在java中新建对象使用new操作符，对应的Bytecode为_new，通过解释器或者模板执行的代码在开启了tlab时会先在tlab进行fast分配，不满足分配条件才会到heap公共空间分配
在templateTable_x86.cpp中查看_new代码,另外通过搜索可以看到bytecodeinterpreter.cpp中同样也有实现的_new方法，按照引用查看后者是zgc使用的

//注释有用就不删除了,代码主要看流程，bind是打标签，jmp是goto
void TemplateTable::_new() {
  transition(vtos, atos);
  __ get_unsigned_2_byte_index_at_bcp(rdx, 1);
  Label slow_case;
  Label slow_case_no_pop;
  Label done;
  Label initialize_header;
  Label initialize_object;  // including clearing the fields

  __ get_cpool_and_tags(rcx, rax);

  // Make sure the class we're about to instantiate has been resolved.
  // This is done before loading InstanceKlass to be consistent with the order
  // how Constant Pool is updated (see ConstantPool::klass_at_put)
  //判断常量，是就跳到tlab外
  const int tags_offset = Array<u1>::base_offset_in_bytes();
  __ cmpb(Address(rax, rdx, Address::times_1, tags_offset), JVM_CONSTANT_Class);
  __ jcc(Assembler::notEqual, slow_case_no_pop);

  // get InstanceKlass
  __ load_resolved_klass_at_index(rcx, rcx, rdx);
  __ push(rcx);  // save the contexts of klass for initializing the header

  // make sure klass is initialized & doesn't have finalizer
  // make sure klass is fully initialized
  //类已经初始化clinit否则跳出tlab
  __ cmpb(Address(rcx, InstanceKlass::init_state_offset()), InstanceKlass::fully_initialized);
  __ jcc(Assembler::notEqual, slow_case);

  // get instance_size in InstanceKlass (scaled to a count of bytes)
  __ movl(rdx, Address(rcx, Klass::layout_helper_offset()));
  // test to see if it has a finalizer or is malformed in some way
  //并且不能有回收方法
  __ testl(rdx, Klass::_lh_instance_slow_path_bit);
  __ jcc(Assembler::notZero, slow_case);

  // Allocate the instance:
  //  If TLAB is enabled:
  //    Try to allocate in the TLAB.
  //    If fails, go to the slow path.
  //  Else If inline contiguous allocations are enabled:
  //    Try to allocate in eden.
  //    If fails due to heap end, go to slow path.
  //
  //  If TLAB is enabled OR inline contiguous is enabled:
  //    Initialize the allocation.
  //    Exit.
  //
  //  Go to slow path.

  const bool allow_shared_alloc =
    Universe::heap()->supports_inline_contig_alloc();

  const Register thread = LP64_ONLY(r15_thread) NOT_LP64(rcx);
#ifndef _LP64
  if (UseTLAB || allow_shared_alloc) {
    __ get_thread(thread);
  }
#endif // _LP64

  if (UseTLAB) {
  //tlab分配，将slow case标记点传入，分配失败走slow
  //rax在get_cpool_and_tags中设置了对象在常量池的地址，可以理解成class中常量池，全限定名对应的对象地址
  //rdx是对象大小，基本都是一些地址和8种基本数据类型的集合
    __ tlab_allocate(thread, rax, rdx, 0, rcx, rbx, slow_case);
    if (ZeroTLAB) {
      // the fields have been already cleared
      __ jmp(initialize_header);
    } else {
      // initialize both the header and fields
      __ jmp(initialize_object);
    }
  } else {
    // Allocation in the shared Eden, if allowed.
    // rdx: instance size in bytes
    //tlab走不到这里,不开也会跳到slow_case
    __ eden_allocate(thread, rax, rdx, 0, rbx, slow_case);
  }

  // If UseTLAB or allow_shared_alloc are true, the object is created above and
  // there is an initialize need. Otherwise, skip and go to the slow path.
  //分配成功才会进入这里
  if (UseTLAB || allow_shared_alloc) {
     // The object is initialized before the header.  If the object size is
    // zero, go directly to the header initialization.
    __ bind(initialize_object);
    __ decrement(rdx, sizeof(oopDesc));
    __ jcc(Assembler::zero, initialize_header);

    // Initialize topmost object field, divide rdx by 8, check if odd and
    // test if zero.
    __ xorl(rcx, rcx);    // use zero reg to clear memory (shorter code)
    __ shrl(rdx, LogBytesPerLong); // divide by 2*oopSize and set carry flag if odd

    // rdx must have been multiple of 8
#ifdef ASSERT
    // make sure rdx was multiple of 8
    Label L;
    // Ignore partial flag stall after shrl() since it is debug VM
    __ jcc(Assembler::carryClear, L);
    __ stop("object size is not multiple of 2 - adjust this code");
    __ bind(L);
    // rdx must be > 0, no extra check needed here
#endif

    // initialize remaining object fields: rdx was a multiple of 8
    { Label loop;
    __ bind(loop);
    __ movptr(Address(rax, rdx, Address::times_8, sizeof(oopDesc) - 1*oopSize), rcx);
    NOT_LP64(__ movptr(Address(rax, rdx, Address::times_8, sizeof(oopDesc) - 2*oopSize), rcx));
    __ decrement(rdx);
    __ jcc(Assembler::notZero, loop);
    }

    // initialize object header only.
    __ bind(initialize_header);
    if (UseBiasedLocking) {
      __ pop(rcx);   // get saved klass back in the register.
      __ movptr(rbx, Address(rcx, Klass::prototype_header_offset()));
      __ movptr(Address(rax, oopDesc::mark_offset_in_bytes ()), rbx);
    } else {
      __ movptr(Address(rax, oopDesc::mark_offset_in_bytes ()),
                (intptr_t)markWord::prototype().value()); // header
      __ pop(rcx);   // get saved klass back in the register.
    }
#ifdef _LP64
    __ xorl(rsi, rsi); // use zero reg to clear memory (shorter code)
    __ store_klass_gap(rax, rsi);  // zero klass gap for compressed oops
#endif
    Register tmp_store_klass = LP64_ONLY(rscratch1) NOT_LP64(noreg);
    __ store_klass(rax, rcx, tmp_store_klass);  // klass

    {
      SkipIfEqual skip_if(_masm, &DTraceAllocProbes, 0);
      // Trigger dtrace event for fastpath
      __ push(atos);
      __ call_VM_leaf(
           CAST_FROM_FN_PTR(address, SharedRuntime::dtrace_object_alloc), rax);
      __ pop(atos);
    }

    __ jmp(done);
  }

  // slow case
  __ bind(slow_case);
  __ pop(rcx);   // restore stack pointer to what it was when we came in.
  __ bind(slow_case_no_pop);

  Register rarg1 = LP64_ONLY(c_rarg1) NOT_LP64(rax);
  Register rarg2 = LP64_ONLY(c_rarg2) NOT_LP64(rdx);

  __ get_constant_pool(rarg1);
  __ get_unsigned_2_byte_index_at_bcp(rarg2, 1);
  //slow分配都要走这里
  call_VM(rax, CAST_FROM_FN_PTR(address, InterpreterRuntime::_new), rarg1, rarg2);
   __ verify_oop(rax);

  // continue
  __ bind(done);
}
//InterpreterRuntime::_new中的分配空间方法
instanceOop InstanceKlass::allocate_instance(TRAPS) {
  bool has_finalizer_flag = has_finalizer(); // Query before possible GC
  int size = size_helper();  // Query before forming handle.

  instanceOop i;

  i = (instanceOop)Universe::heap()->obj_allocate(this, size, CHECK_NULL);
  if (has_finalizer_flag && !RegisterFinalizersAtInit) {
    i = register_finalizer(i, CHECK_NULL);
  }
  return i;
}
//heap()->obj_allocate
inline oop CollectedHeap::obj_allocate(Klass* klass, int size, TRAPS) {
  ObjAllocator allocator(klass, size, THREAD);
  return allocator.allocate();
}
// allocator.allocate()
oop MemAllocator::allocate() const {
  oop obj = NULL;
  {
    Allocation allocation(*this, &obj);
    HeapWord* mem = mem_allocate(allocation);
    if (mem != NULL) {
      obj = initialize(mem);
    } else {
      // The unhandled oop detector will poison local variable obj,
      // so reset it to NULL if mem is NULL.
      obj = NULL;
    }
  }
  return obj;
}
//HeapWord* mem = mem_allocate(allocation);
HeapWord* MemAllocator::mem_allocate(Allocation& allocation) const {
  if (UseTLAB) {
  //在tlab分配，如果失败进行tlab扩容
    HeapWord* result = allocate_inside_tlab(allocation);
    if (result != NULL) {
      return result;
    }
  }
  //heap中分配
  return allocate_outside_tlab(allocation);
}

结合java常识大意可以看出，new创建对象时
1.先判断是常量,未进行类初始化，有回收方法的，都要从heap分配
2.tlab快速分配，成功后跳出
3.heap分配，开启了tlab后，heap也会尝试在tlab分配，失败后进行扩容再分配
4.未开启tlab，走heap分配