RFR: JDK-8221766: Load-reference barriers for Shenandoah

Roman Kennke rkennke at redhat.com
Tue Apr 2 21:28:02 UTC 2019

Thanks, Erik!


> Build change looks good.
> /Erik
> On 2019-04-02 14:12, Roman Kennke wrote:
>> (I am cross-posting this to build-dev and compiler-dev because this
>> contains some (trivial-ish) shared build and C2 changes. The C2 changes
>> are almost all reversals of Shenandoah-specific paths that have been
>> introduced in initial Shenandoah push.)
>> I would like to propose that we switch to what we came to call 'load
>> reference barrier' as new barrier scheme for Shenandoah GC.
>> The main difference is that instead of ensuring correct invariant when
>> we store anything into the heap (e.g. read-barrier before reads,
>> write-barrier before writes, plus a bunch of other stuff), we ensure the
>> strong invariance on objects when they get loaded, by employing what is
>> currently our write-barrier.
>> The reason why I'm proposing it is:
>> - simpler barrier interface
>> - easier to get good performance out of it
>>    ==> good for upcoming Graal (sup)port
>> - reduced maintenance burden (I intend to backport it all the way)
>> This has a number of advantages:
>> - Strong invariant means it's a lot easier to reason about the state of
>> GC and objects
>> - Much simpler barrier interface. Infact, a lot of stuff that we added
>> to barrier interfaces after JDK11 will now become unused: no need for
>> barriers on primitives, no need for object equality barriers, no need
>> for resolve barriers, etc. Also, some C2 stuff that we added for
>> Shenandoah can now be removed again. (Those are what comprise most
>> shared C2 changes.)
>> - Optimization is much easier: we currently put barriers 'down low'
>> close to their uses (which might be inside a hot loop), and then work
>> hard to optimize barriers upwards, e.g. out of loops. By using
>> load-ref-barriers, we would place them at the outermost site already.
>> Look how much code is removed from shenandoahSupport.cpp!
>> - No more need for object equals barriers.
>> - No more need for 'resolve' barriers.
>> - All barriers are now conditional, which opens up opportunity for
>> further optimization later on.
>> - we can re-enable the fast JNI getfield stuff
>> - we no longer need the nmethod initializer that initializes embedded
>> oops to to-space
>> - We no longer have the problem to use two registers for 'the same'
>> value (pre- and post-barrier).
>> The 'only' optimizations that we do in C2 are:
>> - Look upwards and see if barrier input indicates we don't actually need
>> the barrier. Would be the case for: constants, nulls, method parameters,
>> etc (anything that is not like a load). Even though we insert barriers
>> after loads, you'd be surprised to see how many loads actually disappear.
>> - Look downwards to check uses of the barrier. If it doesn't feed into
>> anything that requires a barrier, we can remove it.
>> Performance doesn't seem to be negatively impacted at all. Some
>> benchmarks benefit positively from it.
>> Testing: Testing: hotspot_gc_shenandoah, SPECjvm2008, SPECjbb2015, all
>> of them many times. This patch has baked in shenandoah/jdk for 1.5
>> months, undergone our rigorous CI, received various bug-fixes, we have
>> had a close look at the generated code to verify it is sane. jdk/submit
>> job expected good before push.
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8221766
>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8221766/webrev.00/
>> Can I please get reviews for this change?
>> Roman

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