(L) Prelim RFR: 8132510: Replace ThreadLocalStorage with compiler/language-based thread-local variables

[David Holmes]

bug: https://bugs.openjdk.java.net/browse/JDK-8132510

Open webrev: http://cr.openjdk.java.net/~dholmes/8132510/webrev.v2/

A simple (in principle) but wide-ranging change which should appeal to 
our Code Deletion Engineer's. We implement Thread::current() using a 
compiler/language-based thread-local variable eg:


  static __thread Thread *_thr_current;

  inline Thread* Thread::current() {
    return _thr_current;
  }

with an appropriate setter of course. By doing this we can completely 
remove the platform-specific ThreadLocalStorage implementations, and the 
associated os::thread_local_storage* calls, plus all the uses of 
ThreadLocalStorage::thread() and ThreadLocalStorage::get_thread_slow(). 
This extends the previous work done on Solaris to implement 
ThreadLocalStorage::thread() using compiler-based thread-locals.

We can also consolidate nearly all the os_cpu versions of 
MacroAssembler::get_thread on x86 into one cpu specific one ( a special 
variant is still needed for 32-bit Windows).

As a result of this change we have further potential cleanups:
- all the src/os/<os>/vm/thread_<os>.inline.hpp files are now completely 
empty and could also be removed
- the MINIMIZE_RAM_USAGE define (which avoids use of the linux sp-map 
"cache" on 32-bit) now has no affect and so could be completely removed 
from the build system

I plan to do the MINIMIZE_RAM_USAGE removal as a follow up CR, but could 
add the removal of the "inline" files to this CR if people think it 
worth removing them.

I have one missing piece on Aarch64 - I need to change 
MacroAssembler::get_thread to simply call Thread::current() as on other 
platforms, but I don't know how to write that. I would appreciate it if 
someone could give me the right code for that.

I would also appreciate comments/testing by the AIX and PPC64 folk as well.

A concern about memory-leaks had previously been raised, but experiments 
using simple C code on linux 86 and Solaris showed no issues. Also note 
that Aarch64 already uses this kind of thread-local.

Thanks,
David