On Mon, Jan 15, 2024 at 06:10:55PM -0500, Jason Merrill wrote: > 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. >
Ah, right; I definitely misread what was going on. > 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 > I didn't consider messing with 'CLASSTYPE_INTERFACE_ONLY' because the modules code currently always sets 'lang->interface_unknown = true' on read with the comment "Redetermine interface". But I see now as well this comment: // Interfaceness is recalculated upon reading. May have to revisit? // How do dllexport and dllimport interact across a module? // lang->interface_only // lang->interface_unknown So it seems reasonable to reconsider this now. I hadn't considered declarations in the GMF but I think you're right that we'll want to propagate it; I guess in general things in the GMF should try to behave as close to how they would have if they were #included directly in the importing TU. I suspect this fix will be more involved but I'll try to wrap my head around it and see what I can come up with.
