Question regarding "native-compiled frame"

[Chris Plummer]

On 9/13/17 6:10 AM, Felix Yang wrote:
> On 7 September 2017 at 19:04, Andrew Dinn <adinn at redhat.com> wrote:
>
>> On 07/09/17 11:35, Felix Yang wrote:
>>>    Thanks for the reply.
>>>    Then when will the last return statement of frame::sender got a chance
>> to
>>> be executed?
>>>    As I see it, when JVM does something in safepoint state and need to
>>> traverse Java thread stack, we never calculate the sender of a
>>> native-compiled frame.
>> It is possible for Java to call out into the JVM and then for the JVM to
>> call back into Java. For example, when a class is loaded the JVM calls
>> into Java to run the class initializer. This re-entry may happen
>> multiple times.
>>
>> In that case a stack walk under the re-entry may find a Java start fame
>> and it's parent frame will be the native frame where Java entered the VM.
>>
> Yes, that's the frame structure.
>
>
>> Note that the native frame will always be returned by the call to
>> sender_for_entry_frame(map). That method skips all the C frames between
>> the Java entry frame and the native frame which exited Java.
>>
> True. And this is handled by the three IF statements of frame::sender
> function.
> It seems to me that the last return statement is not involved in the stack
> walking process, isn't it?
Hi Felix,

Are you talking about the following:

   // Must be native-compiled frame, i.e. the marshaling code for native
   // methods that exists in the core system.
   return frame(sender_sp(), link(), sender_pc());

If so, the Oracle arm port used to have this same code. However, 2-3 
years ago I was doing quite a few fixes to arm stack/frame walking, and 
suspected this code was never executed. In 9 I changed to:

   assert(false, "should not be called for a C frame");
   return frame();

We've never hit this assert in all our testing since. Possibly this 
change could also be made to x86. However, the details of why I thought 
this change was ok has escaped me.

cheers,

Chris
>
> Thanks for your help,
> Felix