Re: Deprecating arithmetic on std::atomic<void*>

[Jonathan Wakely <jwakely@redhat.com>]

On 20/04/17 10:25 +0100, Jonathan Wakely wrote:
>On 20/04/17 11:21 +0200, Jakub Jelinek wrote:
>>On Thu, Apr 20, 2017 at 10:18:09AM +0100, Jonathan Wakely wrote:
>>>On 20/04/17 08:19 +0200, Florian Weimer wrote:
>>>> On 04/19/2017 07:07 PM, Jonathan Wakely wrote:
>>>> > I know it's a bit late, but I'd like to propose deprecating the
>>>> > libstdc++ extension that allows arithmetic on std::atomic<void*>.
>>>> > Currently we make it behave like arithmetic on void*, which is also a
>>>> > GNU extension (https://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html).
>>>> > We also allow arithmetic on types such as std::atomic<void(*)()> which
>>>> > is probably not useful (PR 69769).
>>>>
>>>> Why is it acceptable to have the extension for built-in types, but not
>>>> for library types wrapping them?  Why be inconsistent about this?
>>>
>>>C++17 [atomic.types.pointer] paragraph 4 says:
>>>
>>> Requires: T shall be an object type, otherwise the program is
>>> ill-formed. [Note: Pointer arithmetic on void* or function pointers
>>> is ill-formed. — end note]
>>>
>>>That doesn't give us any leeway to support it.
>>
>>Can't the support or lack thereof depend on -pedantic/-pedantic-errors?
>
>In theory maybe.
>
>>I mean, with -pedantic-errors we already error on void * arighmetics
>>or function pointer arithmetics.  If std::atomic<void*> would use
>>the void * arithmetics, it would also reject it.  Or does it use integer
>>arithmetics instead?
>
>No, it does it on void*, but the __atomic built-ins still perform that
>arithmetic even with -pedantic-errors.


If we can make the built-ins do the right thing then I'd be happy with
that.

However, currently the built-ins treat all pointer arithmetic as void*
(or maybe char*, I'm not sure) i.e. arithmetic on T* ignores
sizeof(T). We already work around this in libstdc++, and PR 64843
points out <stdatomic.h> needs to do the same.