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


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.


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