RFR: JDK-8073321: assert(q > prev_q) failed: we should be moving forward through memory

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?

http://cr.openjdk.java.net/~brutisso/8073321/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8073321

Background:

The assert happens in scan_and_adjust_pointers() and it could happen if 
scan_and_forward() is not behaving properly. What could happen is that 
the all object up to _first_dead should not be moved, which means that 
they should all be "unmarked" even though they are under the _first_dead 
value. However, the code just checks the first object. So, if 
scan_and_forward() would for some reason happen to mark the first object 
but not the rest we will read a mark word instead of a pointer to a live 
object in scan_and_adjust_pointers().

I have not been able to reproduce this problem. But I changed the code 
in scan_and_adjust_pointers() to not just check the first object, but 
instead treat all objects up to _first_dead as non-moved objects. This 
should make the code more stable.

To be able to understand the code while I was trying to look for a 
problem in it I had to rename some variables and make some refactorings. 
I think it is worth pushing this renamed and refactored code. And I 
think it is worth closing JDK-8073321 even if I have not been able to 
prove that my patch actually fixes the specific issue in the bug report. 
The problem has only ever happened twice so I think that with the better 
assert message added by JDK-8153203 we have a better chance of catching 
any other problems if it happens again and JDK-8073321 is closed.

Here are some incremental webrevs to make it easier to review this change:

Renaming variables in scan_and_adjust_pointers():
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_and_adjust_pointers-rename/

The fix to make sure to treat all objects below _first_dead correctly:
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_and_adjust_pointers-refactor/

Renaming variable in scan_and_forward() (and some small refactoring):
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_and_forward-rename/

Refactoring of scan_and_forward(), in particular introducing a 
DeadSpacer class to handle the special case for the non-G1 case:
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_and_forward-refactor/
(BTW, I would very much appreciate suggestions for a better name for 
DeadSpacer.)

Renaming variables in scan_and_compact():
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_and_compact-rename/

Refactoring scan_and_compact():
http://cr.openjdk.java.net/~brutisso/8073321/webrev.00-scan_amd_compact-refactor/

Thanks,
Bengt