On Tue, 11 Mar 2025 at 23:11, Brian Bi <bbi5291_at_[hidden]> wrote:

> On Tue, Mar 11, 2025 at 10:04 AM Yongwei Wu via Std-Discussion <
> std-discussion_at_[hidden]> wrote:
>
>> On Mon, 10 Mar 2025 at 17:36, Giuseppe D'Angelo via Std-Discussion
>> <std-discussion_at_[hidden]> wrote:
>> >
>> > On 25/02/2025 12:45, Yongwei Wu via Std-Discussion wrote:
>> > > Currently, Obj::scratch_buffer will contain indeterminate values. In
>> > > C++26, will there be a way for it to opt out of the erroneous value
>> > > initialization, sans requiring the user to change the client-side
>> > > code?
>> >
>> > I think there's a misunderstanding here: EB doesn't mandate that the
>> > storage for an automatic variable gets initialized in any specific way.
>> > It just changes the semantics of what happens if you read from an
>> > uninitialized automatic variable (you get EB). The [[indeterminate]]
>> > attribute restores the pre-C++26 semantics (you get UB).
>>
>> The problem is not the avoidance of UB, but the potential harm to
>> performance. And I do not want the user to use the [[indeterminate]]
>> attribute, which is awkward and simply wrong in semantics.
>>
>> > Do you have a use case for always wanting UB instead of EB for
>> > uninitialized reads of a non-static data member?
>>
>> Was my example not good enough? OK, a more real one. But keep in mind
>> I do not want UB, just that I do not want the compiler to initialize
>> some uninitialized data members.
>>
>> I wrote a BigFixedString for test purposes, and google-benchmarked it.
>> Two versions are posted online:
>>
>> https://quick-bench.com/q/9QkL9cSkqHSi2pb5A-TTkglbaDU
>> https://quick-bench.com/q/MmE9IwFaXqjz_gFnEmSK8s3qFrA
>>
>> The difference is that one has a member `char buffer_[128];`, and the
>> other `char buffer_[128]{};`. Of course, the uninitialized data are
>> never read.
>>
>> Currently the benchmark results show the performance difference is
>> about 90:54. Always initializing seems to have a 65% penalty on
>> performance.
>
>
> I'm uncomfortable with the idea of a class author being able to override
> the user's choice to *not* declare a local variable [[indeterminate]], by
> placing [[indeterminate]] on the declaration of a non-static data member
> and thus making part of the object's storage indeterminate anyway. Note
> that even if it's just in a private member, there will still be UB if the
> object is used as the source of `memcpy`.
>

When a user writes `[[indeterminate]]`, it is their intention to exploit
the potential efficiency of not initializing certain storage. When I, as
class author, write `[[indeterminate]]` on data members, it is my intention
(and responsibility) to exploit the efficiency and make sure I do not make
mistakes.

If a reasonable use of my code requires users to write `[[indeterminate]]`,
it is an overburden for users. If they are cautious, they will have to
check the implementation details of my class, which defeats the purpose of
encapsulation.

-- 
Yongwei Wu
URL: http://wyw.dcweb.cn/