ASTWriter: Cache some DenseMaps we use repeatedly.

- This reduces our total # of allocations building a PCH for Cocoa.h by almost
   a whopping 50%.
 - A SmallPtrMap would be cleaner, but since we don't have one yet...

llvm-svn: 151697

clang/include/clang/Serialization/ASTWriter.h

@@ -207,6 +207,18 @@ private:
   /// IdentifierInfo.
   llvm::DenseMap<const IdentifierInfo *, serialization::IdentID> IdentifierIDs;
 
+  /// @name FlushStmt Caches
+  /// @{
+
+  /// \brief Set of parent Stmts for the currently serializing sub stmt.
+  llvm::DenseSet<Stmt *> ParentStmts;
+
+  /// \brief Offsets of sub stmts already serialized. The offset points
+  /// just after the stmt record.
+  llvm::DenseMap<Stmt *, uint64_t> SubStmtEntries;
+
+  /// @}
+
   /// \brief Offsets of each of the identifier IDs into the identifier
   /// table.
   std::vector<uint32_t> IdentifierOffsets;

clang/lib/Serialization/ASTWriterStmt.cpp

@@ -1586,11 +1586,10 @@ void ASTWriter::WriteSubStmt(Stmt *S,
 void ASTWriter::FlushStmts() {
   RecordData Record;
 
-  /// \brief Set of parent Stmts for the currently serializing sub stmt.
-  llvm::DenseSet<Stmt *> ParentStmts;
-  /// \brief Offsets of sub stmts already serialized. The offset points
-  /// just after the stmt record.
-  llvm::DenseMap<Stmt *, uint64_t> SubStmtEntries;
+  // We expect to be the only consumer of the two temporary statement maps,
+  // assert that they are empty.
+  assert(SubStmtEntries.empty() && "unexpected entries in sub stmt map");
+  assert(ParentStmts.empty() && "unexpected entries in parent stmt map");
 
   for (unsigned I = 0, N = StmtsToEmit.size(); I != N; ++I) {
     WriteSubStmt(StmtsToEmit[I], SubStmtEntries, ParentStmts);