RFR 8140685: Fix backtrace building to not rely on constant pool merging

[David Holmes]

Hi Coleen,

Just a superficial review as I'm not familiar with this code.

On 31/10/2015 8:16 AM, Coleen Phillimore wrote:
>
> On 10/30/15 6:01 PM, Coleen Phillimore wrote:
>> Summary: Save Throwable in a list which is a weak GC root for walking
>> during MetadataOnStackMark

I'm unclear on terminology here. At the Java level an object that is 
only weakly reachable can be gc'd. But here we seem to use a "weak" root 
to prevent something being gc'd - ???

>> This was the original implementation that I had for finding Method* in
>> backtraces but it was slow because I had used jweak to save the
>> Throwable object.   Using internal vm weak GC roots, this does not
>> cause a performance regression in refWorkload.  This fix is more
>> straightforward and does not rely on the constant pool index of the
>> merged constant pool during redefinition to find the method's name.
>> This is one fix that enables removing merged constant pools during
>> redefinition (work in progress).  It also is would be used as a model
>> for JEP 259: Stack Walking API
>> https://bugs.openjdk.java.net/browse/JDK-8043814.
>>
>> The code that registers MemberNames for replacing Method* during
>> redefinition may need to use the same mechanism for performance, if
>> MemberName arrays are used for the Stack Walking API.
>>
>> I assume this will generate comments from the GC group.
>>
>> bug link https://bugs.openjdk.java.net/browse/JDK-8140685
>>
>
> open webrev at http://cr.openjdk.java.net/~coleenp/8140685.01/
> bug link https://bugs.openjdk.java.net/browse/JDK-8140685

Some minor comments:

src/share/vm/classfile/javaClasses.hpp

Typo:   // Mark methods as that are

src/share/vm/classfile/backtrace.hpp

_backtraces_head should be volatile as you update via CAS.

src/share/vm/classfile/backtrace.cpp

Minor nit: the "exchanged" variable in add should really be called "found".

Thanks,
David
-----

>>
>> Tested with RBT quick (former "quick" tests), jdk/java/lang/instrument
>> tests and the test that I wrote for handling backtraces with
>> redefinition in hotspot/test/runtime/RedefineTests.
>>
>> Thanks,
>> Coleen
>