RFR: 8132510: Replace ThreadLocalStorage with compiler/language-based thread-local variables

Bertrand Delsart bertrand.delsart at oracle.com
Mon Nov 23 10:14:05 UTC 2015

Hi David,

Overall looks good.
Thanks for the #ifndef USE_LIBRARY_BASED_TLS_ONLY :-)

One doubt, in case this has not been discussed before.

I'm still catching up on x86_64 (after years of ARM assembly :-)) but it 
seems that there are some stack alignment constraints on runtime calls, 
at least for some x86_64 ABIs.

Some of the x86 MacroAssembler::get_thread implementations had code to 
align the stack before calling pthread_getspecific. See for instance

This alignment is no longer performed in the new (shared) implementation 

Now, Solaris was not performing the alignment and Windows has a separate 
path for x86_64. Did we really need the alignment for linux x86_64 and 
bsd_x86_64 ? Might it be needed for other ports ?

IMHO, it might be safer to align the stack by default, knowing it should 
not be expensive since we call get_thread rarely for x86_64 (result is 
cached in r15). I'll let you see whether it is worth adding an ifdef so 
as to explicitly deactivate the alignment on some platforms 
(solaris_x86_64 ?)



On 23/11/2015 08:03, David Holmes wrote:
> After all the preliminary discussions here are final proposed changes:
> bug: https://bugs.openjdk.java.net/browse/JDK-8132510
> Open webrev: http://cr.openjdk.java.net/~dholmes/8132510/webrev.v6/
> 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 os_cpu-specific ThreadLocalStorage implementations, and the
> associated os::thread_local_storage* calls.
> As __thread is not async-signal-safe (either in theory or practice) we
> need a fallback library-based solution as used today. For this we use a
> very simple ThreadLocalStorage class and an implementation thereof for
> POSIX (covers AIX, BSD, Linux, OSX and Solaris) using
> pthread_get/setspecific; and one for Windows using its TLS library.
> While these library routines are not guaranteed async-signal-safe, they
> seem to be in practice and are what we have been using all along.
> We also allow for use of only the library-based TLS for platforms where
> compiler-based thread locals are not supported (as will be needed in the
> Mobile project). This is enabled at build time by defining
> Thanks to Andrew Haley for providing the Aarch64 code; and for Thomas
> Stuefe for testing PPC and AIX.
> Testing:
>   - JPRT (core platforms)
>   - Jtreg tests (linux & solaris)
>   - vm.runtime (core platforms)
> Performance:
>   - still TBD - this is proving to be extremely difficult. If anyone has
> any simple to run microbenchmarks to suggest I'd give them a try as a
> sanity check. But I lack access to hardware for running serious
> benchmarking.
> Footprint:
> - varies by platform and the VM (server, client, minimal)
> - worst-case: ~1% increase for server VM and minimal VM
> - best-case:  0.4% decrease for client VM
> Thanks,
> David

Bertrand Delsart,                     Grenoble Engineering Center
Oracle,         180 av. de l'Europe,          ZIRST de Montbonnot
38330 Montbonnot Saint Martin,                             FRANCE
bertrand.delsart at oracle.com             Phone : +33 4 76 18 81 23

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