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

[Jorn Vernee]

On Fri, 26 Mar 2021 12:20:02 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> 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

This pull request has now been integrated.

Changeset: 07665f6f
Author:    Jorn Vernee <jvernee at openjdk.org>
URL:       https://git.openjdk.java.net/panama-foreign/commit/07665f6f
Stats:     147 lines in 3 files changed: 138 ins; 0 del; 9 mod

8264187: Add a method for creating a slicing MethodHandle

Reviewed-by: mcimadamore

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