static uint64_t LookupFieldBitOffset(CodeGen::CodeGenModule &CGM,
                                     const ObjCInterfaceDecl *OID,
                                     const ObjCImplementationDecl *ID,
                                     const ObjCIvarDecl *Ivar) {
  const ObjCInterfaceDecl *Container = Ivar->getContainingInterface();

  // FIXME: We should eliminate the need to have ObjCImplementationDecl passed
  // in here; it should never be necessary because that should be the lexical
  // decl context for the ivar.

  // If we know have an implementation (and the ivar is in it) then
  // look up in the implementation layout.
  const ASTRecordLayout *RL;
  if (ID && ID->getClassInterface() == Container)
    RL = &CGM.getContext().getASTObjCImplementationLayout(ID);
  else
    RL = &CGM.getContext().getASTObjCInterfaceLayout(Container);

  // Compute field index.
  //
  // FIXME: The index here is closely tied to how ASTContext::getObjCLayout is
  // implemented. This should be fixed to get the information from the layout
  // directly.
  unsigned Index = 0;

  for (const ObjCIvarDecl *IVD = Container->all_declared_ivar_begin(); 
       IVD; IVD = IVD->getNextIvar()) {
    if (Ivar == IVD)
      break;
    ++Index;
  }
  assert(Index < RL->getFieldCount() && "Ivar is not inside record layout!");

  return RL->getFieldOffset(Index);
}

uint64_t CGObjCRuntime::ComputeIvarBaseOffset(CodeGen::CodeGenModule &CGM,
                                              const ObjCImplementationDecl *OID,
                                              const ObjCIvarDecl *Ivar) {
  return LookupFieldBitOffset(CGM, OID->getClassInterface(), OID, Ivar) / 
    CGM.getContext().getCharWidth();
}

CGObjCJit::CGObjCJit(CodeGen::CodeGenModule &cgm)
  : CGObjCRuntime(cgm),
    isUsable(false),
    CGM(cgm),
    VMContext(cgm.getLLVMContext()),
    ObjCTypes(cgm),
    JitInitFunction(0) {

  //puts("Constructing CGObjCJit (host runtime proxy)");

  llvm::PointerType *PtrToInt8Ty =
      llvm::Type::getInt8Ty(VMContext)->getPointerTo();

  NULLPtr = llvm::ConstantPointerNull::get(PtrToInt8Ty);

  // TODO: For now, just searching for pre-loaded objc runtime
  LOAD_FN(_objc_getClass, "objc_getClass");

  // Don't bother to try the rest if the first one wasn't available
  if (_objc_getClass) {
    isUsable = true;
    LOAD_FN(_objc_allocateClassPair,        "objc_allocateClassPair");
    LOAD_FN(_objc_registerClassPair,        "objc_registerClassPair");
    LOAD_FN(_objc_getProtocol,              "objc_getProtocol");
    LOAD_FN(_object_getClass,               "object_getClass");
    LOAD_FN(_sel_registerName,              "sel_registerName");
    LOAD_FN(_class_getClassVariable,        "class_getClassVariable");
    LOAD_FN(_class_addIvar,                 "class_addIvar");
    LOAD_FN(_class_addProtocol,             "class_addProtocol");
    LOAD_FN(_class_getMethodImplementation, "class_getMethodImplementation");
    LOAD_FN(_ivar_getOffset,                "ivar_getOffset");
    LOAD_FN(_NSGetSizeAndAlignment,         "NSGetSizeAndAlignment");

    //puts("Creating .objc_jit_init()");
    llvm::FunctionType *initFuncType =
      llvm::FunctionType::get(llvm::Type::getVoidTy(VMContext), false);

    JitInitFunction =
        cast<llvm::Function>(CGM.getModule().getOrInsertFunction(
                                               llvm::StringRef(".objc_jit_init"),
                                               initFuncType));
    JitInitFunction->setLinkage(llvm::Function::PrivateLinkage);
    JitInitFunction->setCallingConv(llvm::CallingConv::C);
    JitInitBlock =
      llvm::BasicBlock::Create(VMContext, "entry", JitInitFunction);


    // Define VM objc_getClass function

    fn_objc_getClass.init(&CGM, "objc_getClass",
                          ObjCTypes.ClassPtrTy,
                          ObjCTypes.Int8PtrTy,
                          NULL);

    fn_objc_getMetaClass.init(&CGM, "objc_getMetaClass",
                              ObjCTypes.ClassPtrTy,
                              ObjCTypes.Int8PtrTy,
                              NULL);

    fn_objc_getProtocol.init(&CGM, "objc_getProtocol",
                             ObjCTypes.ProtocolPtrTy,
                             ObjCTypes.Int8PtrTy,
                             NULL);

    fn_object_getClass.init(&CGM, "object_getClass",
                            ObjCTypes.ClassPtrTy,
                            ObjCTypes.ObjectPtrTy,
                            NULL);

    // Define VM class_addMethod and class_replaceMethod function calls for
    // the init function

    llvm::Type *ImpParams[] = {ObjCTypes.ObjectPtrTy, ObjCTypes.SelectorPtrTy};
    ImpPtrTy = llvm::FunctionType::get(ObjCTypes.ObjectPtrTy,
                                       ImpParams,
                                       false)->getPointerTo();

    fn_class_addMethod.init(&CGM, "class_addMethod",
                            llvm::Type::getInt8Ty(VMContext),
                            ObjCTypes.ClassPtrTy,
                            ObjCTypes.SelectorPtrTy,
                            ImpPtrTy,
                            ObjCTypes.Int8PtrTy,
                            NULL);

    fn_class_replaceMethod.init(&CGM, "class_replaceMethod",
                                llvm::Type::getInt8Ty(VMContext),
                                ObjCTypes.ClassTy,
                                ObjCTypes.SelectorPtrTy,
                                ImpPtrTy,
                                ObjCTypes.Int8PtrTy,
                                NULL);

    fn_class_getMethodImplementation.init(&CGM, "class_getMethodImplementation",
                                          ImpPtrTy,
                                          ObjCTypes.ClassPtrTy,
                                          ObjCTypes.SelectorPtrTy,
                                          NULL);

    // Unfortunately, even though class_getMethodImplementation is provided by
//.........这里部分代码省略.........

uint64_t CGObjCRuntime::ComputeIvarBaseOffset(CodeGen::CodeGenModule &CGM,
                                              const ObjCInterfaceDecl *OID,
                                              const ObjCIvarDecl *Ivar) {
  return LookupFieldBitOffset(CGM, OID, nullptr, Ivar) /
    CGM.getContext().getCharWidth();
}