[foreign-memaccess+abi] RFR: 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));

src/java.base/share/classes/jdk/internal/foreign/Utils.java line 110:

> 108:         if (handle != null) {
> 109:             return handle;
> 110:         }

Added a cache lookup here so that we don't end up re-creating the var handle if one is already in the cache. Also, I noticed that previously the cache was sensitive to layout names, which it shouldn't I think, since the names are not of consequence to the produced var handle.

src/java.base/share/classes/jdk/internal/foreign/abi/SharedUtils.java line 428:

> 426:         }
> 427:     }
> 428: 

Saw this dead code was still here, so I've removed it.

src/java.base/share/classes/jdk/internal/foreign/layout/AbstractLayout.java line 58:

> 56:     @SuppressWarnings("unchecked")
> 57:     public final L withoutName() {
> 58:         return name.isPresent() ? dup(byteAlignment(), Optional.empty()) : (L) this;

Made this idempotent to make the `withoutName()` call in `makeSegmentViewVarHandle` cheaper.

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PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1235223824
PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1235224486
PR Review Comment: https://git.openjdk.org/panama-foreign/pull/840#discussion_r1235225987