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

[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

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

Commit messages:
 - Add some tests
 - add sliceHandle method

Changes: https://git.openjdk.java.net/panama-foreign/pull/477/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=477&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264187
  Stats: 115 lines in 3 files changed: 113 ins; 0 del; 2 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