(L) Prelim RFR: 8132510: Replace ThreadLocalStorage with compiler/language-based thread-local variables
thomas.stuefe at gmail.com
Thu Nov 5 10:18:00 UTC 2015
looks good and works fine on AIX and Linux Power.
We could now get rid of the thread_<os>_inline.hpp files too, right?
Kind Regards, Thomas
On Thu, Nov 5, 2015 at 5:36 AM, David Holmes <david.holmes at oracle.com>
> Here's updated webrev:
> Changes since v2:
> - include Thomas's AIX fixes
> - Add assertion for not-NULL into Thread::current()
> - Add Thread::current_or_null() for when NULL can be expected, or for
> where failing an assert would cause more problems (eg crash reporting).
> Most uses of ThreadLocalStorage::thread()/get_thread_slow() now call
> - Removed Thread::current_noinline() (it was only used in an assert in
> some G1 code, so the inline-or-not seems irrelevant)
> - Made Thread::clear_thread_current() private
> I'm debating whether the get_thread implementations should call
> Thread::current() or Thread::current_or_null(). We should never get NULL
> but seems unnecessary overhead to check that with an assert in this code.
> Opinions welcomed.
> I still need some assistance from Aarch64 folk to write their get_thread
> function please!
> I still have footprint and performance measurements to make before
> proposing formal RFR.
> I also am still to determine whether to include the ability to hook in a
> pthread_ based implementation instead.
> On 4/11/2015 7:26 PM, David Holmes wrote:
>> On 4/11/2015 6:17 PM, Thomas Stüfe wrote:
>>> Hi David,
>>> On Wed, Nov 4, 2015 at 5:08 AM, David Holmes <david.holmes at oracle.com
>>> <mailto:david.holmes at oracle.com>> wrote:
>>> Hi Thomas,
>>> One question about your changes:
>>> Before, Thread::current() would assert instead of returning
>>> NULL if
>>> called before Thread::initialize_thread_current() or after
>>> Thead::clear_thread_current() . Now, we just return NULL. Is
>>> this intended?
>>> Ah great question ... so before we have a mix of calls to:
>>> - Thread::current() (asserts on NULL as does JavaThread::current)
>>> - ThreadLocalStorage::thread() (can return NULL)
>>> - ThreadLocalStorage::get_thread_slow() (can return NULL)
>>> and now we only have Thread::current() which means we have to allow
>>> returning NULL because it can be intentionally called when a thread
>>> is not attached. That means we won't directly catch calls to
>>> Thread::current() from code that doesn't realize it is calling it
>>> "too soon" - though there do exist numerous assertions in the
>>> callers of Thread::current() that check the result is not NULL.
>>> I could add the assert to Thread::current() and also add
>>> Thread::current_or_null() to be used by code that needs to use it to
>>> check for attachment (ie JNI code). I'd also have to examine all the
>>> changed ThreadLocalStorage::thread/get_thread_slow call-sites to see
>>> if any of those legitimately expect the thread may not be attached.
>>> What do you think?
>>> I would prefer having Thread::current() to assert and to have a
>>> Thread::current_or_null() for cases where NULL could occurr. I tend to
>>> hit that assert a lot in development, it is useful. And the
>>> non-asserting version gets already used in a number of places, also in
>>> our (not OpenJDK) coding.
>> Yes I agree. Most of the TLS::thread() and TLS::get_thread_slow() should
>> actually call Thread::current_or_null(). I also found a couple of
>> existing Thread::current()'s that should be current_or_null(). :)
>> I also need to look at the location of Thread::current in the .hpp
>>> file rather than .inline.hpp and reconcile that with comments
>>> regarding the noinline version (which is only used in
>>> Could we leave just the inline version in thread.hpp and remove the
>>> noinline version altogether? Now that Thread::current() is very simple,
>>> we may just as well keep it in the class body like the other accessors.
>> I'll see if the g1 code can tolerate that.
>> I'll update a prepare a new webrev tomorrow.
>> Thanks, Thomas
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