On Wed, 18 Oct 2023 23:56:17 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org>
wrote:
>> 8318364: Add an FFM-based implementation of harfbuzz OpenType layout
>
> src/java.desktop/share/classes/sun/font/HBShaper.java line 110:
>
>> 108:
>> 109: private static VarHandle getVarHandle(StructLayout struct, String
>> name) {
>> 110: VarHandle h = struct.varHandle(PathElement.groupElement(name));
>
> Note that, strictly speaking, these combiners are _not_ required. If you just
> call `MemoryLayout::varHandle`, you get back a var handle that takes a
> `MemorySegment` and a `long` offset. So, you can, if you want, adapt the var
> handle so that the offset parameter becomes something else. But you could
> also just leave the var handle as is. Then, in the loop that is doing the
> access, you can do this:
>
>
> for (int i = 0 ; i < limit ; i++) {
> x_offsetHandle.get(segment, PositionLayout.scale(0, i));
> y_offsetHandle.get(segment, PositionLayout.scale(0, i));
> ...
> }
>
> That is, use the offset hole to your advantage, to pass the struct base
> offset (obtained by scaling the enclosing struct layout by the value of the
> loop induction variable).
>
> (That said, I understand that working with logical indices is a common
> operation, and that this is made a bit harder by the recent API changes. We
> should consider, as @JornVernee mentioned, adding back a more general
> `MemoryLayout::arrayElementVarHandle` which will give you the var handle you
> need - with coordinates `MemorySegment` and `long` - a logical index, not an
> offset).
A PR which adds `MemoryLayout::arrayElementVarHandle` can be found here:
https://github.com/openjdk/jdk/pull/16272
With this, you can call the new method in order to create the var handle. The
returned var handle will accept *two* long coordinate - the first is a base
offset (as usual), the second in a logical index (what you need). The PR also
adds plenty of narrative text describing how access to variable-length arrays
should be performed using layouts (and also shows cases where the offset
parameter is used in a non-trivial fashion).
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/15476#discussion_r1365898029
