[Bug c++/107745] long double constexprs don't work with * or /, but work with + and - (JUST ON PPC)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107745

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Xi Ruoyao from comment #6)
> (In reply to Sebastian "spaetz" Spaeth from comment #5)
> > I fully understand that nobody wants to invest time into fixing this. What
> > would be nice though, is if this were really just a missed optimization and
> > not rejecting to compile valid code.
> > 
> > powerpc could ignore the constexpr in this case, rather than failing to
> > build?
> 
> It will be an violation of the standard (at least in some cases).

Yeah, the suggestion doesn't really make sense in general. If you don't care
whether the initialization is constexpr ... don't use constexpr. It's not about
optimization, it's about guaranteeing compile-time calculations.

I suppose it might be possible to implicitly change the variable to const
instead of constexpr, which would then give errors if you tried to use that in
any constant expressions. I would guess that won't help much real code, because
if you didn't want to use it in constant expressions, you wouldn't usually
declare it constexpr anyway.

In the specific case of
https://github.com/google/s2geometry/blob/2ff824474f0c4dfb157a0d056e4a6bb76bfa690f/src/s2/s2edge_crossings.cc#L115
it would compile, because constexpr apparently is being used as an
optimization, it doesn't need to be done at compile time.

But again, somebody needs to spend time to do that work. The people who require
this to work on their hardware should be the ones to do (or fund) the work on
it. The people unaffected by it probably aren't going to do anything about it.

It might be simpler to implement a "this is powerpc double double and we know
we can't do some arithmetic at compile time so treat this is const not
constexpr and see if that allows us to continue" feature than to implement full
compile-time arithmetic for double double.