On Thu, 19 Oct 2023 16:52:53 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
> The 21 iteration of the FFM API used to contain a method, namely > `ValueLayout::arrayElementVarHandle`. This var handle factory was useful when > accessing elements in a flat array of variable size. > > In 22, this method has been removed because: > * It was not general enough (it only worked with value layouts) > * A more useful/general version could be obtained with the following code: > > > MethodHandles.collectCoordinates(varHandle(elements), 1, scaleHandle()); > > > Given that accessing variable-length array is rather common, and given that > combining var handles is perceived as hard, we noticed that developers often > tried to workaround by creating a sequence layout of maximal size, and then > deriving a var handle from it. This feels suboptimal (as a new layout is > created just so that we can select from it, and using this layout for > anything else will likely lead to exceptions, because of its size). > > For these reasons, we would like to expose this functionality as a new method > in the `MemoryLayout` interface, namely > `MemoryLayout::arrayElementVarHandle`. This method generalizes the old method > in the 21 API, and is implemented in the obvious way, using a combinator (see > above). > > This PR also adds a lot of narrative text to describe how to deal with > variable-length arrays, which should clarify the role of the new method. This pull request has now been integrated. Changeset: 848ecc16 Author: Maurizio Cimadamore <mcimadam...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/848ecc1621c347ab12dd3f421af82cb55c71e075 Stats: 163 lines in 6 files changed: 119 ins; 21 del; 23 mod 8318538: Add a way to obtain a strided var handle from a layout Reviewed-by: jvernee, pminborg ------------- PR: https://git.openjdk.org/jdk/pull/16272
