When we deserialize a declaration from a module file, allocate extra

storage for the global declaration ID. Declarations that are parsed
(rather than deserialized) are unaffected, so the number of
declarations that pay this cost tends to be relatively small (since
relatively few declarations are ever deserialized).

This replaces a largish DenseMap within the AST reader. It's not
strictly a win in terms of memory use---not every declaration was
added to that DenseMap in the first place---but it's cleaner to have
this information available for every deserialized declaration, so that
future clients can rely on it.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147617 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/DeclBase.h

@@ -532,6 +532,14 @@ public:
   /// a precompiled header or module) rather than having been parsed.
   bool isFromASTFile() const { return FromASTFile; }
 
+  /// \brief Retrieve the global declaration ID associated with this 
+  /// declaration, which specifies where in the 
+  unsigned getGlobalID() const { 
+    if (isFromASTFile())
+      return *((const unsigned*)this - 1);
+    return 0;
+  }
+              
   unsigned getIdentifierNamespace() const {
     return IdentifierNamespace;
   }

include/clang/Serialization/ASTReader.h

@@ -675,13 +675,6 @@ private:
   /// \brief Keeps track of the elements added to PendingDeclChains.
   llvm::SmallSet<serialization::DeclID, 16> PendingDeclChainsKnown;
 
-  /// \brief Reverse mapping from declarations to their global declaration IDs.
-  /// 
-  /// FIXME: This data structure is currently only used for ObjCInterfaceDecls
-  /// and ObjCProtocolDecls to support declaration merging. If we must have 
-  /// this for other declarations, allocate it along with the Decl itself.
-  llvm::DenseMap<Decl *, serialization::GlobalDeclID> DeclToID;
-  
   typedef llvm::DenseMap<Decl *, llvm::SmallVector<serialization::DeclID, 2> >
     MergedDeclsMap;
     

lib/AST/DeclBase.cpp

@@ -44,7 +44,20 @@ static bool StatSwitch = false;
 void *Decl::AllocateDeserializedDecl(const ASTContext &Context, 
                                      unsigned ID,
                                      unsigned Size) {
-  return Context.Allocate(Size);
+  // Allocate an extra pointer's worth of storage, which ensures that 
+  //   (1) We have enough storage to stash the global declaration ID, and
+  //   (2) We maintain pointer alignment.
+  //
+  // Note that this wastes 4 bytes on x86-64, which we'll undoubtedly end up
+  // finding a use for later.
+  void *Start = Context.Allocate(Size + sizeof(void*));
+  void *Result = (char*)Start + sizeof(void*);
+  
+  // Store the global declaration ID 
+  unsigned *IDPtr = (unsigned*)Result - 1;
+  *IDPtr = ID;
+  
+  return Result;
 }
 
 const char *Decl::getDeclKindName() const {

lib/Serialization/ASTReaderDecl.cpp

@@ -402,9 +402,6 @@ void ASTDeclReader::VisitTypeDecl(TypeDecl *TD) {
 }
 
 void ASTDeclReader::VisitTypedefNameDecl(TypedefNameDecl *TD) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[TD] = ThisDeclID;
-  
   RedeclarableResult Redecl = VisitRedeclarable(TD);
   VisitTypeDecl(TD);
   
@@ -421,9 +418,6 @@ void ASTDeclReader::VisitTypeAliasDecl(TypeAliasDecl *TD) {
 }
 
 void ASTDeclReader::VisitTagDecl(TagDecl *TD) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[TD] = ThisDeclID;
-    
   RedeclarableResult Redecl = VisitRedeclarable(TD);
   VisitTypeDecl(TD);
   
@@ -490,9 +484,6 @@ void ASTDeclReader::VisitDeclaratorDecl(DeclaratorDecl *DD) {
 }
 
 void ASTDeclReader::VisitFunctionDecl(FunctionDecl *FD) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[FD] = ThisDeclID;
-  
   RedeclarableResult Redecl = VisitRedeclarable(FD);
   VisitDeclaratorDecl(FD);
 
@@ -673,9 +664,6 @@ void ASTDeclReader::VisitObjCContainerDecl(ObjCContainerDecl *CD) {
 }
 
 void ASTDeclReader::VisitObjCInterfaceDecl(ObjCInterfaceDecl *ID) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[ID] = ThisDeclID;
-  
   RedeclarableResult Redecl = VisitRedeclarable(ID);
   VisitObjCContainerDecl(ID);
   TypeIDForTypeDecl = Reader.getGlobalTypeID(F, Record[Idx++]);
@@ -746,9 +734,6 @@ void ASTDeclReader::VisitObjCIvarDecl(ObjCIvarDecl *IVD) {
 }
 
 void ASTDeclReader::VisitObjCProtocolDecl(ObjCProtocolDecl *PD) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[PD] = ThisDeclID;
-  
   RedeclarableResult Redecl = VisitRedeclarable(PD);
   VisitObjCContainerDecl(PD);
   mergeRedeclarable(PD, Redecl);
@@ -880,9 +865,6 @@ void ASTDeclReader::VisitIndirectFieldDecl(IndirectFieldDecl *FD) {
 }
 
 void ASTDeclReader::VisitVarDecl(VarDecl *VD) {
-  // Record the declaration -> global ID mapping.
-  Reader.DeclToID[VD] = ThisDeclID;
-  
   RedeclarableResult Redecl = VisitRedeclarable(VD);
   VisitDeclaratorDecl(VD);
   
@@ -1561,7 +1543,7 @@ void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *D,
         // Introduce ExistingCanon into the set of pending declaration chains,
         // if in fact it came from a module file.
         if (ExistingCanon->isFromASTFile()) {
-          GlobalDeclID ExistingCanonID = Reader.DeclToID[ExistingCanon];
+          GlobalDeclID ExistingCanonID = ExistingCanon->getGlobalID();
           assert(ExistingCanonID && "Unrecorded canonical declaration ID?");
           if (Reader.PendingDeclChainsKnown.insert(ExistingCanonID))
             Reader.PendingDeclChains.push_back(ExistingCanonID);

lib/Serialization/ASTWriter.cpp

@@ -2914,7 +2914,7 @@ void ASTWriter::WriteMergedDecls() {
   for (ASTReader::MergedDeclsMap::iterator I = Chain->MergedDecls.begin(),
                                         IEnd = Chain->MergedDecls.end();
        I != IEnd; ++I) {
-    DeclID CanonID = I->first->isFromASTFile()? Chain->DeclToID[I->first]
+    DeclID CanonID = I->first->isFromASTFile()? I->first->getGlobalID()
                                               : getDeclID(I->first);
     assert(CanonID && "Merged declaration not known?");