[Lldb-commits] [PATCH] D86660: Modifying ImportDeclContext(...) to ensure that we also handle the case when the FieldDecl is an ArrayType whose ElementType is a RecordDecl

[Gabor Marton via Phabricator via lldb-commits]

martong added inline comments.


================
Comment at: clang/lib/AST/ASTImporter.cpp:1755-1759
+          QualType FromTy = ArrayFrom->getElementType();
+          QualType ToTy = ArrayTo->getElementType();
+
+          FromRecordDecl = FromTy->getAsRecordDecl();
+          ToRecordDecl = ToTy->getAsRecordDecl();
----------------
labath wrote:
> shafik wrote:
> > martong wrote:
> > > labath wrote:
> > > > What about 2- or n-dimensional arrays?
> > > @labath, this is a very good question! And made me realize that D71378 is fundamentally flawed (@shafik, please take no offense). Let me explain.
> > > 
> > > So, with D71378, we added the `if (ImportedOrErr) { ... }` block to import definitions specifically of fields' Record types. But we forget to handle arrays. Now we may forget to handle multidimensional arrays ... and we may forget to handle other language constructs. So, we would finally end up in re-implementing the logic of `ASTNodeImporter::VisitFieldDecl`.
> > > 
> > > So all this should have been handled properly by the preceding import call of the FieldDecl! Here
> > > ```
> > > 1735: ExpectedDecl ImportedOrErr = import(From);
> > > ```
> > > I have a suspicion that real reason why this import call fails in case of the public ASTImporter::ImportDefinition() is that we fail to drive through the import kind (`IDK_Everything`) during the import process.
> > > Below we set IDK_Everything and start a complete import process.
> > > ```
> > >   8784   if (auto *ToRecord = dyn_cast<RecordDecl>(To)) {
> > >   8785     if (!ToRecord->getDefinition()) {
> > >   8786       return Importer.ImportDefinition(   // ASTNodeImporter::ImportDefinition !
> > >   8787           cast<RecordDecl>(FromDC), ToRecord,
> > >   8788           ASTNodeImporter::IDK_Everything);
> > >   8789     }
> > >   8790   }
> > > ```
> > > However, there may be many places where we fail to channel through that we want to do a complete import. E.g here
> > > ```
> > > 1957           ImportDefinitionIfNeeded(Base1.getType()->getAsCXXRecordDecl()))
> > > ```
> > > we do another definition import and IDK_Everything is not set. So we may have a wrong kind of import since the "minimal" flag is set.
> > > 
> > > The thing is, it is really confusing and error-prone to have both the `ASTImporter::Minimal` flag and the `ImportDefinitionKind`. They seem to be in contradiction to each other some times.
> > > I think we should get rid of the Minimal flag. And Kind should be either a full completion (IDK_Everythink) or just a minimal (IDK_Basic). So, I'd scrap the IDK_Default too. Alternatively we could have a Kind member in AST**//Node//**Importer.
> > > I think we should go into this direction to avoid similar problems during CodeGen in the future. WDYT?
> > No offense, you definitely understand the Importer better than I, so I value your input here. I don't believe that should have other fields where we could have a record that effects the layout but the concern is still valid and yes I did miss multi-dimensional arrays.
> > 
> > Your theory on `IDK_Everything` not be pushed through everywhere, I did a quick look and it seems pretty reasonable. 
> > 
> > I agree that the `ASTImporter::Minimal` flag and the `ImportDefinitionKind` seem to inconsistent or perhaps a work-around. That seems like a bigger change with a lot more impact beyond this fix but worth looking into longer term. 
> > 
> > If pushing through `IDK_Everything` everywhere fixes the underlying issue I am very happy to take that approach. If not we can discuss alternatives. 
> I've been looking at this code, but I'm still not very familiar with it, so what I am asking may be obvious, but... What is the expected behavior for non-minimal imports for code like this?
> ```
> struct A { ...};
> struct B { A* a; }; // Note the pointer
> ```
> Should importing B also import the definition of the A struct ? As I think that should not happen in the minimal import... If we get rid of the minimal flag, and rely solely on argument passing, we will need to be careful, as it shouldn't be passed _everywhere_ (it should stop at pointers for instance). But then it may not be possible to reproduce the current non-minimal import, as it (I think) expects that A will be fully imported too...
> 
> > I don't believe that should have other fields where we could have a record that effects the layout
> This isn't exactly layout related, but there is the question of covariant methods. If a method is covariant, then its return type must be complete. Currently we handle the completion of these in LLDB, but that solution is a bit hacky. It might be interesting if that could be handled by the ast importer as well.
> What is the expected behavior for non-minimal imports for code like this?
In this case we import the definition of A fully, even though that seems unnecessary. This may be an existing technical debt in clang::ASTNodeImporter::VisitPointerType. Perhaps instead of a full import we could just do a minimal import for the PointeeType always. Similarly to references. But I am not sure about whether we would know during a subsequent import (where the full type is really needed) that the type had been minimally imported before.
Actually, we had a similar discussion with Jaroslav about this before:
http://lists.llvm.org/pipermail/lldb-dev/2019-November/015738.html

> If we get rid of the minimal flag, and rely solely on argument passing, we will need to be careful as it shouldn't be passed _everywhere_ (it should stop at pointers for instance)
Yes, I agree. Currently, the problem is that even though the flag is set, we require complete imports via the public ImportDefinition function. And during this supposedly full import we sometimes make import decisions based on the flag, ouch. Perhaps we could keep the flag, but in case of the ASTImporter::ImportDefinition we should set that to false at entry and before returning we should set that back to true .(?)
Anyhow, I really think we shall have only one mechanism to decide whether we want a full or just a partial import, this should be either the flag or the Kind.

> This isn't exactly layout related, but there is the question of covariant methods. If a method is covariant, then its return type must be complete.
We already import all the base classes of a derived class (at least in full import). But, we do not import all the derived classes during the import of the base. Is this what you do in LLDB to support covariant return types?


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