Re: c++/modules: Emit definitions of ODR-used static members imported from modules [PR112899]

[Nathaniel Shead <nathanieloshead@gmail.com>]

On Thu, Jan 04, 2024 at 05:42:34PM -0500, Jason Merrill wrote:
> On 1/4/24 17:24, Nathaniel Shead wrote:
> > On Thu, Jan 04, 2024 at 03:31:50PM -0500, Jason Merrill wrote:
> > > On 1/2/24 17:40, Nathaniel Shead wrote:
> > > > Static data members marked 'inline' should be emitted in TUs where they
> > > > are ODR-used.  We need to make sure that statics imported from modules
> > > > are correctly added to the 'pending_statics' map so that they get
> > > > emitted if needed, otherwise the attached testcase fails to link.
> > > 
> > > Hmm, this seems wrong to me; I'd think that static data members marked
> > > inline should be emitted in the module, and not in importers.
> > 
> > That's what I'd initially thought too, but this is at least consistent
> > with non-class inlines (variables and functions), which similarly only
> > get emitted in TUs that they're ODR-used rather than always (and only)
> > being emitted within the module.
> > 
> > I guess an alternative would be to change it around so that all
> > exported definitions are marked as needed in the module interface file
> > (and emitted there), and then setting some flag so that they're never
> > emitted in importers.
> 
> Yes, that would be my expectation.  What do other modules implementations
> do?

Clang only emits ODR-used declarations (same as GCC currently).

MSVC emits all inline variables (whether exported or not) but no inline
functions.

> > I'm not entirely sure what flag that would be
> > though, I still haven't quite wrapped my head what controls what with
> > regards to this, and I'm not convinced it wouldn't break template
> > instantiations.
> 
> I would guess avoid emitting if DECL_MODULE_IMPORT_P &&
> DECL_MODULE_ATTACH_P.

Ah yup, that would make sense. I guess, thinking about it more, we
should then also ensure that all TREE_PUBLIC declarations are emitted in
the module interface even if not exported, since they may be needed in
implementation units?

> > I wonder if this might also be related to the issue Nathan noted with
> > regards to block-scope class methods, which I haven't completely worked
> > out how to solve yet otherwise (see
> > https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638223.html).
> 
> Indeed.
> 
> Jason
> 

I'll give implementing this a try then, if you think that would be
sensible. (Where by "this" I mean "emit all public declarations in
module interface files, whether used or not".)