RFR: 8033764: Remove the usage of StarTask from BufferingOopClosure

[Stefan Karlsson]

Hi all,

please review the following patch:
http://cr.openjdk.java.net/~stefank/8033764/webrev.00/

for the RFE:
https://bugs.openjdk.java.net/browse/JDK-8033764

This is a first step to simplify/unify the code root scanning, something 
we want for the G1 Class Unloading project:
http://openjdk.java.net/jeps/156

Background:

G1 uses the BufferingOopClosure to separate the root iteration from the 
object copying. The oop*/narrowOop* roots are gathered in a buffer and 
then bulk processed. By only timing the processing part and not the root 
iteration, the object copying time can be measured.

Currently, the BufferingOopClosure uses StarTask, from the taskqueue 
code, to differentiate between oop* and narrowOop* roots. The StarTask 
uses the least significant bit to mark whether the address contains an 
oop or a narrowOop. This works for most of the roots, since the 
addresses are aligned and the LSB is always 0. However, oops can be 
embedded as immediates in the JITed code and the addresses for these are 
not necessarily aligned. This prevents us from using StarTasks with oops 
in the CodeCache.

See this comment from g1_process_strong_roots:
...
BufferingOopClosure buf_scan_non_heap_roots(scan_non_heap_roots);

// Walk the code cache/strong code roots w/o buffering, because StarTask
// cannot handle unaligned oop locations.
CodeBlobToOopClosure eager_scan_code_roots(scan_non_heap_roots, true /* 
do_marking */);
...

I suggest that we replace the StartTask usage with another 
implementation that allows the BufferingOopClosures to be used for the 
CodeCache scanning.

thanks,
StefanK
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