RFR: JDK-8229871: Imporve performance of Method.copy() and leafCopy()

[Kazunori Ogata]

Hi,

May I have review for "JDK-8229871: Imporve performance of Method.copy() 
and leafCopy()"?

Method.copy() and leafCopy() creates a copy of a Method object with 
sharing MethodAccessor object. Since the methodAccessor field is a 
volatile variable, copying this field needs memory fence to ensure the 
field is visible to all threads on the weak memory platforms such as POWER 
and ARM.

When the methodAccessor of the root object is null (i.e., not initialized 
yet), we do not need to copy the null value because this field of the 
copied object has been initialized to null in the constructor. We can 
reduce overhead of the memory fence only when the root's methodAccessor is 
non-null. This change improved performance by 5.8% using a micro benchmark 
that repeatedly invokes Class.getMethods().

Bug: https://bugs.openjdk.java.net/browse/JDK-8229871

Webrev: http://cr.openjdk.java.net/~ogatak/8229871/webrev.00/


By the way, why Method.methodAccessor is volatile, while 
Field.fieldAccessor and Field.overrideFieldAccessor are not volatile?  I 
know the use of volatile reduces probability of creating duplicated method 
accessor, but the chance still exists.  I couldn't find the difference 
between Method and Field classes to make Method.methodAccessor volatile. 
If we can make it non-volatile, it is more preferable than a quick hack 
above.


Regards,
Ogata