Another very small iteration which fixes a gap in the javadoc
specification for MemorySegment::toArray (thanks Chris!)
Webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v5/webrev
Delta webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v5/webrev_delta/
Cheers
Maurizio
On 13/05/2020 13:12, Maurizio Cimadamore wrote:
Another iteration which addresses the latest CSR comments (the CSR has
now been approved):
* make MemorySegment::withAccessModes/hasAccessMode throw
IllegalArgumentException in cases where the provided mask is invalid
(this required a test tweak)
* sprinkled a couple of references to the JLS in the package javadoc,
as per CSR suggestions
* Fixed the ParallelSum::findAny_bulk benchmarks, which were
(erroneously) not testing all the elements in the segment
Webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v4/webrev
Delta webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v4/webrev_delta/
Cheers
Maurizio
On 01/05/2020 12:06, Maurizio Cimadamore wrote:
Latest iteration - the follow issues were addressed:
* fix a bug with alignment of native segments triggering spurious
failures (contributed by Jorn)
* fix javadoc and tests for access modes (contributed by Chris)
* added benchmarks for Stream::findAny using segment spliterator
(contributed by Peter)
* addressed CSR comments
Webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v3/webrev
Delta webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v3/webrev_delta/
Javadoc:
http://cr.openjdk.java.net/~mcimadamore/8243491_v3/javadoc
Specdiff:
http://cr.openjdk.java.net/~mcimadamore/8243491_v3/specdiff/overview-summary.html
Cheers
Maurizio
On 27/04/2020 13:13, Maurizio Cimadamore wrote:
Another iteration, which addresses the following issues:
* wrong copyright headers in certain tests
* issue with TestNative when running in debug mode caused by
mismatched malloc/os::free (contributed by Jorn)
* clarify javadoc of MemoryHandles::withStride
* improved implementation of MemoryAccessVarHandleGenerator to use
hidden classes rather than Unsafe.dAC (contributed by Mandy)
Webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v2/webrev
Delta webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v2/webrev_delta/
Javadoc:
http://cr.openjdk.java.net/~mcimadamore/8243491_v2/javadoc
Specdiff:
http://cr.openjdk.java.net/~mcimadamore/8243491_v2/specdiff/overview-summary.html
Cheers
Maurizio
On 23/04/2020 21:33, Maurizio Cimadamore wrote:
Hi,
time has come for another round of foreign memory access API
incubation (see JEP 383 [3]). This iteration aims at polishing some
of the rough edges of the API, and adds some of the functionalities
that developers have been asking for during this first round of
incubation. The revised API tightens the thread-confinement
constraints (by removing the MemorySegment::acquire method) and
instead provides more targeted support for parallel computation via
a segment spliterator. The API also adds a way to create a custom
native segment; this is, essentially, an unsafe API point, very
similar in spirit to the JNI NewDirectByteBuffer functionality [1].
By using this bit of API, power-users will be able to add support,
via MemorySegment, to *their own memory sources* (e.g. think of a
custom allocator written in C/C++). For now, this API point is
called off as "restricted" and a special read-only JDK property
will have to be set on the command line for calls to this method to
succeed. We are aware there's no precedent for something like this
in the Java SE API - but if Project Panama is to remain true about
its ultimate goal of replacing bits of JNI code with (low level)
Java code, stuff like this has to be *possible*. We anticipate
that, at some point, this property will become a true launcher
flag, and that the foreign restricted machinery will be integrated
more neatly into the module system.
A list of the API, implementation and test changes is provided
below. If you have any questions, or need more detailed
explanations, I (and the rest of the Panama team) will be happy to
point at existing discussions, and/or to provide the feedback
required.
Thanks
Maurizio
Webrev:
http://cr.openjdk.java.net/~mcimadamore/8243491_v1/webrev
Javadoc:
http://cr.openjdk.java.net/~mcimadamore/8243491_v1/javadoc
Specdiff:
http://cr.openjdk.java.net/~mcimadamore/8243491_v1/specdiff/overview-summary.html
CSR:
https://bugs.openjdk.java.net/browse/JDK-8243496
API changes
===========
* MemorySegment
- drop support for acquire() method - in its place now you can
obtain a spliterator from a segment, which supports divide-and-conquer
- revamped support for views - e.g. isReadOnly - now segments
have access modes
- added API to do serial confinement hand-off
(MemorySegment::withOwnerThread)
- added unsafe factory to construct a native segment out of an
existing address; this API is "restricted" and only available if
the program is executed using the -Dforeign.unsafe=permit flag.
- the MemorySegment::mapFromPath now returns a MappedMemorySegment
* MappedMemorySegment
- small sub-interface which provides extra capabilities for
mapped segments (load(), unload() and force())
* MemoryAddress
- added distinction between *checked* and *unchecked* addresses;
*unchecked* addresses do not have a segment, so they cannot be
dereferenced
- added NULL memory address (it's an unchecked address)
- added factory to construct MemoryAddress from long value
(result is also an unchecked address)
- added API point to get raw address value (where possible - e.g.
if this is not an address pointing to a heap segment)
* MemoryLayout
- Added support for layout "attributes" - e.g. store metadata
inside MemoryLayouts
- Added MemoryLayout::isPadding predicate
- Added helper function to SequenceLayout to rehape/flatten
sequence layouts (a la NDArray [4])
* MemoryHandles
- add support for general VarHandle combinators (similar to MH
combinators)
- add a combinator to turn a long-VH into a MemoryAddress VH (the
resulting MemoryAddress is also *unchecked* and cannot be
dereferenced)
Implementation changes
======================
* add support for VarHandle combinators (e.g. IndirectVH)
The idea here is simple: a VarHandle can almost be thought of as a
set of method handles (one for each access mode supported by the
var handle) that are lazily linked. This gives us a relatively
simple idea upon which to build support for custom var handle
adapters: we could create a VarHandle by passing an existing var
handle and also specify the set of adaptations that should be
applied to the method handle for a given access mode in the
original var handle. The result is a new VarHandle which might
support a different carrier type and more, or less coordinate
types. Adding this support was relatively easy - and it only
required one low-level surgery of the lambda forms generated for
adapted var handle (this is required so that the "right" var handle
receiver can be used for dispatching the access mode call).
All the new adapters in the MemoryHandles API (which are really
defined inside VarHandles) are really just a bunch of MH adapters
that are stitched together into a brand new VH. The only caveat is
that, we could have a checked exception mismatch: the VarHandle API
methods are specified not to throw any checked exception, whereas
method handles can throw any throwable. This means that,
potentially, calling get() on an adapted VarHandle could result in
a checked exception being thrown; to solve this gnarly issue, we
decided to scan all the filter functions passed to the VH
combinators and look for direct method handles which throw checked
exceptions. If such MHs are found (these can be deeply nested,
since the MHs can be adapted on their own), adaptation of the
target VH fails fast.
* More ByteBuffer implementation changes
Some more changes to ByteBuffer support were necessary here. First,
we have added support for retrieval of "mapped" properties
associated with a ByteBuffer (e.g. the file descriptor, etc.). This
is crucial if we want to be able to turn an existing byte buffer
into the "right kind" of memory segment.
Conversely, we also have to allow creation of mapped byte buffers
given existing parameters - which is needed when going from
(mapped) segment to a buffer. These two pieces together allow us to
go from segment to buffer and back w/o losing any information about
the underlying memory mapping (which was an issue in the previous
implementation).
Lastly, to support the new MappedMemorySegment abstraction, all the
memory mapped supporting functionalities have been moved into a
common helper class so that MappedMemorySegmentImpl can reuse that
(e.g. for MappedMemorySegment::force).
* Rewritten memory segment hierarchy
The old implementation had a monomorphic memory segment class. In
this round we aimed at splitting the various implementation classes
so that we have a class for heap segments (HeapMemorySegmentImpl),
one for native segments (NativeMemorySegmentImpl) and one for
memory mapped segments (MappedMemorySegmentImpl, which extends from
NativeMemorySegmentImpl). Not much to see here - although one
important point is that, by doing this, we have been able to speed
up performances quite a bit, since now e.g. native/mapped segments
are _guaranteed_ to have a null "base". We have also done few
tricks to make sure that the "base" accessor for heap segment is
sharply typed and also NPE checked, which allows C2 to speculate
more and hoist. With these changes _all_ segment types have
comparable performances and hoisting guarantees (unlike in the old
implementation).
* Add workarounds in MemoryAddressProxy, AbstractMemorySegmentImpl
to special case "small segments" so that VM can apply bound check
elimination
This is another important piece which allows to get very good
performances out of indexes memory access var handles; as you might
know, the JIT compiler has troubles in optimizing loops where the
loop variable is a long [2]. To make up for that, in this round we
add an optimization which allows the API to detect whether a
segment is *small* or *large*. For small segments, the API realizes
that there's no need to perform long computation (e.g. to perform
bound checks, or offset additions), so it falls back to integer
logic, which in turns allows bound check elimination.
* renaming of the various var handle classes to conform to "memory
access var handle" terminology
This is mostly stylistic, nothing to see here.
Tests changes
=============
In addition to the tests for the new API changes, we've also added
some stress tests for var handle combinators - e.g. there's a flag
that can be enabled which turns on some "dummy" var handle
adaptations on all var handles created by the runtime. We've used
this flag on existing tests to make sure that things work as expected.
To sanity test the new memory segment spliterator, we have wired
the new segment spliterator with the existing spliterator test
harness.
We have also added several micro benchmarks for the memory segment
API (and made some changes to the build script so that native
libraries would be handled correctly).
[1] -
https://docs.oracle.com/en/java/javase/14/docs/specs/jni/functions.html#newdirectbytebuffer
[2] - https://bugs.openjdk.java.net/browse/JDK-8223051
[3] - https://openjdk.java.net/jeps/383
[4] -
https://docs.scipy.org/doc/numpy/reference/generated/numpy.reshape.html#numpy.reshape
