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.
>
> > On the other hand: even in pre-C++26 modes, compilers have hardening
> > options that fill the storage for automatic variables with certain bit
> > patterns (such as -ftrivial-auto-var-init), and also have
> > compiler-specific attributes to opt-out in case this filling gets too
> > expensive (such as [[gnu::uninitialized]]). For certain use cases it may
> > make sense to disable such filling, and to do so in a centralized manner
> > (by applying the attribute to a class, or a constructor, or a data
> > member, etc., that's open for debate).
>
> The problem is that EB (and thus automatic initialization of automatic
> objects) will become the default in C++26, and I want a
> user-transparent way to get out of this default behaviour for certain
> data members (only). Yup, making the attribute applicable to
> non-static data members is exactly what I want.
>

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`.


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-- 
*Brian Bi*