RFR JDK-8234049: Implementation of Memory Access API (Incubator)
raffaello.giulietti at gmail.com
Mon Dec 9 10:11:16 UTC 2019
will there be a
MemoryAddress.move(MemoryAddress src, MemoryAddress dst, long bytes)
method with POSIX memmove(3) semantics at some point?
That would be useful, e.g., to "open a hole" into an array by shifting
existing elements towards higher indices (provided there's room).
MemoryAddress.copy(), with its lower-to-higher semantics, doesn't really
help here, so without move() one would need to code an explicit loop for
such a case, I guess. Not a big deal, just a little bit annoying.
On 2019-12-07 00:51, Maurizio Cimadamore wrote:
> On 06/12/2019 18:29, Raffaello Giulietti wrote:
>> great job!
>> I think that the doc of MemoryAddress.copy() should be explicit about
>> the direction of the copying, so it should either:
> Thanks! - I'll rectify the doc to specify lower-to-higher.
>> * explicitly specify a direction, e.g., lower-to-higher addresses
>> * or specify that in the case of an overlap the copying is smart
>> enough to not destroy the src bytes before they have landed in dst
>> * or accept a negative third argument to encode a higher-to-lower
>> addresses copying direction.
>>> as part of the effort to upstream the changes related to JEP 370
>>> (foreign memory access API) , I'd like to ask for a code review
>>> for the corresponding core-libs and hotspot changes:
>>> A javadoc for the memory access API is also available here:
>>> Note: the patch passes tier1, tier2 and tier3 testing (**)
>>> Here is a brief summary of the changes in java.base and hotspot (the
>>> remaining new files are implementation classes and tests for the new
>>> * ciField.cpp - this one is to trust final fields in the foreign
>>> memory access implementation (otherwise VM doesn't trust memory
>>> segment bounds)
>>> * Modules.gmk - these changes are needed to require that the
>>> incubating module is loaded by the boot loader (otherwise the above
>>> changes are useless)
>>> * library_call.cpp - this one is a JIT compiler change to treat
>>> Thread.currentThread() as a well-known constant - which helps a lot
>>> in the confinement checks (thanks Vlad!)
>>> * various Buffer-related changes; these changes are needed because
>>> the memory access API allows a memory segment to be projected into a
>>> byte buffer, for interop reasons. As such, we need to insert a
>>> liveness check in the various get/put methods. Previously we had an
>>> implementation strategy where a BB was 'decorated' by a subclass
>>> called ScopedBuffer - but doing so required some changes to the BB
>>> API (e.g. making certain methods non-final, so that we could decorate
>>> them). Here I use an approach (which I have discussed with Alan)
>>> which doesn't require any public API changes, but needs to add a
>>> 'segment' field in Buffer - and then have constructors which keep
>>> track of this extra parameter.
>>> * FileChannel changes - these changes are required so that we can
>>> reuse the Unmapper class from the MemorySegment implementation, to
>>> deterministically deallocate a mapped memory segment. This should be
>>> a 'straight' refactoring, no change in behavior should occur here.
>>> Please double check.
>>> * VarHandles - this class now provides a factory to create memory
>>> access VarHandle - this is a bit tricky, since VarHandle cannot
>>> really be implemented outside java.base (e.g. VarForm is not public).
>>> So we do the usual trick where we define a bunch of proxy interfaces
>>> (see jdk/internal/access/foreign) have the classes in java.base refer
>>> to these - and then have the implementation classes of the memory
>>> access API implement these interfaces.
>>> * JavaNIOAccess, JavaLangInvokeAccess - because of the above, we need
>>> to provide access to otherwise hidden functionalities - e.g. creating
>>> a new scoped buffer, or retrieving the properties of a memory access
>>> handle (e.g. offset, stride etc.), so that we can implement the
>>> memory access API in its own separate module
>>> * GensrcVarHandles.gmk - these changes are needed to enable the
>>> generation of the new memory address var handle implementations;
>>> there's an helper class per carrier (e.g.
>>> VarHandleMemoryAddressAsBytes, ...). At runtime, when a memory access
>>> var handle is needed, we dynamically spin a new VH implementation
>>> which makes use of the right carrier. We need to spin because the VH
>>> can have a variable number of access coordinates (e.g. depending on
>>> the dimensions of the array to be accessed). But, under the hood, all
>>> the generated implementation will be using the same helper class.
>>> * tests - we've tried to add fairly robust tests, often checking all
>>> possible permutations of carriers/dimensions etc. Because of that,
>>> the tests might not be the easiest to look at, but they have proven
>>> to be pretty effective at shaking out issues.
>>> I think that covers the main aspects of the implementation and where
>>> it differs from vanilla JDK.
>>> In the CSR review , Joe raised a fair point - which is
>>> MemoryAddress has both:
>>> offset(long) --> move address of given offset
>>> offset() --> return the offset of this address in its owning segment
>>> And this was considered suboptimal, given both methods use the same
>>> name but do something quite different (one is an accessor, another is
>>> a 'wither'). one obvious option is to rename the first to
>>> 'withOffset'. But I think that would lead to verbose code (since that
>>> is a very common operation). Other options are to:
>>> * rename offset(long) to move(long), advance(long), or something else
>>> * drop offset() - but then add an overload of MemorySegment::asSlice
>>> which takes an address instead of a plain long offset
>>> I'll leave the choice to the reviewers :-)
>>> Finally, I'd like to thank Mark, Brian, John, Alan, Paul, Vlad,
>>> Stuart, Roger, Joe and the Panama team for the feedback provided so
>>> far, which helped to get the API in the shape it is today.
>>> (**) There is one failure, for "java/util/TimeZone/Bug6329116.java" -
>>> but that is unrelated to this patch, and it's a known failing test.
>>>  - https://openjdk.java.net/jeps/370
>>>  - https://bugs.openjdk.java.net/browse/JDK-8234050
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