Slice a MemorySegment using SequenceLayout's element boundaries

[Maurizio Cimadamore]

Hi Sebastian,
look at the MemoryLayouy::sliceHandle - I believe it should do what you 
want? It takes a sequence of PathElements and returns a MethodHandle 
which takes a MemorySegment and some long coordinates and return the 
segment slice at that coordinates, which seems to be what you are trying 
to do.

Cheers
Maurizio

On 11/01/2022 20:48, Sebastian Stenzel wrote:
> Hi,
>
> I've just updated some of my experiments [1] using the latest API in the upcoming JDK 18. Which lead to a question:
>
> Given:
>
> 1. a MemoryAddress (parameter in an upcall stub) representing a pointer to an array of structs
> 2. a matching SequenceLayout of GroupLayouts for this array of structs
>
> Is there any convenient API to obtain a MemorySegment of the nth array element? Currently I'm calculating memory boundaries manually and slice my segment accordingly:
>
> ```
> private void upcall(MemoryAddress pointer) {
>     try (var scope = ResourceScope.newConfinedScope()) {
>        var structLayout = ...;
>        var sequenceLayout = MemoryLayout.sequenceLayout(42, structLayout);
>        var segment = MemorySegment.ofAddress(pointer, sequenceLayout.byteSize(), scope);
>
>        // either:
>        var offset0 = sequenceLayout.byteOffset(MemoryLayout.PathElement.sequenceElement(0));
>        var offset1 = sequenceLayout.byteOffset(MemoryLayout.PathElement.sequenceElement(1));
>        var elem0 = segment.asSlice(offset0, structLayout.byteSize());
>        var elem1 = segment.asSlice(offset1, structLayout.byteSize());
>
>        // or alternatively:
>        var elem0 = segment.asSlice(0, structLayout.byteSize());
>        var elem1 = segment.asSlice(structLayout.byteSize(), structLayout.byteSize());
>     }
> }
> ```
>
> But since a SequenceLayout knows how to calculate the offset and knows the size of each of its elements, I guess there is a more convenient way to slice a segment.
>
> Or if there isn't: Consider this a feature request ��
>
> Cheers!
> Sebastian
>
> [1]: https://github.com/skymatic/fuse-panama/blob/develop/mac/src/main/java/de/skymatic/fusepanama/mac/MacFuseOperationsMapper.java#L178-L187