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

[Jeremy Manson]

Something that's bitten me with __thread: it isn't async-safe when called
from a shared object on Linux.  Have you vetted to make sure this doesn't
make HS less async-safe?

Jeremy

On Sun, Nov 1, 2015 at 10:40 PM, David Holmes <david.holmes at oracle.com>
wrote:

> 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
>