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

[Jorn Vernee]

On Wed, 21 Jun 2023 13:22:10 GMT, Maurizio Cimadamore <mcimadamore 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));
>
> src/java.base/share/classes/java/lang/foreign/MemorySegment.java line 163:
> 
>> 161:  * {@link MemoryLayout#varHandle(MemoryLayout.PathElement...)}, as well as the method handle returned by
>> 162:  * {@link MemoryLayout#byteOffsetHandle(MemoryLayout.PathElement...)} and {@link MemoryLayout#sliceHandle(MemoryLayout.PathElement...)}
>> 163:  * feature a <em>base offset</em> parameter. This parameter represents a base offset for the offset computation. This extra
> 
> Maybe drop "extra" - e.g. I wonder if we're writing "extra" as a justification as to why it's there coming from the old API

Yeah, good point

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1238854223