Is an uninitialized object in the stack assignable to a reference type?

[陈雨亭]

Hi, dear all,

One of my tests happens to reveal a verification difference between HotSpot
and J9. J9 rejects the code and says that " Type 'uninitialized' (current
frame, stack[0]) is not assignable to 'reference' type ". HotSpot allows.
Can anyone tell me whether an 'uninitialized' object can be loaded from the
stack (and assigned to a reference).

0: new           #2                  // class Search
3: new           #2                  // class Search
6: astore_0
7: ifnonnull     14
10: aload_0
11: ifnonnull     14

Regards,
Yuting

class Search
  minor version: 0
  major version: 52
  flags: ACC_SUPER
Constant pool:
   #1 = Utf8               Search
   #2 = Class              #1             // Search
   #3 = Utf8               java/lang/Object
   #4 = Class              #3             // java/lang/Object
   #5 = Utf8               x
   #6 = Utf8               LSearch;
   #7 = Utf8               y
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = NameAndType        #8:#9          // "<init>":()V
  #11 = Methodref          #4.#10         // java/lang/Object."<init>":()V
  #12 = Utf8               main
  #13 = Utf8               ([Ljava/lang/String;)V
  #14 = Utf8               Code
  #15 = Utf8               StackMapTable
{
  public Search x;
    descriptor: LSearch;
    flags: ACC_PUBLIC

  public static Search y;
    descriptor: LSearch;
    flags: ACC_PUBLIC, ACC_STATIC

  public Search();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #11                 // Method
java/lang/Object."<init>":()V
         4: return

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: new           #2                  // class Search
         3: new           #2                  // class Search
         6: astore_0
         7: ifnonnull     14
        10: aload_0
        11: ifnonnull     14
        14: return
      StackMapTable: number_of_entries = 1
        frame_type = 255 /* full_frame */
          offset_delta = 14
          locals = [ uninitialized 3 ]
          stack = []
}

J9: 
Exception in thread "main" java.lang.VerifyError: JVMVRFY012 stack shape
inconsistent; class=Search, method=main([Ljava/lang/String;)V, pc=11
Exception Details:
  Location:
    Search.main([Ljava/lang/String;)V @11: JBifnonnull
  Reason:
    Type 'uninitialized' (current frame, stack[0]) is not assignable to
'reference' type
  Current Frame:
    bci: @11
    flags: { }
    locals: { 'uninitialized' }
    stack: { 'uninitialized' }
  Stackmap Table:
    full_frame(@14,{Object[#3]},{})