[ASTImporter] Reorder fields after structure import is finished

We reorder declarations in RecordDecls because they may have another order
in the "to" context than they have in the "from" context. This may happen
e.g when we import a class like this:
   struct declToImport {
       int a = c + b;
       int b = 1;
       int c = 2;
   };
During the import of `a` we import first the dependencies in sequence,
thus the order would be `c`, `b`, `a`. We will get the normal order by
first removing the already imported members and then adding them in the
order as they apper in the "from" context.

Keeping field order is vital because it determines structure layout.

Reviewers: a_sidorin, shafik

Tags: #clang

Differential Revision: https://reviews.llvm.org/D44100

llvm-svn: 366997

clang/lib/AST/ASTImporter.cpp

@@ -1645,7 +1645,6 @@ ASTNodeImporter::ImportDeclContext(DeclContext *FromDC, bool ForceImport) {
   bool AccumulateChildErrors = isa<TagDecl>(FromDC);
 
   Error ChildErrors = Error::success();
-  llvm::SmallVector<Decl *, 8> ImportedDecls;
   for (auto *From : FromDC->decls()) {
     ExpectedDecl ImportedOrErr = import(From);
     if (!ImportedOrErr) {
@@ -1657,6 +1656,59 @@ ASTNodeImporter::ImportDeclContext(DeclContext *FromDC, bool ForceImport) {
     }
   }
 
+  // We reorder declarations in RecordDecls because they may have another order
+  // in the "to" context than they have in the "from" context. This may happen
+  // e.g when we import a class like this:
+  //    struct declToImport {
+  //        int a = c + b;
+  //        int b = 1;
+  //        int c = 2;
+  //    };
+  // During the import of `a` we import first the dependencies in sequence,
+  // thus the order would be `c`, `b`, `a`. We will get the normal order by
+  // first removing the already imported members and then adding them in the
+  // order as they apper in the "from" context.
+  //
+  // Keeping field order is vital because it determines structure layout.
+  //
+  // Here and below, we cannot call field_begin() method and its callers on
+  // ToDC if it has an external storage. Calling field_begin() will
+  // automatically load all the fields by calling
+  // LoadFieldsFromExternalStorage(). LoadFieldsFromExternalStorage() would
+  // call ASTImporter::Import(). This is because the ExternalASTSource
+  // interface in LLDB is implemented by the means of the ASTImporter. However,
+  // calling an import at this point would result in an uncontrolled import, we
+  // must avoid that.
+  const auto *FromRD = dyn_cast<RecordDecl>(FromDC);
+  if (!FromRD)
+    return ChildErrors;
+
+  auto ToDCOrErr = Importer.ImportContext(FromDC);
+  if (!ToDCOrErr) {
+    consumeError(std::move(ChildErrors));
+    return ToDCOrErr.takeError();
+  }
+
+  DeclContext *ToDC = *ToDCOrErr;
+  // Remove all declarations, which may be in wrong order in the
+  // lexical DeclContext and then add them in the proper order.
+  for (auto *D : FromRD->decls()) {
+    if (isa<FieldDecl>(D) || isa<FriendDecl>(D)) {
+      assert(D && "DC contains a null decl");
+      Decl *ToD = Importer.GetAlreadyImportedOrNull(D);
+      // Remove only the decls which we successfully imported.
+      if (ToD) {
+        assert(ToDC == ToD->getLexicalDeclContext() && ToDC->containsDecl(ToD));
+        // Remove the decl from its wrong place in the linked list.
+        ToDC->removeDecl(ToD);
+        // Add the decl to the end of the linked list.
+        // This time it will be at the proper place because the enclosing for
+        // loop iterates in the original (good) order of the decls.
+        ToDC->addDeclInternal(ToD);
+      }
+    }
+  }
+
   return ChildErrors;
 }
 

clang/unittests/AST/ASTImporterTest.cpp

@@ -1472,7 +1472,7 @@ TEST_P(ASTImporterOptionSpecificTestBase,
 }
 
 TEST_P(ASTImporterOptionSpecificTestBase,
-       DISABLED_CXXRecordDeclFieldOrderShouldNotDependOnImportOrder) {
+       CXXRecordDeclFieldOrderShouldNotDependOnImportOrder) {
   Decl *From, *To;
   std::tie(From, To) = getImportedDecl(
       // The original recursive algorithm of ASTImporter first imports 'c' then
@@ -2795,6 +2795,16 @@ TEST_P(ImportDecl, ImportEnumSequential) {
       "main.c", enumDecl(), VerificationMatcher);
 }
 
+TEST_P(ImportDecl, ImportFieldOrder) {
+  MatchVerifier<Decl> Verifier;
+  testImport("struct declToImport {"
+             "  int b = a + 2;"
+             "  int a = 5;"
+             "};",
+             Lang_CXX11, "", Lang_CXX11, Verifier,
+             recordDecl(hasFieldOrder({"b", "a"})));
+}
+
 const internal::VariadicDynCastAllOfMatcher<Expr, DependentScopeDeclRefExpr>
     dependentScopeDeclRefExpr;