Introduce a constant for the number of predefined declarations in an

AST file, along with an enumeration naming those predefined
declarations. No functionality change, but this will make it easier to
introduce new predefined declarations, when/if we need them.

llvm-svn: 136781

clang/include/clang/Serialization/ASTBitCodes.h

@@ -56,8 +56,9 @@ namespace clang {
     /// \brief An ID number that refers to a declaration in an AST file.
     ///
     /// The ID numbers of declarations are consecutive (in order of
-    /// discovery) and start at 2. 0 is reserved for NULL, and 1 is
-    /// reserved for the translation unit declaration.
+    /// discovery), with values below NUM_PREDEF_DECL_IDS being reserved. 
+    /// At the start of a chain of precompiled headers, declaration ID 1 is 
+    /// used for the translation unit declaration.
     typedef uint32_t DeclID;
 
     /// \brief a Decl::Kind/DeclID pair.
@@ -661,6 +662,23 @@ namespace clang {
       SPECIAL_TYPE_AUTO_RREF_DEDUCT            = 18
     };
 
+    /// \brief Predefined declaration IDs.
+    ///
+    /// These declaration IDs correspond to predefined declarations in the AST
+    /// context, such as the NULL declaration ID. Such declarations are never
+    /// actually serialized, since they will be built by the AST context when 
+    /// it is created.
+    enum PredefinedDeclIDs {
+      /// \brief The NULL declaration.
+      PREDEF_DECL_NULL_ID       = 0,
+    };
+
+    /// \brief The number of declaration IDs that are predefined.
+    ///
+    /// For more information about predefined declarations, see the
+    /// \c PredefinedDeclIDs type and the PREDEF_DECL_*_ID constants.
+    const unsigned int NUM_PREDEF_DECL_IDS = 1;
+    
     /// \brief Record codes for each kind of declaration.
     ///
     /// These constants describe the declaration records that can occur within

clang/lib/Serialization/ASTReader.cpp

@@ -4119,26 +4119,32 @@ TranslationUnitDecl *ASTReader::GetTranslationUnitDecl() {
 
 serialization::DeclID 
 ASTReader::getGlobalDeclID(Module &F, unsigned LocalID) const {
-  if (LocalID == 0)
+  if (LocalID < NUM_PREDEF_DECL_IDS)
     return LocalID;
 
   ContinuousRangeMap<uint32_t, int, 2>::iterator I
-    = F.DeclRemap.find(LocalID - 1);
+    = F.DeclRemap.find(LocalID - NUM_PREDEF_DECL_IDS);
   assert(I != F.DeclRemap.end() && "Invalid index into decl index remap");
   
   return LocalID + I->second;
 }
 
 Decl *ASTReader::GetDecl(DeclID ID) {
-  if (ID == 0)
+  if (ID < NUM_PREDEF_DECL_IDS) {    
+    switch ((PredefinedDeclIDs)ID) {
+    case serialization::PREDEF_DECL_NULL_ID:
+      return 0;
+    }
+    
     return 0;
-
+  }
+  
   if (ID > DeclsLoaded.size()) {
     Error("declaration ID out-of-range for AST file");
     return 0;
   }
 
-  unsigned Index = ID - 1;
+  unsigned Index = ID - NUM_PREDEF_DECL_IDS;
   if (!DeclsLoaded[Index]) {
     ReadDeclRecord(ID);
     if (DeserializationListener)

clang/lib/Serialization/ASTWriter.cpp

@@ -2039,7 +2039,7 @@ void ASTWriter::WriteTypeDeclOffsets() {
   Record.clear();
   Record.push_back(DECL_OFFSET);
   Record.push_back(DeclOffsets.size());
-  Record.push_back(FirstDeclID - 1);
+  Record.push_back(FirstDeclID - NUM_PREDEF_DECL_IDS);
   Stream.EmitRecordWithBlob(DeclOffsetAbbrev, Record, data(DeclOffsets));
 }
 
@@ -2740,7 +2740,7 @@ void ASTWriter::SetSelectorOffset(Selector Sel, uint32_t Offset) {
 
 ASTWriter::ASTWriter(llvm::BitstreamWriter &Stream)
   : Stream(Stream), Chain(0), SerializationListener(0), 
-    FirstDeclID(1), NextDeclID(FirstDeclID),
+    FirstDeclID(NUM_PREDEF_DECL_IDS), NextDeclID(FirstDeclID),
     FirstTypeID(NUM_PREDEF_TYPE_IDS), NextTypeID(FirstTypeID),
     FirstIdentID(1), NextIdentID(FirstIdentID), FirstSelectorID(1),
     NextSelectorID(FirstSelectorID), FirstMacroID(1), NextMacroID(FirstMacroID),