[clang] fa4a0f1 - [modules] Add a flag for TagDecl if it was a definition demoted to a declaration.

[Volodymyr Sapsai via cfe-commits]

Author: Volodymyr Sapsai
Date: 2022-02-14T16:04:40-08:00
New Revision: fa4a0f1d31e2f797c3e27eb7cad15755e46a4726

URL: https://github.com/llvm/llvm-project/commit/fa4a0f1d31e2f797c3e27eb7cad15755e46a4726
DIFF: https://github.com/llvm/llvm-project/commit/fa4a0f1d31e2f797c3e27eb7cad15755e46a4726.diff

LOG: [modules] Add a flag for TagDecl if it was a definition demoted to a declaration.

For redeclaration chains we maintain an invariant of having only a
single definition in the chain. In a single translation unit we make
sure not to create duplicates. But modules are separate translation
units and they can contain definitions for the same symbol
independently. When we load such modules together, we need to demote
duplicate definitions to keep the AST invariants.

Some AST clients are interested in distinguishing
declaration-that-was-demoted-from-definition and
declaration-that-was-never-a-definition. For that purpose introducing
`IsThisDeclarationADemotedDefinition`. No functional change intended.

rdar://84677782

Differential Revision: https://reviews.llvm.org/D118855

Added: 
    

Modified: 
    clang/include/clang/AST/Decl.h
    clang/include/clang/AST/DeclBase.h
    clang/lib/AST/Decl.cpp
    clang/lib/Serialization/ASTReaderDecl.cpp

Removed: 
    


################################################################################
diff  --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 862e8899d2751..7611bac83419d 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -3486,6 +3486,24 @@ class TagDecl : public TypeDecl,
   /// parameters.
   bool isDependentType() const { return isDependentContext(); }
 
+  /// Whether this declaration was a definition in some module but was forced
+  /// to be a declaration.
+  ///
+  /// Useful for clients checking if a module has a definition of a specific
+  /// symbol and not interested in the final AST with deduplicated definitions.
+  bool isThisDeclarationADemotedDefinition() const {
+    return TagDeclBits.IsThisDeclarationADemotedDefinition;
+  }
+
+  /// Mark a definition as a declaration and maintain information it _was_
+  /// a definition.
+  void demoteThisDefinitionToDeclaration() {
+    assert(isCompleteDefinition() &&
+           "Should demote definitions only, not forward declarations");
+    setCompleteDefinition(false);
+    TagDeclBits.IsThisDeclarationADemotedDefinition = true;
+  }
+
   /// Starts the definition of this tag declaration.
   ///
   /// This method should be invoked at the beginning of the definition

diff  --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 06d2f17d14300..a89f776248c1f 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1443,10 +1443,14 @@ class DeclContext {
     /// Has the full definition of this type been required by a use somewhere in
     /// the TU.
     uint64_t IsCompleteDefinitionRequired : 1;
+
+    /// Whether this tag is a definition which was demoted due to
+    /// a module merge.
+    uint64_t IsThisDeclarationADemotedDefinition : 1;
   };
 
   /// Number of non-inherited bits in TagDeclBitfields.
-  enum { NumTagDeclBits = 9 };
+  enum { NumTagDeclBits = 10 };
 
   /// Stores the bits used by EnumDecl.
   /// If modified NumEnumDeclBit and the accessor

diff  --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 060a6d1ad5ed5..030da7f55fac4 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -4301,6 +4301,7 @@ TagDecl::TagDecl(Kind DK, TagKind TK, const ASTContext &C, DeclContext *DC,
   setEmbeddedInDeclarator(false);
   setFreeStanding(false);
   setCompleteDefinitionRequired(false);
+  TagDeclBits.IsThisDeclarationADemotedDefinition = false;
 }
 
 SourceLocation TagDecl::getOuterLocStart() const {

diff  --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 1ab26e58a4040..25d7e9e6a2e68 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -773,7 +773,7 @@ void ASTDeclReader::VisitEnumDecl(EnumDecl *ED) {
     }
     if (OldDef) {
       Reader.MergedDeclContexts.insert(std::make_pair(ED, OldDef));
-      ED->setCompleteDefinition(false);
+      ED->demoteThisDefinitionToDeclaration();
       Reader.mergeDefinitionVisibility(OldDef, ED);
       if (OldDef->getODRHash() != ED->getODRHash())
         Reader.PendingEnumOdrMergeFailures[OldDef].push_back(ED);
@@ -828,7 +828,7 @@ void ASTDeclReader::VisitRecordDecl(RecordDecl *RD) {
     }
     if (OldDef) {
       Reader.MergedDeclContexts.insert(std::make_pair(RD, OldDef));
-      RD->setCompleteDefinition(false);
+      RD->demoteThisDefinitionToDeclaration();
       Reader.mergeDefinitionVisibility(OldDef, RD);
     } else {
       OldDef = RD;