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

Erik Joelsson erik.joelsson at oracle.com
Tue Apr 2 21:25:06 UTC 2019


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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