[foreign-memaccess+abi] RFR: 8315769: Add support for sliced allocation [v4]

[Jorn Vernee]

On Thu, 7 Sep 2023 10:31:02 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This PR adds a new method in `SegmentAllocator`:
>> 
>> 
>> default MemorySegment allocateFrom(ValueLayout elementLayout, MemorySegment source,
>>                                        ValueLayout sourceElementLayout, long sourceOffset, long srcElementCount) {
>> 
>> 
>> This method allows clients to allocate a new memory segment and copy the contents of a portion of an existing segment into the newly allocated region of memory. As such it can be used to address the following use cases:
>> 
>> * allocate from a `ByteBuffer`
>> * allocate from another memory segment
>> * allocate from a Java array slice
>> 
>> All these cases were not covered by the existing API points, which meant that developers had to use a more general allocation request (such as `allocate(long, long)`) and then pay a performance cost (because of memory zeroing). In other words, the new method in this PR completes the allocation API, by providing a flexible way to allocate a new segment from an existing source (another segment) with given offset and length.
>> 
>> Given that the new method is more general than the existing array-accepting `allocateFrom`, this PR rewires the existing array-accepting method to be rewritten on top of the new overload (and tweaks the javadoc of such methods accordingly).
>> 
>> One detail to note is that the new method takes _two_ element layouts - one is the layout of the newly allocated segment, whereas the other is the layout of the source segment. Such layouts must have same alignment and same carrier - but they can have different endianness (in which case a bulk copy with swap is performed). This is not too different from the most general `MemorySegment::copy` static overload.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use explicit layouts in array-accepting allocateFrom overloads

Marked as reviewed by jvernee (Committer).

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PR Review: https://git.openjdk.org/panama-foreign/pull/878#pullrequestreview-1615129180