[Bug c++/100897] Symmetric transfer does not prevent stack-overflow for C++20 coroutines

["iains at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100897

--- Comment #3 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Leonard von Merzljak from comment #2)
> (In reply to Iain Sandoe from comment #1)

> Thank you for your comment. I tried it out and can confirm that I don't get
> a stack-overflow anymore if I omit -fsanitize=address and use an
> optimization level > 1.

So that's a workaround (on platforms that support indirect tail calls at all).

> If the issues with coroutines and sanitizers are
> already known, then this bug report can be marked as resolved.

For the present, I will leave this open - until (at least) there's a chance to
confirm the hypothesis and determine if the problems are the same ones as
mentioned in other PRs.

> Of course, it would be nice if the stack-overflow would not occur even when
> using an optimization level <= 1, but this probably does not qualify as a
> bug.

Note that the inability to support indirect tail calls is not usually a failing
in GCC - but that some platform ABIs cannot support it (e.g. because they
require initialisation of some per DSO data).

For platforms that support indirect tail calls, it is actually feasible to
support the symmetric transfer at O0 (at least as per my local testing) - the
front end can demand a tailcall "for correctness".  The issue is that
coroutines are not a target-specific implementation, and therefore demanding
the tailcall will cause compile fails on targets that cannot support it.

Of course, one can argue that the code will *probably* fail on those targets if
there is arbitrary recursion needed - but it was decided to not to make this
demand until a solution is found to supporting continuations on all target.

JFTR, my outline sketch for this would be to allocate some area in the
coroutine frame that is reserved for target-specific continuation support, and
then to use a builtin to implement the continuation rather than relying on the
indirect tailcall mechanism.