HotSpot and IBM's J9 behave a little different when verifying this class

[Yuting Chen]

It seems that J9 forgot to check the flags when athrow follows.

On Thu, Apr 13, 2017 at 4:21 PM, Michael Rasmussen
<michael.rasmussen at zeroturnaround.com> wrote:
> On 14 April 2017 at 02:15, Yuting Chen <chenyt.cs.sjtu at gmail.com> wrote:
>> Thanks, Remi and Michael.
>>
>> The difference appears only when all of the paths end in athrow. Then
>> what does the comment ("Return TRUE if all code paths starting with
>> start_bc_offset end in bytecode athrow or loop") in verifier.cpp mean?
>> It seems that J9 follows this comment.
>>
>> Both verifiers can decline the class if "athrow" is changed to "return".
>
> Yes, for constructors, if exit is via return, then there cannot be any
> uninitializedThis, but allowed if exit is via athrow.
>
> But, your problem is, that your two frame aren't assignable to each other.
>>>> Current Frame:
>>>>    bci: @1
>>>>    flags: { flagThisUninit }
>>>>    locals: { uninitializedThis, integer, integer }
>>>>    stack: { integer }
>>>>  Stackmap Frame:
>>>>    bci: @11
>>>>    flags: { }
>>>>    locals: { top, integer, integer }
>>>>    stack: { }
>
> While the locals are compatible, the flags are not, you cannot go from
> { flagThisUninit } to {}
> http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/tip/src/share/vm/classfile/stackMapFrame.cpp#l190
>
> /Michael