RFR(XL): 8224675: Late GC barrier insertion for ZGC

Roman Kennke rkennke at redhat.com
Thu May 23 14:31:37 UTC 2019

Quick glance showed a problem: you are renaming/moving
BarrierSetC2::add_users_to_worklist() but ShenandoahBarrierSetC2 is not
updated accordingly.


> Hi,
> In ZGC we use load barriers on references. In the original
> implementation these where added as macro nodes at parse time. The load
> barrier node consumes and produces control flow in order to be able to
> be lowered into a check with a slow path late. The load barrier nodes
> are fixed in the control flow, and extensions to different optimizations
> are need the barriers out of loop and past other unrelated control flow.
> With this patch the barriers are instead added after the loop
> optimizations, before macro node expansion. This makes the entire
> pipeline until that point oblivious about the barriers. A dump of the IR
> with ZGC or EpsilonGC will be basically identical at that point, and the
> diff compared to serialGC or ParallelGC that use write barriers is
> really small.
> Benefits
> - A major complexity reduction. One can reason about and implement loop
> optimization without caring about the barriers. The escape analysis
> doesn't need to know about the barriers. Loads float freely like they
> are supposed to.
> - Less nodes early. The inlining will become more deterministic. A
> barrier heavy GC will not run into node limits earlier. Also node limit
> bounded optimization like unrolling and peeling will not be penalized by
> barriers.
> - Better test coverage, or reduce testing cost when the same
> optimization doesn't need to be verified with every GC.
> - Better control on where barriers end up. It is trivial to guarantee
> that the load and barriers are not separated by a safepoint.
> Design
> The implementation uses an extra phase that piggy back on PhaseIdealLoop
> which provides control and dominator information for all loads. This
> extra phase is needed because we need to splice the control flow when
> adding the load barriers.
> Barriers are inserted on the loads nodes in post order (any successor
> first). This is to guarantee the dominator information above every
> insertion is correct. This is also important within blocks. Two loads in
> the same block can float in relation to each other. The addition of
> barriers serializes their order. Any def-use relationship is upheld by
> expanding them post order.
> Barrier insertion is done in stages. In this first stage a single macro
> node that represents the barrier is added with all dependencies that is
> required. In the macro expansion phase the barrier nodes is expanded
> into the final shape, adding nodes that represent the conditional load
> barrier check. (Write barriers in other GCs could possibly be expanded
> here directly)
> All the barriers that are needed for unsafe reference operations (cas,
> swap, cmpx) are also expanded late. They already have control flow, so
> the expansion is straight forward.
> The barriers for the unsafe reference operations (cas, getandset, cmpx)
> have also been simplified. The cas-load-cas dance have been replaced by
> a pre-load. The pre-load is a load with a barrier, that is kept alive by
> an extra (required) edge on the unsafe-primitive-nodes (specialized as
> ZCompareAndSwap, ZGetAndSet, ZCompareAndExchange).
> One challenge that was encountered early and that have caused
> considerable work is that nodes (like loads) can end up between calls
> and their catch projections. This is usually handled after matching, in
> PhaseCFG::call_catch_cleanup, where the nodes after the call are cloned
> to all catch blocks. At this stage they are in an ordered list, so that
> is a straight forward process. For late barrier insertion we need to
> splice in control earlier, before matching, and control flow between
> calls and catches is not allowed. This requires us to add a
> transformation pass where all loads and their dependent instructions are
> cloned out to the catch blocks before we can start splicing in control
> flow. This transformation doesn't replace the legacy call_catch_cleanup
> fully, but it could be a future goal.
> In the original barrier implementation there where two different load
> barrier implementations: the basic and the optimized. With the new
> approach to barriers on unsafe, the basic is no longer required and has
> been removed. (It provided options for skipping the self healing, and
> passed the ref in a register, guaranteeing that the oop wasn't reloaded.)
> The wart that was fixup_partial_loads in zHeap has also been made
> redundant.
> Dominating barriers are no longer removed on weak loads. Weak barriers
> doesn't guarantee self-healing.
> Follow up work:
> - Consolidate all uses of GrowableArray::insert_sorted to use the new
> version
> - Refactor the phases. There are a lot of simplifications and
> verification that can be done with more well defined phases.
> - Simplify the remaining barrier optimizations. There might still be
> code paths that are no longer needed.
> Testing:
> Hotspot tier 1-6, CTW, jcstress, micros, runthese, kitchensink, and then
> some. All with -XX:+ZVerifyViews.
> Bug: https://bugs.openjdk.java.net/browse/JDK-8224675
> Webrev: http://cr.openjdk.java.net/~neliasso/8224675/webrev.01/
> Please review,
> Regards,
> Nils

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