[foreign-memaccess] RFR: JDK-8242011: Add support for memory address combinator

[Jorn Vernee]

On Thu, 2 Apr 2020 00:22:58 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Following recent changes (see https://github.com/openjdk/panama-foreign/pull/77)which backport some of the goodies in
>> foreign-abi into foreign-memaccess, this patch brings support for a VarHandle combinator which turns a regular memory
>> access var handle into a var handle which gets/sets a MemoryAddress (e.g. instead of a long).
>> This patch also addresses the general problem of the co-existence between combinators in MemoryHandles and the general
>> var handle combinators in MethodHandles.
>> This coexistence has always been tricky, because the combinators in MemoryHandles like to create a new 'flattened'
>> memory access var handle, which provides best possible performances, and also performs certain alignment checks
>> upfront.  However, if a memory access handle is adapted (e.g. the carrier type is changed) this simplistic approach no
>> longer works, as, by reconstructing the memory access var handle from scratch we will also lose all the adaptations.
>> The solution is to detect as to wheter the target handle is a "direct" memory access var handle or not (special
>> provisions are made for the ubiquitous adaptation added on all memory access var handle creation as a workaround for
>> JDK-8237349). If that's the case, and, if the stride or the offset matches the alignment constraint, then we go the
>> fast/flattened path and we can adapt by reconstructing the handle from scratch. If any of these two conditions are not
>> met (there's complex adaption that would be dropped on the floor, or the offset/stride parameter doesn't match with
>> alignment constraint), then the slow path is taken - the target var handle is kept around and is adapted using the
>> standard combinator API. This leads to a less performant VarHandle (because, unfortunately, calling addOffset()
>> currently breaks C2 optimizations), but also guarantees that alignment constraints will be checked in full (this is
>> because the memory access var handle implementation always checks the alignment of the base address passed to it - only
>> for the offset part is the alignment check skipped - on the basis that this has already been verified by
>> construction).  The result is that we can lift a lot of the restrictions surrounding the combinators in MemoryHandles;
>> such combinators can now work on _any_ var handle (provided the var handle has a first coordinate type of type
>> MemoryAddress). Also, I've lifted the alignment restrictions, since these will either be enforced dynamically (by
>> taking the slow path), or they won't be enforced because we have already statically proven that the constraints are
>> satisfied (fast path).  I've also took the chance to rename some of the classes surrounding memory access var handles
>> to use the "memory access var handle" terminology, which is the one that stuck (currently, some classes use the word
>> "address var hande" which is ambiguous).
>
> src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryAddress.java line 133:
> 
>> 132:      */
>> 133:     MemoryAddress NULL = MemorySegmentImpl.NOTHING.baseAddress();
>> 134:
> 
> In the end, this was added back :-)

:)

-------------

PR: https://git.openjdk.java.net/panama-foreign/pull/84