5 files changed, 28 insertions, 14 deletions

diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index d663dbeea87..13921a132cf 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/clang/include/clang/AST/DeclCXX.h
@@ -464,6 +464,8 @@ class CXXRecordDecl : public RecordDecl {
     /// \brief Whether we are currently parsing base specifiers.
     unsigned IsParsingBaseSpecifiers : 1;
 
+    unsigned HasODRHash : 1;
+
     /// \brief A hash of parts of the class to help in ODR checking.
     unsigned ODRHash;
 
@@ -712,8 +714,7 @@ public:
     return data().IsParsingBaseSpecifiers;
   }
 
-  void computeODRHash();
-  unsigned getODRHash() const { return data().ODRHash; }
+  unsigned getODRHash() const;
 
   /// \brief Sets the base classes of this struct or class.
   void setBases(CXXBaseSpecifier const * const *Bases, unsigned NumBases);
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index a1aed332a13..2e5cec9c108 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -73,8 +73,9 @@ CXXRecordDecl::DefinitionData::DefinitionData(CXXRecordDecl *D)
       ImplicitCopyAssignmentHasConstParam(true),
       HasDeclaredCopyConstructorWithConstParam(false),
       HasDeclaredCopyAssignmentWithConstParam(false), IsLambda(false),
-      IsParsingBaseSpecifiers(false), ODRHash(0), NumBases(0), NumVBases(0),
-      Bases(), VBases(), Definition(D), FirstFriend() {}
+      IsParsingBaseSpecifiers(false), HasODRHash(false), ODRHash(0),
+      NumBases(0), NumVBases(0), Bases(), VBases(), Definition(D),
+      FirstFriend() {}
 
 CXXBaseSpecifier *CXXRecordDecl::DefinitionData::getBasesSlowCase() const {
   return Bases.get(Definition->getASTContext().getExternalSource());
@@ -381,16 +382,23 @@ CXXRecordDecl::setBases(CXXBaseSpecifier const * const *Bases,
   data().IsParsingBaseSpecifiers = false;
 }
 
-void CXXRecordDecl::computeODRHash() {
-  if (!DefinitionData)
-    return;
+unsigned CXXRecordDecl::getODRHash() const {
+  assert(hasDefinition() && "ODRHash only for records with definitions");
 
-  ODRHash Hash;
-  Hash.AddCXXRecordDecl(this);
+  // Previously calculated hash is stored in DefinitionData.
+  if (DefinitionData->HasODRHash)
+    return DefinitionData->ODRHash;
 
+  // Only calculate hash on first call of getODRHash per record.
+  ODRHash Hash;
+  Hash.AddCXXRecordDecl(getDefinition());
+  DefinitionData->HasODRHash = true;
   DefinitionData->ODRHash = Hash.CalculateHash();
+
+  return DefinitionData->ODRHash;
 }
 
+
 void CXXRecordDecl::addedClassSubobject(CXXRecordDecl *Subobj) {
   // C++11 [class.copy]p11:
   //   A defaulted copy/move constructor for a class X is defined as
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 02cbca8b2bb..c6a0b0101d3 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13797,10 +13797,8 @@ void Sema::ActOnTagFinishDefinition(Scope *S, Decl *TagD,
       RD->completeDefinition();
   }
 
-  if (auto *RD = dyn_cast<CXXRecordDecl>(Tag)) {
+  if (isa<CXXRecordDecl>(Tag)) {
     FieldCollector->FinishClass();
-    if (Context.getLangOpts().Modules)
-      RD->computeODRHash();
   }
 
   // Exit this scope of this tag's definition.
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index db7d55ec0b8..2fb882c1bec 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1536,6 +1536,7 @@ void ASTDeclReader::ReadCXXDefinitionData(
   Data.HasDeclaredCopyConstructorWithConstParam = Record.readInt();
   Data.HasDeclaredCopyAssignmentWithConstParam = Record.readInt();
   Data.ODRHash = Record.readInt();
+  Data.HasODRHash = true;
 
   if (Record.readInt()) {
     Reader.BodySource[D] = Loc.F->Kind == ModuleKind::MK_MainFile
@@ -1673,7 +1674,6 @@ void ASTDeclReader::MergeDefinitionData(
   OR_FIELD(HasDeclaredCopyConstructorWithConstParam)
   OR_FIELD(HasDeclaredCopyAssignmentWithConstParam)
   MATCH_FIELD(IsLambda)
-  MATCH_FIELD(ODRHash)
 #undef OR_FIELD
 #undef MATCH_FIELD
 
@@ -1697,6 +1697,10 @@ void ASTDeclReader::MergeDefinitionData(
     // when they occur within the body of a function template specialization).
   }
 
+  if (D->getODRHash() != MergeDD.ODRHash) {
+    DetectedOdrViolation = true;
+  }
+
   if (DetectedOdrViolation)
     Reader.PendingOdrMergeFailures[DD.Definition].push_back(MergeDD.Definition);
 }
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index ec59250e192..6f0d0eed5b9 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -5769,7 +5769,10 @@ void ASTRecordWriter::AddCXXDefinitionData(const CXXRecordDecl *D) {
   Record->push_back(Data.ImplicitCopyAssignmentHasConstParam);
   Record->push_back(Data.HasDeclaredCopyConstructorWithConstParam);
   Record->push_back(Data.HasDeclaredCopyAssignmentWithConstParam);
-  Record->push_back(Data.ODRHash);
+
+  // getODRHash will compute the ODRHash if it has not been previously computed.
+  Record->push_back(D->getODRHash());
+
   bool ModularCodegen = Writer->Context->getLangOpts().ModularCodegen &&
                         Writer->WritingModule && !D->isDependentType();
   Record->push_back(ModularCodegen);