[foreign-memaccess+abi] Integrated: 8310362: Improve composability of handle derived from layouts

[Jorn Vernee]

On Tue, 20 Jun 2023 12:49:41 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> Me and Maurizio have been discussing how to better address certain use cases like accessing structs with variable length array members, or matrices that have a dynamic row and column size.
> 
> The solution we came up with is captured in this patch. It adds an additional base offset parameter to the handles produced by `MemoryLayout::varHandle`, `MemoryLayout::sliceHandle` and `MemoryLayout::byteOffsetHandle`. This allows these handles to be composed with other offset computations that are not necessarily derived from layouts.
> 
> This patch also adds a `scale` method to MemoryLayout, which can be used to scale and index by the size of a layout. Essentially: `offset + layout.byteSize() * index`. This is useful for expressing ad-hoc offset computations. A `scaleHandle` method is also added for convience, which returns a MethodHandle for `scale` bound to the receiver layout.
> 
> Both these changes allowed for several simplifications:
> - `MethodHandles::memorySegmentViewVarHandle` can be removed, as it's now exactly the same as calling ML::varHandle on a ValueLayout with an empty layout path. (the former method also didn't take advantage of the access handle cache found on ValueLayouts).
> - `ValueLayout::arrayHandle` is removed, as most use cases can now simply be expressed using ML::varHandle + ML::scaleHandle. e.g.:
> 
> MethodHandles.collectCoordinates(layout.varHandle(),
>             1, MethodHandles.insertArguments(layout.scaleHandle(), 0, 0L));

This pull request has now been integrated.

Changeset: 2a794ce3
Author:    Jorn Vernee <jvernee at openjdk.org>
URL:       https://git.openjdk.org/panama-foreign/commit/2a794ce3373038b73be77144c9010f33cee3d51e
Stats:     858 lines in 42 files changed: 257 ins; 260 del; 341 mod

8310362: Improve composability of handle derived from layouts

Reviewed-by: mcimadamore

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PR: https://git.openjdk.org/panama-foreign/pull/840