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

[Maurizio Cimadamore]

On Wed, 21 Jun 2023 13:24:19 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/ValueLayout.java line 98:
> 
>> 96: 
>> 97:     /**
>> 98:      * {@return a var handle which can be used to access values represented by this value layout, in a given memory segment.}
> 
> "values represented by this layout" I don't think is a concept we have. Perhaps a slightly more colloquial "values described by this layout" would work?

I think we should say somewhere that this is similar (but more efficient) to `varHandle(new PathElement[0])` to remove any ambiguity.

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1237001282