Slice a MemorySegment using SequenceLayout's element boundaries

[Sebastian Stenzel]

Hi Maurizio,

thanks for the fast reply. While your suggested method may achieve the same, it doesn't really appeal to me. Creating a method handle and invoking it (i.e. without any type safety) isn't any less complicated than my original code. Or maybe I didn't understand it correct yet?


> On 11. Jan 2022, at 22:08, Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
> 
> 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