[foreign-memaccess] RFR: 8252504: Add a method to MemoryLayout which returns a offset-computing method handle

[Maurizio Cimadamore]

On Thu, 19 Nov 2020 15:53:44 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This PR adds two new methods: `bitOffsetHandle` and `byteOffsetHandle` to `MemoryLayout`, that can be used to create method handles that can compute the effective offset of a layout element given a set of additional `long` indexes.
> 
> I also cleaned up a leftover in the javadoc of bitOffset and byteOffset where an API note was stating that `sequenceElement()` layout path elements would be interpreted as `sequenceElement(0)`, but they were actually being rejected outright (as they should).
> 
> I also added some tests to cover the corner case where an UnsupportedOperationException was being thrown. (some of the churn in the tests is from introducing static imports. I hope it's not too noisy).

src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/LayoutPath.java line 205:

> 203:             if (cur.enclosing.layout instanceof SequenceLayout
> 204:                 && cur.elementIndex == UNSPECIFIED_ELEM_INDEX) { // sequenceElement()
> 205:                 MethodHandle collector = MethodHandles.insertArguments(MH_ADD_SCALED_OFFSET, 2, cur.layout.bitSize());

You already have the `strides` array for doing this? E.g. I don't think there's a need to walk the layout paths upwards and to get their size - all you need is in the stride array.

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PR: https://git.openjdk.java.net/panama-foreign/pull/396