【JVM】模板解释器–字节码的resolve过程

1、背景

上文探讨了:【JVM】模板解释器–如何根据字节码生成汇编码?

本篇,我们来关注下字节码的resolve过程。

2、问题及准备工作

上文虽然探讨了字节码到汇编码的过程,但是:

mov %rax,%(rcx,rbx,1) // 0x89 0x04 0x19

其中为什么要指定0×04和0×19呢?

搬出我们的代码:

public int swap2(CallBy a,CallBy b) {
    int t = a.value;
    a.value = b.value;
    b.value  = t;
    return t;
}

换句话讲,我们的汇编代码是要将b.value赋给a.value:

//b.value怎么来的呢?
a.value = b.value

b.value是个整形的field,上述代码的关键字节码是putfield,而模板解释器在初始化的时候(非运行时,这也是模板的意义所在)会调用下面的函数来生成对应的汇编码:

void TemplateTable::putfield_or_static(int byte_no, bool is_static) {
  transition(vtos, vtos);

  const Register cache = rcx;
  const Register index = rdx;
  const Register obj   = rcx;
  const Register off   = rbx;
  const Register flags = rax;
  const Register bc    = c_rarg3;

  /********************************
  * 关键:这个函数在做什么?
  ********************************/
  resolve_cache_and_index(byte_no, cache, index, sizeof(u2));

  jvmti_post_field_mod(cache, index, is_static);

  // 上面resolve后,直接从cp cache中对应的entry中就可以获取到field
  load_field_cp_cache_entry(obj, cache, index, off, flags, is_static);

  // [jk] not needed currently
  // volatile_barrier(Assembler::Membar_mask_bits(Assembler::LoadStore |
  //                                              Assembler::StoreStore));

  Label notVolatile, Done;
  __ movl(rdx, flags);
  __ shrl(rdx, ConstantPoolCacheEntry::is_volatile_shift);
  __ andl(rdx, 0x1);

  // field address
  const Address field(obj, off, Address::times_1);

  Label notByte, notInt, notShort, notChar,
        notLong, notFloat, notObj, notDouble;

  __ shrl(flags, ConstantPoolCacheEntry::tos_state_shift);

  assert(btos == 0, "change code, btos != 0");
  __ andl(flags, ConstantPoolCacheEntry::tos_state_mask);
  __ jcc(Assembler::notZero, notByte);

  // btos
  // ...

  // atos
  // ...

  // itos
  {

  	/***************************************
  	*  itos类型,我们的b.value是个整形,
  	*  所以对应的机器级别的类型是i,表示整形
  	****************************************/

    __ pop(itos);
    if (!is_static) pop_and_check_object(obj);

    // 这里就是生成汇编码,也就是上篇博文探讨的主要内容了
    __ movl(field, rax);

	if (!is_static) {
      patch_bytecode(Bytecodes::_fast_iputfield, bc, rbx, true, byte_no);
    }
    __ jmp(Done);
  }

  __ bind(notInt);
  __ cmpl(flags, ctos);
  __ jcc(Assembler::notEqual, notChar);

  // ctos
  // ...

  // stos
  // ...

  // ltos
  // ...

  // ftos
  // ...

  // dtos
  // ...

  // Check for volatile store
  // ...
}

3、field、class的符号解析及链接

3.1、resolve_cache_and_index

来看看上面代码中的关键点:

// 1. 根据不同的字节码,选择对应的resolve函数.
// 2. 调用resolve函数.
// 3. 根据resolve后的结果,更新寄存器信息,做好衔接.
void TemplateTable::resolve_cache_and_index(int byte_no,
                                            Register Rcache,
                                            Register index,
                                            size_t index_size) {
  const Register temp = rbx;
  assert_different_registers(Rcache, index, temp);

  Label resolved;
    assert(byte_no == f1_byte || byte_no == f2_byte, "byte_no out of range");

    /****************
    * 关键点1
    *****************/

    __ get_cache_and_index_and_bytecode_at_bcp(Rcache, index, temp, byte_no, 1, index_size);
    __ cmpl(temp, (int) bytecode());  // have we resolved this bytecode?
    __ jcc(Assembler::equal, resolved);

  // resolve first time through
  address entry;
  switch (bytecode()) {
  case Bytecodes::_getstatic:
  case Bytecodes::_putstatic:
  case Bytecodes::_getfield:
  case Bytecodes::_putfield:

    /****************
    * 关键点2
    *****************/

    entry = CAST_FROM_FN_PTR(address, InterpreterRuntime::resolve_get_put);
    break;

  // ...

  default:
    fatal(err_msg("unexpected bytecode: %s", Bytecodes::name(bytecode())));
    break;
  }

  // 
  __ movl(temp, (int) bytecode());
  __ call_VM(noreg, entry, temp);

  //
  // Update registers with resolved info
  __ get_cache_and_index_at_bcp(Rcache, index, 1, index_size);
  __ bind(resolved);
}

上面的代码又有两个关键点:

3.2、get_cache_and_index_and_bytecode_at_bcp

get_cache_and_index_and_bytecode_at_bcp函数,主要做的一些工作如下文所述。

cp cache指ConstantPoolCache,注意这不是一个一般意义上的缓存,其目的是用于解释器执行时,对字节码进行resolve的。

  1. 对给定的bytecode,在cp cache中查找是否已经存在,如果不存在要进行resolve.至于cp cache问题,最后再说。
  2. 进行resolve的主要内容:
    – InterpreterRuntime::resolve_get_put
    – InterpreterRuntime::resolve_invoke
    – InterpreterRuntime::resolve_invokehandle
    – InterpreterRuntime::resolve_invokedynamic

3.3、resolve_get_put

因为我们的putfield字节码会选择函数resolve_get_put来进行resolve,来关注这个过程:

IRT_ENTRY(void, InterpreterRuntime::resolve_get_put(JavaThread* thread, Bytecodes::Code bytecode))
  // resolve field
  fieldDescriptor info;
  constantPoolHandle pool(thread, method(thread)->constants());
  bool is_put    = (bytecode == Bytecodes::_putfield  || bytecode == Bytecodes::_putstatic);
  bool is_static = (bytecode == Bytecodes::_getstatic || bytecode == Bytecodes::_putstatic);

  {
    JvmtiHideSingleStepping jhss(thread);

    /*******************
    * 关键点
    ********************/

    LinkResolver::resolve_field_access(info, pool, get_index_u2_cpcache(thread, bytecode),
                                       bytecode, CHECK);
  } // end JvmtiHideSingleStepping

  // check if link resolution caused cpCache to be updated
  if (already_resolved(thread)) return;

  // compute auxiliary field attributes
  TosState state  = as_TosState(info.field_type());

  Bytecodes::Code put_code = (Bytecodes::Code)0;

  InstanceKlass* klass = InstanceKlass::cast(info.field_holder());
  bool uninitialized_static = ((bytecode == Bytecodes::_getstatic || bytecode == Bytecodes::_putstatic) &&
                               !klass->is_initialized());
  Bytecodes::Code get_code = (Bytecodes::Code)0;

  if (!uninitialized_static) {
    get_code = ((is_static) ? Bytecodes::_getstatic : Bytecodes::_getfield);
    if (is_put || !info.access_flags().is_final()) {
      put_code = ((is_static) ? Bytecodes::_putstatic : Bytecodes::_putfield);
    }
  }

  // 设置cp cache entry
  // 1. field的存/取字节码.
  // 2. field所属的InstanceKlass(Java类在VM层面的抽象)指针.
  // 3. index和offset
  // 4. field在机器级别的类型状态.因为机器级别只有i(整)、a(引用)、v(void)等类型,这一点也可以帮助理解为什么解释器在生成汇编代码时,需要判断tos.
  // 5. field是否final的.
  // 6. field是否volatile的.
  // 7. 常量池的holder(InstanceKlass*类型).
  cache_entry(thread)->set_field(
    get_code,
    put_code,
    info.field_holder(),
    info.index(),
    info.offset(),
    state,
    info.access_flags().is_final(),
    info.access_flags().is_volatile(),
    pool->pool_holder()
  );
IRT_END

注意tos这个点:

其中,tos是指 T op– O f– S tack,也就是操作数栈(vm实现中是expression stack)顶的东东的类型.

上面的代码中又标出一个关键点:

3.4、resolve_field_access

看代码:

// 对field进行resolve,并检查其可访问性等信息
void LinkResolver::resolve_field_access(fieldDescriptor& result, constantPoolHandle pool, int index, Bytecodes::Code byte, TRAPS) {
  // Load these early in case the resolve of the containing klass fails

  // 从常量池中获取field符号
  Symbol* field = pool->name_ref_at(index);

  // 从常量池中获取field的签名符号
  Symbol* sig   = pool->signature_ref_at(index);

  // resolve specified klass
  KlassHandle resolved_klass;

  // 关键点1
  resolve_klass(resolved_klass, pool, index, CHECK);

  // 关键点2
  KlassHandle  current_klass(THREAD, pool->pool_holder());
  resolve_field(result, resolved_klass, field, sig, current_klass, byte, true, true, CHECK);
}

注意到上面的代码还调用了resolve_klassresolve_field,我们一个一个看,

3.5、resolve_klass:

// resolve klass
void LinkResolver::resolve_klass(KlassHandle& result, constantPoolHandle pool, int index, TRAPS) {
  Klass* result_oop = pool->klass_ref_at(index, CHECK);
  result = KlassHandle(THREAD, result_oop);
}

上面的代码很简单,从常量池取出对应的klass,并同当前线程一起,封装为一个KlassHandle。

3.6、resolve_field:

再接着看resolve_field:

// field的解析及链接
// 此过程将完成:
//
//   1. field的可访问性验证.
//   2. field所属的类的可访问性验证.
//   3. field所属的类的ClassLoaderData及当前执行的方法(Method)所属的类的ClassLoaderData的验证.
//   4. field所属的类中,如果对其它的类有依赖,要进行装载、解析和链接,如果没有找到,比如classpath中不包含,那么就报类似ClassDefNotFoundError的异常.
//    如果Jar包冲突,也在这里检测到,并报异常.
//    如果field所属的类,及其依赖的类都找到了,那么将ClassLoaderData的约束constraint进行合并.
//   5. 当前正在调用的方法的签名,从callee角度和caller角度来比较是否一致.

// 关于classLoader的问题,后续文章再展开吧,不是一句两句能说的清。
void LinkResolver::resolve_field(fieldDescriptor& fd, KlassHandle resolved_klass, Symbol* field, Symbol* sig,
                                 KlassHandle current_klass, Bytecodes::Code byte, bool check_access, bool initialize_class,
                                 TRAPS) {
  assert(byte == Bytecodes::_getstatic || byte == Bytecodes::_putstatic ||
         byte == Bytecodes::_getfield  || byte == Bytecodes::_putfield  ||
         (byte == Bytecodes::_nop && !check_access), "bad field access bytecode");

  bool is_static = (byte == Bytecodes::_getstatic || byte == Bytecodes::_putstatic);
  bool is_put    = (byte == Bytecodes::_putfield  || byte == Bytecodes::_putstatic);

  // Check if there's a resolved klass containing the field
  if (resolved_klass.is_null()) {
    ResourceMark rm(THREAD);
    THROW_MSG(vmSymbols::java_lang_NoSuchFieldError(), field->as_C_string());
  }

  /************************
  * 关键点1
  *************************/
  // Resolve instance field
  KlassHandle sel_klass(THREAD, resolved_klass->find_field(field, sig, &fd));

  // check if field exists; i.e., if a klass containing the field def has been selected
  if (sel_klass.is_null()) {
    ResourceMark rm(THREAD);
    THROW_MSG(vmSymbols::java_lang_NoSuchFieldError(), field->as_C_string());
  }

  if (!check_access)
    // Access checking may be turned off when calling from within the VM.
    return;

  /************************
  * 关键点2
  *************************/
  // check access
  check_field_accessability(current_klass, resolved_klass, sel_klass, fd, CHECK);

  // check for errors
  if (is_static != fd.is_static()) {

    // ...

    THROW_MSG(vmSymbols::java_lang_IncompatibleClassChangeError(), msg);
  }

  // Final fields can only be accessed from its own class.
  if (is_put && fd.access_flags().is_final() && sel_klass() != current_klass()) {
    THROW(vmSymbols::java_lang_IllegalAccessError());
  }

  // initialize resolved_klass if necessary
  // note 1: the klass which declared the field must be initialized (i.e, sel_klass)
  //         according to the newest JVM spec (5.5, p.170) - was bug (gri 7/28/99)
  //
  // note 2: we don't want to force initialization if we are just checking
  //         if the field access is legal; e.g., during compilation
  if (is_static && initialize_class) {
    sel_klass->initialize(CHECK);
  }

  if (sel_klass() != current_klass()) {
    HandleMark hm(THREAD);
    Handle ref_loader (THREAD, InstanceKlass::cast(current_klass())->class_loader());
    Handle sel_loader (THREAD, InstanceKlass::cast(sel_klass())->class_loader());
    {
      ResourceMark rm(THREAD);

      /************************
      * 关键点3
      *************************/
      Symbol* failed_type_symbol =
        SystemDictionary::check_signature_loaders(sig,
                                                  ref_loader, sel_loader,
                                                  false,
                                                  CHECK);
      if (failed_type_symbol != NULL) {

        // ...

        THROW_MSG(vmSymbols::java_lang_LinkageError(), buf);
      }
    }
  }

  // return information. note that the klass is set to the actual klass containing the
  // field, otherwise access of static fields in superclasses will not work.
}

上面的代码,我们梳理出三个跟本主题相关的关键点,已在注释中标出,我们来看:

// 关键点1 :
// 获取field所属的类或接口对应的klass,或者NULL,如果是NULL就抛异常了
KlassHandle sel_klass(THREAD, resolved_klass->find_field(field, sig, &fd));

// 1. 如果是resolved_klass中的field,返回resolved_klass
// 2. 如果1不满足,尝试返回接口或接口的超类(super interface)对应的klass(递归)
// 3. 如果1、2点都不满足,尝试返回父类或超类对应的klass(递归)或者NULL.
Klass* InstanceKlass::find_field(Symbol* name, Symbol* sig, fieldDescriptor* fd) const {
  // search order according to newest JVM spec (5.4.3.2, p.167).
  // 1) search for field in current klass
  if (find_local_field(name, sig, fd)) {
    return const_cast<InstanceKlass*>(this);
  }
  // 2) search for field recursively in direct superinterfaces
  { Klass* intf = find_interface_field(name, sig, fd);
    if (intf != NULL) return intf;
  }
  // 3) apply field lookup recursively if superclass exists
  { Klass* supr = super();
    if (supr != NULL) return InstanceKlass::cast(supr)->find_field(name, sig, fd);
  }
  // 4) otherwise field lookup fails
  return NULL;
}

// 关键点2:
// 1. resolved_klass来自当前线程所执行的当前方法的当前字节码所属的常量池.
// 2. sel_klass是field所属的类或接口对应的klass
// 3. current_klass是常量池所属的klass(pool_holder).
// 4. 3种klass可以相同,也可以不同.可以想象一个调用链,依赖的各个class.
check_field_accessability(current_klass, resolved_klass, sel_klass, fd, CHECK);

// 关键点3:
// ref_loader代表了current_klass的classLoader
Handle ref_loader (THREAD, InstanceKlass::cast(current_klass())->class_loader());
// sel_loader代表了sel_klass的classLoader
    Handle sel_loader (THREAD, InstanceKlass::cast(sel_klass())->class_loader());
// 根据签名符号sig、ref_loader、sel_loader来检查classLoader的约束是否一致,如果不一致就会抛异常,所谓一致不是相同但包含相同的情况,如果一致,那么就合并约束,同时还要进行依赖(depedencies)链的维护.
// 由于内容比较多,本篇不展开.
Symbol* failed_type_symbol =
        SystemDictionary::check_signature_loaders(sig,
                                                  ref_loader, sel_loader,
                                                  false,
                                                  CHECK);

上面的关键点解析都在注释中了,其中有的地方内容太多,不宜在本篇展开。

那么,如何获取当前执行的字节码对应的cp cache entry呢?

3.7、如何获取cp cache entry:

关键代码如下:

// 获取当前正在执行的bytecode对应的cp cache entry
static ConstantPoolCacheEntry* cache_entry(JavaThread *thread) { 
  return cache_entry_at(thread, Bytes::get_native_u2(bcp(thread) + 1)); 
}

// ↓

// 获取解释器当前的(B)yte (C)ode (P)ointer,也就是当前指令地址,以指针表达
static address   bcp(JavaThread *thread)           { 
  return last_frame(thread).interpreter_frame_bcp(); 
}

// ↓

// 获取cp cache entry
static ConstantPoolCacheEntry* cache_entry_at(JavaThread *thread, int i)  { 
  return method(thread)->constants()->cache()->entry_at(i); 
}

// ↓

// 获取当前正在执行的方法
static Method*   method(JavaThread *thread) { 
  return last_frame(thread).interpreter_frame_method(); 
}

// ↓

// 获取interpreterState->_method,也就是当前正在执行的方法
Method* frame::interpreter_frame_method() const {
  assert(is_interpreted_frame(), "interpreted frame expected");
  Method* m = *interpreter_frame_method_addr();
  assert(m->is_method(), "not a Method*");
  return m;
}

// ↓

// 获取interpreterState->_method的地址
inline Method** frame::interpreter_frame_method_addr() const {
  assert(is_interpreted_frame(), "must be interpreted");
  return &(get_interpreterState()->_method);
}

// ↓

// 获取interpreterState
inline interpreterState frame::get_interpreterState() const {
  return ((interpreterState)addr_at( -((int)sizeof(BytecodeInterpreter))/wordSize ));
}

// ↓

// interpreterState实际是个BytecodeInterpreter型指针
typedef class BytecodeInterpreter* interpreterState;

上述过程总结下:

1、获取bcp,也就是解释器当前正在执行的字节码的地址,以指针形式返回.

2、bcp是通过当前线程的调用栈的最后一帧来获取的,并且是个解释器栈帧.为什么是最后一帧?

方法1 栈帧1 
调用 -> 方法2 栈帧2
...
调用 -> 方法n 栈帧n // 最后一帧

每个方法在调用时都会用一个栈帧frame来描述调用的状态信息,最后调用的方法就是当前方法,所以是取最后一帧.

3、当前方法的地址是通过栈帧中保存的interpreterState来获取的,而这个interpreterState是个BytecodeInterpreter型的解释器,不是模板解释器。

4、获取到方法的地址后,就可以获取到方法所属的常量池了,接着从常量池对应的cp cache中就可以获取到对应的entry了。

5、第4点提到对应,怎么个对应法?想象数组的下标,这个下标是什么呢?就是对bcp的一个整形映射。

3.8、BytecodeInterpreter的一些关键字段

注意BytecodeInterpreter和TemplateInterpreter不是一码事.

BytecodeInterpreter的一些关键字段,帮助理解bcp、thread、cp、cp cache在解释器栈帧中意义:

private:
    JavaThread*           _thread;        // the vm's java thread pointer
    address               _bcp;           // instruction pointer
    intptr_t*             _locals;        // local variable pointer
    ConstantPoolCache*    _constants;     // constant pool cache
    Method*               _method;        // method being executed
    DataLayout*           _mdx;           // compiler profiling data for current bytecode
    intptr_t*             _stack;         // expression stack
    messages              _msg;           // frame manager <-> interpreter message
    frame_manager_message _result;        // result to frame manager
    interpreterState      _prev_link;     // previous interpreter state
    oop                   _oop_temp;      // mirror for interpreted native, null otherwise
    intptr_t*             _stack_base;    // base of expression stack
    intptr_t*             _stack_limit;   // limit of expression stack
    BasicObjectLock*      _monitor_base;  // base of monitors on the native stack

在进行resolve后,字节码就在ConstantPoolCache对应的Entry中了,下一次再执行就不需要resolve。

至于BytecodeInterpreter是个什么解释器,和模板解释器有啥关系,后面再说吧。

4、结语

本文简要探讨了:

字节码的resolve过程。

终。



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