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

[Maurizio Cimadamore]

On Thu, 19 Nov 2020 19:58:12 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> 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.
>
> We need to walk to inspect the element indices to see if we have a sequenceElement path element with an unspecified index. Could replace the `cur.layout.bitSize` with a lookup into the strides array maybe, but I think that would require keeping track of a separate index as well. (I'll take a look).

My recollection of this code is that when you get to generating the handle, you already have the unspecified paths in the stride array. Look at the places for when a new stride is set - I think it should match the places where you need the info as well (that is `PathElement::sequenceElement()` and `PathElement::sequenceElement(long, long)`).

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