Request for review:8020309:Remove InstanceKlass::_cached_class_file_len and record the length together with the cached class file bytes

[Jiangli Zhou]

Hi,

Please review the webrev for JDK-8020309 
<https://jbs.oracle.com/bugs/browse/JDK-8020309>:

   http://cr.openjdk.java.net/~jiangli/8020309/webrev.00/

The _cached_class_file_bytes and _cached_class_file_len are not set/used 
if no jvmti agent is attached. The _cached_class_file_len field can be 
eliminated and the file length can be recorded as part of the cached 
class data in _cached_class_file_bytes for redefined class. The above 
change allocates extra 4bytes when allocating memory for cached class 
file and stores the length as the first 4bytes and the file data as the 
rest. It saves 4 bytes per class. For classes redefined, the memory 
usage is the same.

Tested with following tests and JPRT:

     jdk/test/java/lang/instrument
     jdk/test/com/sun/jdi
     nsk.jvmti
     nsk.jdi
     nsk.hprof

Thanks,
Jiangli
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