Do we even need to spell such permission out? The language doesn't mandate
those calls at all, it just specifies how exceptions transfer control.

On Thu, Apr 29, 2021, 19:52 Marcin Jaczewski via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> czw., 29 kwi 2021 o 20:38 Edward Catmur via Std-Proposals
> <std-proposals_at_[hidden]> napisał(a):
> >
> > On Thu, 29 Apr 2021 at 19:13, Antony Polukhin <antoshkka_at_[hidden]>
> wrote:
> >>
> >> On Thu, Apr 29, 2021, 20:01 Edward Catmur via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
> >>>
> >>> On Thu, 29 Apr 2021 at 17:46, Andrey Semashev via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
> >>>>
> >>>> On 4/29/21 7:32 PM, Edward Catmur wrote:
> >>>> > It works in the most performance-sensitive case.
> >>>>
> >>>> No, it doesn't. It only works in your toy example where you call
> >>>> std::current_exception_stacktrace directly from the catch block and
> >>>> nothing else. This breaks in practically any other case that is even
> >>>> remotely close to anything real-world, where you do something useful
> in
> >>>> the exception handler that involves a cross-TU call. No, that doesn't
> >>>> sound like "works" to me, sorry.
> >>>
> >>>
> >>> I may have been unclear. The logic is that the compiler sets the flag
> on the handler to request stacktrace unless it can prove that the catch
> block does not call anything that might require it.
> >>
> >>
> >> Unfortunately compilers do not optimize exceptions even in very trivial
> cases https://godbolt.org/z/TKEjTfs1o
> >
> >
> > Well, absolutely - unless libgcc is compiled LTO and statically linked,
> there's no way for the compiler to know what __cxa_throw et al are going to
> do! Your LD_PRELOAD hooks the exception handling machinery to take a
> stacktrace and then rtld_next; someone else's LD_PRELOAD could do something
> else entirely.
> >
>
> Why not allow similar permission like for `memcpy` or `new`? This
> could be only an opaque symbol for the compiler but As-If rule could
> allow elide some calls to it.
>
> >> We are planning to write a benchmark to see, how the stacktraces affect
> performance of exceptions:
> >> * if the effect is not noticeable - stick to the current design
> >> * otherwise - provide a
> std::this_thread::capture_stacktraces_on_throw(bool) function to enable the
> proposed functionality, that would be disabled by default
> >
> >
> > Ah, that's a sensible compromise.
> > --
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