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

[Pavel Labath via Phabricator via cfe-commits]

labath 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();
----------------
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.


================
Comment at: lldb/test/API/commands/expression/codegen-crash-import-def-arraytype-element/main.cpp:22-27
+  union {
+    struct {
+      unsigned char *_s;
+    } t;
+    char *tt[1];
+  } U;
----------------
shafik wrote:
> labath wrote:
> > What's the significance of this union?
> Not sure, I was not able to discern why this is important to reproduce the crash. I wanted feedback on the approach so I left to further investigation later on. 
Yeah, I was afraid of that. :) BTW, something I've been discussing with with @teemperor a few weeks ago might be interesting here: add something like the clangs `-dump-ast` flag to the `expr` command. Then we could test that the type A got properly imported even if we did not trigger the exact code path which causes the crash.

That said, I would expect that an assertion like "type X got imported" should be expressible in the ASTImporter unit tests, without any lldb involvement.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D86660/new/

https://reviews.llvm.org/D86660