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

[Nathan Sidwell <nathan@acm.org>]

Richard Smith & I discussed whether we should use the module interface's 
capability of giving vague linkage entities a strong location. I didn't want to 
go messing with that, 'cos it was changing yet more stuff.

But, perhaps we should revisit that?  Any keyless polymorphic class in module 
purview gets its vtables etc emitted in the module's object file?  Likewise 
these kinds of entities.

cc'ing Iain, who probably knows more about Clang's state here.

nathan


On 1/4/24 21:06, Jason Merrill wrote:
> On 1/4/24 18:02, Nathaniel Shead wrote:
>> 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.
> 
> Hmm, not a strong vote for my direction.
> 
>>>> 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?
> 
> That would also make sense to me; since we know the module interface unit is 
> compiled to an object file, everything vague linkage in it can go there.
> 
>>>> 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.
>>
>> 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".)
> 
> I'd like to hear Nathan's thoughts on the matter first, since he's the modules 
> implementation designer.
> 
> Jason
> 

-- 
Nathan Sidwell