RFR: 8189871: Refactor GC barriers to use declarative semantics

[David Holmes]

Hi Erik,

I really like the level of abstraction and encapsulation this provides.

Can't comment on the GC specific details or the template mechanics 
directly, of course. :)

A couple of comments:

src/hotspot/share/oops/klass.hpp

  412   // Is an oop/narrowOop null or subtype of this Klass?
  413   template <typename T>
  414   bool is_covariant(T element);

I find "is_covariant" a very obscure way to name this. It may be 
academically accurate but it's really just asking if the element is of a 
type that is a subclass of the current klass. The null handling 
complicates it, but it seems to me that:

template <typename T>
bool Klass::is_instanceof_or_null(T element);

would be more consistent with how we normally refer to things in the VM 
(though the _or_null can be dropped from the name).

---

src/hotspot/share/oops/objArrayOop.cpp

Klass* objArrayOopDesc::covariant_bound()

There's that word again. :) If you really think you need to use 
covariance within these API's you really need to add some comments to 
the method declarations to explain them. Most of us probably have a 
minimal recollection of covariance and contravariance from discussing 
type-safety for method parameters and return types. :)

---

src/hotspot/share/prims/unsafe.cpp

The changes from jobjects to oops made me uneasy, but I'm assuming the 
places where MemoryAccess and GuardedMemoryAccess are used are 
affectively all leave routines with no chance of hitting anything that 
would respond to a safepoint request?

Thanks,
David
-----

On 10/11/2017 3:00 AM, Erik Österlund wrote:
> Hi,
> 
> In an effort to remove explicit calls to GC barriers (and other 
> orthogonal forms of barriers, like encoding/decoding oops for compressed 
> oops and fencing for memory ordering), I have built an API that I call 
> "Access". Its purpose is to perform accesses with declarative semantics, 
> to handle multiple orthogonal concerns that affect how an access is 
> performed, including memory ordering, compressed oops, GC barriers for 
> marking, reference strength, etc, and as a result making GCs more 
> modular, and as a result allow new concurrently compacting GC schemes 
> utilizing load barriers to live in harmony in hotspot without everyone 
> going crazy manually inserting barriers if UseBlahGC is enabled.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8189871
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8189871/webrev.00/
> 
> So there are three views of this I suppose:
> 
> 1) The frontend: how this is actually used in shared code
> 2) The backends: how anyone writing a GC sticks their required barriers 
> in there
> 3) The internals: how accesses find their way from the frontend to the 
> corresponding backend
> 
> == Frontend ==
> 
> Let's start with the frontend. I hope I made this fairly simple! You can 
> find it in runtime/access.hpp
> Each access annotates its declarative semantics with a set of 
> "decorators", which is the name of the attributes/properties affecting 
> how an access is performed.
> There is an Access<decorator> API that makes the declarative semantics 
> possible.
> 
> For example, if I want to perform a load acquire of an oop in the heap 
> that has "weak" strength, I would do something like:
> oop result = Access<MO_ACQUIRE | IN_HEAP | 
> ON_WEAK_OOP_REF>::oop_load_at(obj, offset);
> 
> The Access API would then send the access through some GC backend, that 
> overrides the whole access and tells it to perform a "raw" load acquire, 
> and then possibly keep it alive if necessary (G1 SATB enqueue barriers).
> 
> To make life easier, there are some helpers for the most common access 
> patterns that merely add some default decorator for the involved type of 
> access. For example, there is a RawAccess for performing AS_RAW accesses 
> (that bypasses runtime checks and GC barriers), HeapAccess sets the 
> IN_HEAP decorator and RootAccess sets the IN_ROOT decorator for 
> accessing root oops. So for the previous call, I could simply do:
> 
> oop result = HeapAccess<MO_ACQUIRE | ON_WEAK_OOP_REF>::oop_load_at(obj, 
> offset);
> 
> The access.hpp file introduces each decorator (belonging to some 
> category) with an explanation what it is for. It also introduces all 
> operations you can make with access (loads, stores, cmpxchg, xchg, 
> arraycopy and clone).
> 
> This changeset mostly introduces the Access API but is not complete in 
> annotating the code more than where it gets very awkward if I don't.
> 
> == Backend ==
> 
> For a GC maintainer, the BarrierSet::AccessBarrier is the top level 
> backend that provides basic accesses that may be overridden. By default, 
> it just performs raw accesses without any GC barriers, that handle 
> things like compressed oops and memory ordering only. The ModRef barrier 
> set introduces the notion of pre/post write barriers, that can be 
> overridden for each GC. The CardTableModRef barrier set overrides the 
> post write barrier to mark cards, and G1 overrides it to mark cards 
> slightly differently and do some SATB enqueueing. G1 also overrides 
> loads to see if we need to perform SATB enqueue on weak references.
> 
> The raw accesses go to the RawAccessBarrier (living in 
> accessBackend.hpp) that performs the actual accesses. It connects to 
> Atomic and OrderAccess for accesses that require that.
> 
> == Internals ==
> 
> Internally, the accesses go through a number of stages in 
> access.inline.hpp as documented at the top.
> 
> 1) set default decorators and get rid of CV qualifiers etc. Sanity 
> checking also happens here: we check that the decorators make sense for 
> the access being performed, and that the passed in types are not bogus.
> 2) reduce types so if we have a different type of the address and value, 
> then either it is not allowed or it implies we use compressed oops and 
> remember that we know something about whether compressed oops are used 
> or not, before erasing address type
> 3) pre-runtime dispatch: figure out if all runtime checks can be 
> bypassed into a raw access
> 4) runtime dispatch: send the access through a function pointer that 
> upon the first invocation resolves the intended GC AccessBarrier 
> accessor on the BarrierSet that handles this access, as well as figures 
> out whether we are using compressed oops or not while we are at it, and 
> then calls it through the post-runtime dispatch
> 5) post-runtime dispatch: fix some erased types that were not known at 
> compile time such as whether the address is a narrowOop* or oop* 
> depending on whether compressed oops was selected at runtime or not, and 
> call the resolved BarrierSet::AccessBarrier accessor (load/store/etc) 
> with all the call-site build-time and run-time resolved decorators and 
> type information that describes the access.
> 
> Testing: mach5 tier1-5
> 
> Thanks,
> /Erik