Change LLVM bitcode linking to use tree reduction to scale better

llvm/tools/lto/LTOCodeGenerator.h

 //===-LTOCodeGenerator.h - LLVM Link Time Optimizer -----------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file declares the LTOCodeGenerator class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LTO_CODE_GENERATOR_H
 #define LTO_CODE_GENERATOR_H
 
 #include "llvm/Linker.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm-c/lto.h"
 #include <string>
+#include <vector>
 
 namespace llvm {
   class LLVMContext;
   class GlobalValue;
   class Mangler;
   class MemoryBuffer;
   class TargetMachine;
   class raw_ostream;
 }
 
 //===----------------------------------------------------------------------===//
 /// LTOCodeGenerator - C++ class which implements the opaque lto_code_gen_t
 /// type.
 ///
 struct LTOCodeGenerator {
   static const char *getVersionString();
 
   LTOCodeGenerator();
   ~LTOCodeGenerator();
 
   bool addModule(struct LTOModule*, std::string &errMsg);
+  // @LOCALMOD-BEGIN
+  // Alternative methods of adding modules, which delay merging modules until
+  // all modules are available.
+  bool gatherModuleForLinking(struct LTOModule*);
+  bool linkGatheredModulesAndDispose(std::string &errMsg);
+  // @LOCALMOD-END
   bool setDebugInfo(lto_debug_model, std::string &errMsg);
   bool setCodePICModel(lto_codegen_model, std::string &errMsg);
 
   void setCpu(const char* mCpu) { _mCpu = mCpu; }
 
   void addMustPreserveSymbol(const char* sym) {
     _mustPreserveSymbols[sym] = 1;
   }
 
   bool writeMergedModules(const char *path, std::string &errMsg);
@@ skipping 27 matching lines @@
   llvm::TargetMachine*        _target;
   bool                        _emitDwarfDebugInfo;
   bool                        _scopeRestrictionsDone;
   lto_codegen_model           _codeModel;
   StringSet                   _mustPreserveSymbols;
   StringSet                   _asmUndefinedRefs;
   llvm::MemoryBuffer*         _nativeObjectFile;
   std::vector<char*>          _codegenOptions;
   std::string                 _mCpu;
   std::string                 _nativeObjectPath;
+
+  // @LOCALMOD
+  std::vector<LTOModule*> _gatheredModules;
 };
 
 #endif // LTO_CODE_GENERATOR_H