RFR: 8080106: Refactor setup of parallel GC threads

[Stefan Karlsson]

Hi all,

Please review these patches to unify the ways we specify the number of 
used worker threads. The main goal for these patches is to get rid of 
CollectedHeap::set_par_threads() and CollectedHeap::n_par_threads().

The RFE has been split into multiple sub-tasks:
  8080109: Use single-threaded code in 
Threads::possibly_parallel_oops_do when running with only one worker thread
  8080110: Remove usage of CollectedHeap::n_par_threads() from root 
processing
  8080111: Remove SubTaskDone::_n_threads
  8080112: Replace and remove the last usages of 
CollectedHeap::n_par_threads()
  8080113: Remove CollectedHeap::set_par_threads()

See the description below for each individual patch:

---
http://cr.openjdk.java.net/~stefank/8080109/webrev.00
8080109: Use single-threaded code in Threads::possibly_parallel_oops_do 
when running with only one worker thread

Today, the root processing code differentiates between two types of 
single-threaded executions:

1) n_par_threads() == 0, used from Serial GC and other code paths that 
executes the root processing from the VM Thread.
2) n_par_threads() == 1, used from non-Serial GCs when the 
root_processing is executed by one GC worker thread.

Today, the only code difference is that the latter will use cmpxchg to 
claim the threads in Threads::possibly_parallel_oops_do.

I propose that we use the same code for both values of n_par_threads(), 
and only use the cmpxchg version if we are running with more than one 
worker thread.

---
http://cr.openjdk.java.net/~stefank/8080110/webrev.00
8080110: Remove usage of CollectedHeap::n_par_threads() from root processing

Remove the explicit usages of CollectedHeap::n_par_threads() from the 
root processing code.

The proposal is to pass the number of worker threads via the 
StrongRootsScope object. The StrongRootsScope object is already setup 
from single-threaded code where we know how many worker threads are 
going to be used.

---
http://cr.openjdk.java.net/~stefank/8080111/webrev.00
8080111: Remove SubTaskDone::_n_threads

Remove the need to keep track of the number of worker threads currently 
executing in the sub-tasks of the root processing. Instead use the 
information that is already stored in StrongRootsScope.

---
http://cr.openjdk.java.net/~stefank/8080112/webrev.00
8080112: Replace and remove the last usages of 
CollectedHeap::n_par_threads()

Replace and remove the last usages of CollectedHeap::n_par_threads().

The number of used threads are passed down as arguments to the functions 
that need to know how many threads are currently executing in parallel.

---
http://cr.openjdk.java.net/~stefank/8080113/webrev.00
8080113: Remove CollectedHeap::set_par_threads()

Remove all the calls to the set_par_threads() functions.
---

The combined webrev can be found at:
http://cr.openjdk.java.net/~stefank/8080106/webrev.00/

Tested with JPRT and gc-test-suite

Thanks,
StefanK