RFR(S) JDK-8206140 [lworld] Move return value null checks into the callee

[Tobias Hartmann]

Hi Ioi,

thanks for looking at this!

On 10.07.2018 07:07, Ioi Lam wrote:
> http://cr.openjdk.java.net/~iklam/valhalla/8206140_lworld_null_check_in_callee.v02/

- doCall.cpp: The cast to NOTNULL is only correct if we know that the return value is a value type
(which can never be null). Currently, you are always casting to NOTNULL which is not correct.
- TestLWorld.java: Where is GetNullAsm and RarelyUsedValueUserAsm defined?

> To improve interpreter speed (so we won't excessively trap into the VM whenever a
> null is returned -- this is especially important for methods that are NOT returning
> a VT), I added 2 bits in Method::_flags. These allow the interpreter to quickly
> check if the areturn is working on a method that returns a VT.
> 
> The flags handling is complicated by the fact that "legacy" classes may return a VT
> (see [1]). Please see my comments around Method::check_returning_vt. I added a new
> test case for this -- see TestLWorld::test78). Anyway, I am not happy with this check,
> so if you can think of a better way, please let me know.

Great that you were able to create a test for this. I don't think we can easily avoid that check but
I think it's sufficient if you add the detailed explanation to the test and remove the else branch
(or add a short comment) in method.cpp.

Someone more familiar with the interpreter/runtime should look at this as well.

Thanks,
Tobias