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

[Maurizio Cimadamore]

On Fri, 26 Mar 2021 15:22:52 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
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments

Looks good - added minor javadoc suggestion

src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryLayout.java line 500:

> 498: 
> 499:     /**
> 500:      * Creates a method handle which, given a memory segment, returns a {@link MemorySegment#asSlice(long,long)}

Suggestion:

     * Creates a method handle which, given a memory segment, returns a {@link MemorySegment#asSlice(long,long) slice}

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Marked as reviewed by mcimadamore (Committer).

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