RFR: 8318364: Add an FFM-based implementation of harfbuzz OpenType layout

[Maurizio Cimadamore]

On Wed, 18 Oct 2023 23:56:17 GMT, Maurizio Cimadamore <mcimadamore at 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).

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PR Review Comment: https://git.openjdk.org/jdk/pull/15476#discussion_r1365898029