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  1. JDK
  2. JDK-8204540

Automatic oop closure devirtualization



    • Type: Enhancement
    • Status: Resolved
    • Priority: P4
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 11
    • Component/s: hotspot
    • Labels:
    • Subcomponent:
    • Resolved In Build:




        The ExtendedOopClosure is the base class of GC closures that are usable by the oopDesc::oop_iterate and friends. The oop_iterate functions call the implementation of the virtual ExtendedOopClosure::do_oop, of the passed in closure. As part of this patch, this closure is renamed OopIterateClosure to aid readers of the code to better understand the purpose of this closure.

        Today there exists a devirtualization layer, which looks at the static type of the closure and turns the call to the virtual do_oop into direct, inlinable calls. This used to be fully implemented by macros, but was partly replaced by templates in JDK-8075955. Because oopDesc::oop_iterate calls the *virtual* Klass::oop_oop_iterate functions, it's non-trivial to pass the closure type all the way through to the do_oop function calls. Therefore, overloads of Klass::oop_oop_iterate for all devirtualized closures were generated by macros.

        The generation of these overloads is finicky and requires the developer to split the closure up into different files and get them registered correctly in the specialized_oops_closure.hpp.

        It also means that we generate implementations of some of the oop_oop_iterate for all closures, even when they are not used. There's even code to exclude compilation of some of these functions when only SerialGC is built.

        I propose a new way to automatically devirtualize these do_oop calls when the compiler detects that static closure type is known to have the do_oop function implemented.

        This will get rid of all these macros, get rid of the *_specialized_oop_closure.* files, stop generating GC specific overloads into oopDesc and the *Klass classes, get rid of virtual calls for more closures, remove the _nv suffixes and template parameters.

        For this to work we need to impose a contract/convention that no OopIterateClosure sub class overrides an implementation of do_oop in any of it's ancestors. Going forward, when we move to a newer C++ standard, we'll be able to mark the do_oop functions as final, to prevent accidental overrides. There might also be a way to automatically detect such breaches with template metaprogramming, but that has not been implemented.

        The discussion above has been about the do_oop functions, but this also applies to the metadata functions as well (do_metadata, do_cld, do_klass).

        The proposed patch implements one dispatch table per OopIterateClosure (and oop_iterate variant) that gets used and devirtualized. Each such table has one entry per sub class of Klass. All these tables get generated during compile time, and installed when the static initializers are run. This gives us a single-call dispatch over the static type of the oop closure type and the dynamic type of the Klass sub class.

        The oop iteration functions also dispatch on UseCompressedOops, and the patch replaces those checks with an initialization-time replacement of the dispatch table, which installs the correct implementation (with or without compressed oops), making this a single-call dispatch over three dimensions. This is similar to how the RuntimeDispatch is done for the Access API.

        With this RFE the developers will be able to do the following to get devirtualized the code in do_oop inlineable into *Klass::oop_oop_iterate:

        class MarkingClosure : public MetadataVisitingOopIterateClosure {
          virtual void do_oop(oop* p) { marking(p); }
          virtual void do_oop(narrowOop* p) { marking(p); }
        MarkingClosure cl;

        I've described the mechanism in more detail in iterate.inline.hpp. See the two larger comment blocks in:


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                stefank Stefan Karlsson
                stefank Stefan Karlsson
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