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

Bertrand Delsart bertrand.delsart at oracle.com
Mon Nov 23 13:46:16 UTC 2015

On 23/11/2015 13:27, David Holmes wrote:
> Hi Bertrand,
> On 23/11/2015 8:14 PM, Bertrand Delsart wrote:
>> 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
>> http://cr.openjdk.java.net/~dholmes/8132510/webrev.v6/src/os_cpu/linux_x86/vm/assembler_linux_x86.cpp.udiff.html
> Sorry I'm not that familiar with the MacroAssembler - is it this odd
> fragment:
> -   push(r10);
> -   // XXX
> -   mov(r10, rsp);
> -   andq(rsp, -16);
> -   push(r10);
> I'm not at all clear what that is doing - and if it somehow changes the
> alignment wouldn't something need to be fixed up when popping the
> previous values ??
> To be honest I'm not even sure what an "unaligned stack" means.

On some platforms, SP may have to be aligned on a 16 bytes boundary when 
calling another method.

After having pushed what needed to be saved, the code above rounds SP 
down and saves the old value. It will then also push r11, which results 
in the expected alignment.

The conterpart, after the VM call, is the:

>> This alignment is no longer performed in the new (shared) implementation
>> http://cr.openjdk.java.net/~dholmes/8132510/webrev.v6/src/cpu/x86/vm/macroAssembler_x86.cpp.udiff.html
>> 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 ?)
> I don't have enough knowledge to even begin to comment on this. I will
> have to rely on our x86_64 macroassembler experts to explain the old
> code and what the requirements are.

OK. This means that the alignment was not removed purposefully.

In that case, you must either keep the per platform code x86_64 (since 
it was not identical for all platforms) or use the safest version, with 
the additional

    // XXX
    mov(r10, rsp);
    andq(rsp, -16);

before the push(r11) and with the


after the pop(r11). It should work on all x86_64 platforms.

FYI, there is another way to do the alignment, based on the fact that
we are at least aligned on 8 bytes (see 
MacroAssembler::call_VM_leaf_base). I assume that this second version is 
more efficient (particularly thanks to specilative branches) but it 
might be safer/simpler to continue using andq in get_thread.


> Thanks,
> David
>> Regards,
>> Bertrand.
>> 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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