Re: [PATCH v2] c++/modules: Differentiate extern templates and TYPE_DECL_SUPPRESS_DEBUG [PR112820]

[Jason Merrill <jason@redhat.com>]

On 1/8/24 10:27, Patrick Palka wrote:
> On Mon, 8 Jan 2024, Nathaniel Shead wrote:
>> On Thu, Jan 04, 2024 at 03:39:15PM -0500, Patrick Palka wrote:
>>> On Sun, 3 Dec 2023, Nathaniel Shead wrote:
>>>>
>>>> The TYPE_DECL_SUPPRESS_DEBUG and DECL_EXTERNAL flags use the same
>>>> underlying bit. This is causing confusion when attempting to determine
>>>> the interface for a streamed-in class type, since the modules code
>>>> currently assumes that all DECL_EXTERNAL types are extern templates.
>>>> However, when -g is specified then TYPE_DECL_SUPPRESS_DEBUG (and hence
>>>> DECL_EXTERNAL) is marked on various other kinds of declarations, such as
>>>> vtables, which causes them to never be emitted.

But a vtable isn't a TYPE_DECL?

I suspect what you mean is that maybe_suppress_debug_info is setting 
TYPE_DECL_SUPPRESS_DEBUG to try to avoid duplication of debug info for 
classes with vtables, and then the modules code is wrongly assuming that 
you can check DECL_EXTERNAL for TYPE_DECL, and that it's set only if 
CLASSTYPE_INTERFACE_ONLY is also set, which is wrong in this case, so we 
avoid emitting the vtable or anything else for that class.

It seems unnecessary to start setting DECL_EXTERNAL on the TYPE_DECL to 
mean the exact same thing as CLASSTYPE_INTERFACE_ONLY.  Rather, the 
modules code should stop trying to check DECL_EXTERNAL on a TYPE_DECL.

Under what circumstances does it make sense for CLASSTYPE_INTERFACE_ONLY 
to be set in the context of modules, anyway?  We probably want to 
propagate it for things in the global module so that various libstdc++ 
explicit instantiations work the same with import std.

For an class imported from a named module, this ties into the earlier 
discussion about vtables and inlines that hasn't resolved yet in the ABI 
committee.  But it's certainly significantly interface-like.  And I 
would expect maybe_suppress_debug_info to suppress the debug info for 
such a class on the assumption that the module unit has the needed debug 
info.

Jason