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

[Phil Race]

On Tue, 17 Oct 2023 11:50:20 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> 8318364: Add an FFM-based implementation of harfbuzz OpenType layout
>
> src/java.desktop/share/classes/sun/font/HBShaper.java line 310:
> 
>> 308:         SequenceLayout glyphInfosLayout = MemoryLayout.sequenceLayout(maxinfo, GlyphInfoLayout);
>> 309:         codePointHandle = getVarHandle(glyphInfosLayout, "codepoint");
>> 310:         clusterHandle = getVarHandle(glyphInfosLayout, "cluster");
> 
> There are better ways to do this in the latest API, by using the extra offset parameter and `MemoryLayout::scaleHandle`.
> 
> I suggest changing this to:
> 
> Suggestion:
> 
>         x_offsetHandle = getVarHandle(PositionLayout, "x_offset");
>         y_offsetHandle = getVarHandle(PositionLayout, "y_offset");
>         x_advanceHandle = getVarHandle(PositionLayout, "x_advance");
>         y_advanceHandle = getVarHandle(PositionLayout, "y_advance");
>         codePointHandle = getVarHandle(GlyphInfoLayout, "codepoint");
>         clusterHandle = getVarHandle(GlyphInfoLayout, "cluster");
> 
> 
> And then in `getVarHandle`:
> 
> 
>     private static VarHandle getVarHandle(MemoryLayout enclosing, String name) {
>         VarHandle h = layout.varHandle(PathElement.groupElement(name));
>         // scale offset by the size of 'enclosing'
>         h = MethodHandles.collectCoordinates(h, 1, enclosing.scaleHandle());
>         /* insert 0 offset so don't need to pass arg every time */
>         return MethodHandles.insertCoordinates(h, 1, 0L).withInvokeExactBehavior();
>     }
> 
> 
> (hope I eyeballed that correctly...)

I've just pushed this change, but I'm unclear why it is "better". It seems more obscure to me.
I've added a somewhat expanded comment of what I guess this does to help
me and others understand what it does. Please take a look to see if it makes sense.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15476#discussion_r1362900089