On Thu, 29 Apr 2021 at 16:20, Andrey Semashev via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> On 4/29/21 5:39 PM, Edward Catmur via Std-Proposals wrote:
> > On Thu, 29 Apr 2021 at 13:28, Andrey Semashev via Std-Proposals
> > Presumably, someone has to tell the runtime to collect the
> stacktrace,
> > and, if I understood you right, that someone is supposed to be the
> > compiler, which sees a std::current_exception_stacktrace call in the
> > catch block.
> >
> > Yes? That sounds entirely feasible.
>
> No, it doesn't sound feasible at all. At least, not with current
> compiler technology. And in case if you bring up link-time code
> generation, no, it is not the answer. As any other optimization, it is
> entirely optional and must not affect the program behavior.
>

Quick sketch: the compiler sets a directive at the catch site to request
stacktrace, which the personality routine observes and allocates a
stacktrace array reachable via the __cxa_exception, then when it is called
again in unwind phase it populates. What's wrong with this technique?
Obviously you need a new directive (or bit flag on an existing directive)
which the compiler can emit and the runtime recognize, but they're supplied
by the same vendor so I can't see that as an obstacle.

Also, the argument of "let's make exceptions slower because they're slow
> anyway" doesn't work. Clearly, people care about their performance, and
> a non-trivial amount of effort was spent to optimize them, even though
> most of the time they are not on the hot path.
>

My understanding is that effort has been spent to optimize the
non-exceptional path.