If a declaration is loaded, and then a module import adds a redeclaration, then

ensure that querying the first declaration for its most recent declaration
checks for redeclarations from the imported module.

This works as follows:
 * The 'most recent' pointer on a canonical declaration grows a pointer to the
   external AST source and a generation number (space- and time-optimized for
   the case where there is no external source).
 * Each time the 'most recent' pointer is queried, if it has an external source,
   we check whether it's up to date, and update it if not.
 * The ancillary data stored on the canonical declaration is allocated lazily
   to avoid filling it in for declarations that end up being non-canonical.
   We'll still perform a redundant (ASTContext) allocation if someone asks for
   the most recent declaration from a decl before setPreviousDecl is called,
   but such cases are probably all bugs, and are now easy to find.

Some finessing is still in order here -- in particular, we use a very general
mechanism for handling the DefinitionData pointer on CXXRecordData, and a more
targeted approach would be more compact.

Also, the MayHaveOutOfDateDef mechanism should now be expunged, since it was
addressing only a corner of the full problem space here. That's not covered
by this patch.

Early performance benchmarks show that this makes no measurable difference to 
Clang performance without modules enabled (and fixes a major correctness issue
with modules enabled). I'll revert if a full performance comparison shows any
problems.

llvm-svn: 209046

1 files changed, 40 insertions, 36 deletions

diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 101b1646248..fc15bdee010 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1616,10 +1616,12 @@ const char *VarDecl::getStorageClassSpecifierString(StorageClass SC) {
   llvm_unreachable("Invalid storage class");
 }
 
-VarDecl::VarDecl(Kind DK, DeclContext *DC, SourceLocation StartLoc,
-                 SourceLocation IdLoc, IdentifierInfo *Id, QualType T,
-                 TypeSourceInfo *TInfo, StorageClass SC)
-    : DeclaratorDecl(DK, DC, IdLoc, Id, T, TInfo, StartLoc), Init() {
+VarDecl::VarDecl(Kind DK, ASTContext &C, DeclContext *DC,
+                 SourceLocation StartLoc, SourceLocation IdLoc,
+                 IdentifierInfo *Id, QualType T, TypeSourceInfo *TInfo,
+                 StorageClass SC)
+    : DeclaratorDecl(DK, DC, IdLoc, Id, T, TInfo, StartLoc),
+      redeclarable_base(C), Init() {
   static_assert(sizeof(VarDeclBitfields) <= sizeof(unsigned),
                 "VarDeclBitfields too large!");
   static_assert(sizeof(ParmVarDeclBitfields) <= sizeof(unsigned),
@@ -1633,12 +1635,13 @@ VarDecl *VarDecl::Create(ASTContext &C, DeclContext *DC,
                          SourceLocation StartL, SourceLocation IdL,
                          IdentifierInfo *Id, QualType T, TypeSourceInfo *TInfo,
                          StorageClass S) {
-  return new (C, DC) VarDecl(Var, DC, StartL, IdL, Id, T, TInfo, S);
+  return new (C, DC) VarDecl(Var, C, DC, StartL, IdL, Id, T, TInfo, S);
 }
 
 VarDecl *VarDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) VarDecl(Var, nullptr, SourceLocation(), SourceLocation(),
-                             nullptr, QualType(), nullptr, SC_None);
+  return new (C, ID)
+      VarDecl(Var, C, nullptr, SourceLocation(), SourceLocation(), nullptr,
+              QualType(), nullptr, SC_None);
 }
 
 void VarDecl::setStorageClass(StorageClass SC) {
@@ -2114,7 +2117,7 @@ ParmVarDecl *ParmVarDecl::Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation IdLoc, IdentifierInfo *Id,
                                  QualType T, TypeSourceInfo *TInfo,
                                  StorageClass S, Expr *DefArg) {
-  return new (C, DC) ParmVarDecl(ParmVar, DC, StartLoc, IdLoc, Id, T, TInfo,
+  return new (C, DC) ParmVarDecl(ParmVar, C, DC, StartLoc, IdLoc, Id, T, TInfo,
                                  S, DefArg);
 }
 
@@ -2127,9 +2130,9 @@ QualType ParmVarDecl::getOriginalType() const {
 }
 
 ParmVarDecl *ParmVarDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) ParmVarDecl(ParmVar, nullptr, SourceLocation(),
-                                 SourceLocation(), nullptr, QualType(),
-                                 nullptr, SC_None, nullptr);
+  return new (C, ID)
+      ParmVarDecl(ParmVar, C, nullptr, SourceLocation(), SourceLocation(),
+                  nullptr, QualType(), nullptr, SC_None, nullptr);
 }
 
 SourceRange ParmVarDecl::getSourceRange() const {
@@ -3234,10 +3237,10 @@ void TagDecl::startDefinition() {
   IsBeingDefined = true;
 
   if (CXXRecordDecl *D = dyn_cast<CXXRecordDecl>(this)) {
-    struct CXXRecordDecl::DefinitionData *Data = 
+    struct CXXRecordDecl::DefinitionData *Data =
       new (getASTContext()) struct CXXRecordDecl::DefinitionData(D);
     for (auto I : redecls())
-      cast<CXXRecordDecl>(I)->DefinitionData = Data;
+      cast<CXXRecordDecl>(I)->DefinitionData.setNotUpdated(Data);
   }
 }
 
@@ -3319,7 +3322,7 @@ EnumDecl *EnumDecl::Create(ASTContext &C, DeclContext *DC,
                            IdentifierInfo *Id,
                            EnumDecl *PrevDecl, bool IsScoped,
                            bool IsScopedUsingClassTag, bool IsFixed) {
-  EnumDecl *Enum = new (C, DC) EnumDecl(DC, StartLoc, IdLoc, Id, PrevDecl,
+  EnumDecl *Enum = new (C, DC) EnumDecl(C, DC, StartLoc, IdLoc, Id, PrevDecl,
                                         IsScoped, IsScopedUsingClassTag,
                                         IsFixed);
   Enum->MayHaveOutOfDateDef = C.getLangOpts().Modules;
@@ -3328,9 +3331,9 @@ EnumDecl *EnumDecl::Create(ASTContext &C, DeclContext *DC,
 }
 
 EnumDecl *EnumDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  EnumDecl *Enum = new (C, ID) EnumDecl(nullptr, SourceLocation(),
-                                        SourceLocation(), nullptr, nullptr,
-                                        false, false, false);
+  EnumDecl *Enum =
+      new (C, ID) EnumDecl(C, nullptr, SourceLocation(), SourceLocation(),
+                           nullptr, nullptr, false, false, false);
   Enum->MayHaveOutOfDateDef = C.getLangOpts().Modules;
   return Enum;
 }
@@ -3389,10 +3392,11 @@ void EnumDecl::setInstantiationOfMemberEnum(ASTContext &C, EnumDecl *ED,
 // RecordDecl Implementation
 //===----------------------------------------------------------------------===//
 
-RecordDecl::RecordDecl(Kind DK, TagKind TK, DeclContext *DC,
-                       SourceLocation StartLoc, SourceLocation IdLoc,
-                       IdentifierInfo *Id, RecordDecl *PrevDecl)
-  : TagDecl(DK, TK, DC, IdLoc, Id, PrevDecl, StartLoc) {
+RecordDecl::RecordDecl(Kind DK, TagKind TK, const ASTContext &C,
+                       DeclContext *DC, SourceLocation StartLoc,
+                       SourceLocation IdLoc, IdentifierInfo *Id,
+                       RecordDecl *PrevDecl)
+    : TagDecl(DK, TK, C, DC, IdLoc, Id, PrevDecl, StartLoc) {
   HasFlexibleArrayMember = false;
   AnonymousStructOrUnion = false;
   HasObjectMember = false;
@@ -3404,8 +3408,8 @@ RecordDecl::RecordDecl(Kind DK, TagKind TK, DeclContext *DC,
 RecordDecl *RecordDecl::Create(const ASTContext &C, TagKind TK, DeclContext *DC,
                                SourceLocation StartLoc, SourceLocation IdLoc,
                                IdentifierInfo *Id, RecordDecl* PrevDecl) {
-  RecordDecl* R = new (C, DC) RecordDecl(Record, TK, DC, StartLoc, IdLoc, Id,
-                                         PrevDecl);
+  RecordDecl *R = new (C, DC) RecordDecl(Record, TK, C, DC,
+                                         StartLoc, IdLoc, Id, PrevDecl);
   R->MayHaveOutOfDateDef = C.getLangOpts().Modules;
 
   C.getTypeDeclType(R, PrevDecl);
@@ -3413,9 +3417,9 @@ RecordDecl *RecordDecl::Create(const ASTContext &C, TagKind TK, DeclContext *DC,
 }
 
 RecordDecl *RecordDecl::CreateDeserialized(const ASTContext &C, unsigned ID) {
-  RecordDecl *R = new (C, ID) RecordDecl(Record, TTK_Struct, nullptr,
-                                         SourceLocation(), SourceLocation(),
-                                         nullptr, nullptr);
+  RecordDecl *R =
+      new (C, ID) RecordDecl(Record, TTK_Struct, C, nullptr, SourceLocation(),
+                             SourceLocation(), nullptr, nullptr);
   R->MayHaveOutOfDateDef = C.getLangOpts().Modules;
   return R;
 }
@@ -3577,12 +3581,12 @@ ImplicitParamDecl *ImplicitParamDecl::Create(ASTContext &C, DeclContext *DC,
                                              SourceLocation IdLoc,
                                              IdentifierInfo *Id,
                                              QualType Type) {
-  return new (C, DC) ImplicitParamDecl(DC, IdLoc, Id, Type);
+  return new (C, DC) ImplicitParamDecl(C, DC, IdLoc, Id, Type);
 }
 
 ImplicitParamDecl *ImplicitParamDecl::CreateDeserialized(ASTContext &C,
                                                          unsigned ID) {
-  return new (C, ID) ImplicitParamDecl(nullptr, SourceLocation(), nullptr,
+  return new (C, ID) ImplicitParamDecl(C, nullptr, SourceLocation(), nullptr,
                                        QualType());
 }
 
@@ -3595,14 +3599,14 @@ FunctionDecl *FunctionDecl::Create(ASTContext &C, DeclContext *DC,
                                    bool hasWrittenPrototype,
                                    bool isConstexprSpecified) {
   FunctionDecl *New =
-      new (C, DC) FunctionDecl(Function, DC, StartLoc, NameInfo, T, TInfo, SC,
-                               isInlineSpecified, isConstexprSpecified);
+      new (C, DC) FunctionDecl(Function, C, DC, StartLoc, NameInfo, T, TInfo,
+                               SC, isInlineSpecified, isConstexprSpecified);
   New->HasWrittenPrototype = hasWrittenPrototype;
   return New;
 }
 
 FunctionDecl *FunctionDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) FunctionDecl(Function, nullptr, SourceLocation(),
+  return new (C, ID) FunctionDecl(Function, C, nullptr, SourceLocation(),
                                   DeclarationNameInfo(), QualType(), nullptr,
                                   SC_None, false, false);
 }
@@ -3668,13 +3672,13 @@ void TypeDecl::anchor() { }
 TypedefDecl *TypedefDecl::Create(ASTContext &C, DeclContext *DC,
                                  SourceLocation StartLoc, SourceLocation IdLoc,
                                  IdentifierInfo *Id, TypeSourceInfo *TInfo) {
-  return new (C, DC) TypedefDecl(DC, StartLoc, IdLoc, Id, TInfo);
+  return new (C, DC) TypedefDecl(C, DC, StartLoc, IdLoc, Id, TInfo);
 }
 
 void TypedefNameDecl::anchor() { }
 
 TypedefDecl *TypedefDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) TypedefDecl(nullptr, SourceLocation(), SourceLocation(),
+  return new (C, ID) TypedefDecl(C, nullptr, SourceLocation(), SourceLocation(),
                                  nullptr, nullptr);
 }
 
@@ -3682,12 +3686,12 @@ TypeAliasDecl *TypeAliasDecl::Create(ASTContext &C, DeclContext *DC,
                                      SourceLocation StartLoc,
                                      SourceLocation IdLoc, IdentifierInfo *Id,
                                      TypeSourceInfo *TInfo) {
-  return new (C, DC) TypeAliasDecl(DC, StartLoc, IdLoc, Id, TInfo);
+  return new (C, DC) TypeAliasDecl(C, DC, StartLoc, IdLoc, Id, TInfo);
 }
 
 TypeAliasDecl *TypeAliasDecl::CreateDeserialized(ASTContext &C, unsigned ID) {
-  return new (C, ID) TypeAliasDecl(nullptr, SourceLocation(), SourceLocation(),
-                                   nullptr, nullptr);
+  return new (C, ID) TypeAliasDecl(C, nullptr, SourceLocation(),
+                                   SourceLocation(), nullptr, nullptr);
 }
 
 SourceRange TypedefDecl::getSourceRange() const {