[foreign-jextract] RFR: 8264187: Add a method for creating a slicing MethodHandle [v2]

[Jorn Vernee]

> Hi,
> 
> This patch adds a `sliceHandle(PathElement...)` method to MemoryLayout, which can be used to create a method handle that, when given a segment, will return a slice of that segment for the layout element selected by the earlier given layout path.
> 
> This for instance makes it easier to iterate over an array of structs by first creating a slice for each element, and then accessing struct fields from there, without having to resolve to manual offset computation. For example:
> 
> SequenceLayout seq = MemoryLayout.ofSequence(5,
>     MemoryLayout.ofStruct(
>        MemoryLayout.ofValueBits(32, ByteOrder.nativeOrder()).withName("x"),
>        MemoryLayout.ofValueBits(32, ByteOrder.nativeOrder()).withName("y")
>     ));
> 
> MethodHandle sliceHandle = seq.sliceHandle(sequenceElement());
> 
> MemoryLayout structLayout = seq.select(sequenceElement());
> VarHandle xHandle = structLayout.varHandle(int.class, groupElement("x"));
> VarHandle yHandle = structLayout.varHandle(int.class, groupElement("y"));
> 
> try (ResourceScope scope = ResourceScope.ofConfined()) {
>     MemorySegment segment = MemorySegment.allocateNative(seq, scope);
>     long count = seq.elementCount().orElseThrow();
>     for (int i = 0; i < count; i++) {
>         // create view segment of single struct element
>         MemorySegment struct = (MemorySegment) sliceHandle.invokeExact(segment, (long) i);
>         xHandle.set(struct, 42);
>         yHandle.set(struct, 84);
>     }
> }
> 
> Thanks,
> Jorn

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/477/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/477/files/5ffc504e..8db12b61

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=477&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=477&range=00-01

  Stats: 51 lines in 3 files changed: 29 ins; 4 del; 18 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/477.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/477/head:pull/477

PR: https://git.openjdk.java.net/panama-foreign/pull/477