[foreign-memaccess+abi] RFR: 8310362: Improve composability of handle derived from layouts [v3]

[Maurizio Cimadamore]

On Thu, 22 Jun 2023 23:14:39 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));
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   more review comments

src/java.base/share/classes/java/lang/foreign/MemoryLayout.java line 341:

> 339:      * @param offset the base offset
> 340:      * @param index the index to be scaled by the byte size of this layout
> 341:      * @throws IllegalArgumentException if {@code offset} is negative

also if `index` is negative?

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1239117383