[cfe-dev] local submodule visibility, namespaces and Decl::getOwningModule()

[David Blaikie via cfe-dev]

I believe we hit this with the first attempt at modular code generation,
fixed it thusly: https://reviews.llvm.org/D29901

Not sure about the namespace situation - don't recall noticing that.

On Mon, Oct 15, 2018 at 10:13 AM Adrian Prantl via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I'm trying to fix a bug in the -gmodules debug info that appears when
> debugging Clang with local submodule visibility enabled. In my reduced
> testcase, I have a module A, defining a template
> "AlignedCharArray<unsigned, unsigned>", and two templates "SmallVector" and
> "Optional" that use it. In module B, I'm instantiating an Optional that
> instantiates an AlignedCharArray<4, 16>. In module C, I'm instantiating a
> SmallVector that also instantiates an AlignedCharArray<4, 16>.
> The -gmodules debug info format saves space by forward-declaring types
> that are defined in an already-imported module and specify which module
> they came from. To point to the module of the definition
> CGDebugInfo::getParentModuleOrNull(const Decl *D) uses
> Decl::getOwningModule(). This mechanism works as expected, but only if the
> TemplateDecl is not inside a namespace.
>
> When compiling the attached testcase with -U WITH_NAMESPACE, the owning
> module for the ClassTemplateSpecializationDecl (of AlignedCharArray<4, 16>)
> in module B is C. (B imports C, so the C specialization is instantiated
> first).
> When compiling the attached testcase with -D WITH_NAMESPACE, the owning
> module for the ClassTemplateSpecializationDecl (of AlignedCharArray<4, 16>)
> in module B is A. (A is where the TemplateDecl that is being specialized
> comes from).
>
> I'm sure that this discrepancy must be because a namespace can be spread
> out over many modules. I can "fix" my issue by always emitting full debug
> info definitions for template specializations inside of namespaces. But I'm
> also really curious whether the behavior I'm experiencing is indicative of
> a general bug/shortcoming of the LSV implementation that we might be
> interested in fixing. The owning module of each declaration is set to the
> owning module of its DeclContext and namespaces seem to interfere with this
> mechanism.
>
>
> thanks,
> Adrian
>
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