unaligned accesses have undefined behaviour; so there must be another
>> reason for the distinction between is_lock_free and is_always_lock_free.
>
> Although I was pretty confident that C++ doesn't make the decision
> between is_lock_free and is_always_lock_free because of the alignement
> I wrote a little program to check this. I've got installed Visual Studio
> with compilers for the Windows-platforms for ARMv8-CPUs. And ARMv8 is
> able to do unaligned memory-accesses but compare & exchange etc. is
> mandated to be aligned. So I thought that when I call is_lock_free on
> a pointer to an atomic is_lock_free wouldn't simply return true of for
> each atomic but only if the atomic is properly aligned. So I wrote a
> little function to check this:
>
> #include <atomic>
> #include <cstddef>
> bool isLockFreeAtomic( std::atomic<std::uint64_t> *a64 )
> {
> return a64->is_lock_free();
> }
>
> When I compile this with assembly-output I get the following code:
>
> |?isLockFreeAtomic@@YA_NPAU?$atomic@_K@std@@@Z| PROC
> movs r0,#1
> |$M12|
> bx lr
> ENDP
>
> So it's like I expected: the runtime-lib doesn't check the pointer.
> So there must be another reason why there's a decision between
> is_lock_free and is_always_lock_free.
It might just be for historic reasons. In C++14 we only have the
function and it is speced:
Returns: True if the object’s operations are lock-free, false otherwise.
It said nothing about whether different objects of the same type always
returned the same value.
In C++17 we also got is_always_lock_free, where *always* is important,
and the function is now supposed to return the same value (says a
non-normative comment...).
Bo Persson
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std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 10:54 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 11:24 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:11 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 14:39 +0100
Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 16:46 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-12 17:40 +0100
Re: std::atomic<T>::is_lock_free() Bo Persson <bo@bo-persson.se> - 2019-11-12 20:23 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:21 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:09 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-13 06:34 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:00 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:26 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 22:33 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:51 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 07:50 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-13 23:14 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 10:43 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-12 15:14 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 13:11 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 07:53 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 16:57 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:16 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:26 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:28 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 08:30 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 17:33 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:41 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:31 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 12:59 -0800
Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-15 21:47 +0000
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:56 -0800
Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 18:07 +0000
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 13:55 -0800
Re: std::atomic<T>::is_lock_free() scott@slp53.sl.home (Scott Lurndal) - 2019-11-16 22:13 +0000
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-16 18:40 -0800
Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-17 18:39 -0500
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 20:59 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:00 -0800
Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-18 08:25 -0500
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-18 18:24 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:59 -0800
Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-16 11:08 +1300
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-15 15:39 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 07:22 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:06 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 17:13 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 08:26 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:27 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 10:37 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-16 19:47 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-16 11:34 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 09:17 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 01:52 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 11:23 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 03:22 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 14:20 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 06:39 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:49 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:54 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:03 +0100
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:39 -0800
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:33 -0800
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-10 22:48 -0800
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-11 00:24 -0800
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-14 07:16 -0800
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2020-01-14 08:55 -0800
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-21 02:01 -0800
Re: std::atomic<T>::is_lock_free() James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-21 06:00 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-17 15:58 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-17 21:01 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:00 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 22:45 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 07:52 +0100
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-18 08:10 +0100
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:23 -0800
Re: std::atomic<T>::is_lock_free() Ian Collins <ian-news@hotmail.com> - 2019-11-18 20:26 +1300
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-17 23:05 -0800
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-11-28 15:44 -0800
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-28 16:43 -0800
Re: std::atomic<T>::is_lock_free() Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-11 06:41 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:33 -0800
Re: std::atomic<T>::is_lock_free() Melzzzzz <Melzzzzz@zzzzz.com> - 2019-11-14 19:08 +0000
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 20:12 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:35 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:32 -0800
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-14 13:37 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-15 09:32 +0100
Re: std::atomic<T>::is_lock_free() "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-11-15 13:06 -0800
Re: std::atomic<T>::is_lock_free() Öö Tiib <ootiib@hot.ee> - 2019-11-14 02:29 -0800
Re: std::atomic<T>::is_lock_free() Bonita Montero <Bonita.Montero@gmail.com> - 2019-11-14 11:45 +0100
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