RFR: 8350118: Simplify the layout access VarHandle [v5]

[Maurizio Cimadamore]

On Thu, 27 Feb 2025 16:00:47 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Simplify the layout access var handles to be direct in some common cases. Also made `VarHandle::isAccessModeSupported` report if an access mode is supported for a VH.
>> 
>> Reduces the instructions to execute this code in a simple main by 47%:
>> 
>> long[] arr = new long[8];
>> var ms = MemorySegment.ofArray(arr);
>> ms.setAtIndex(ValueLayout.JAVA_BYTE, 12, (byte) 3);
>> 
>> 
>> Main overheads in FFM are identified to be:
>> 1. Eager initialization of direct MethodHandle; can be CDS archived
>> 2. MH combinator forms via LambdaFormEditor, not cached right now and always have large overhead
>> 
>> Still need other measures to deal with common user patterns of `MethodHandles.insertCoordinates(vh, 1, 0L)` which currently is still very slow.
>> 
>> Tests: 2 unrelated failures on tier 1-3
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   noStride -> constantOffset, optimize VH classes to have only 2 instead of 3 classes for each type

src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template line 53:

> 51: 
> 52:     static final VarForm CONSTANT_OFFSET_FORM = new VarForm(VarHandleSegmentAs$Type$s.class, MemorySegment.class, $type$.class, long.class);
> 53:     static final VarForm TAKE_OFFSET_FORM = new VarForm(VarHandleSegmentAs$Type$s.class, MemorySegment.class, $type$.class, long.class, long.class);

what does TAKE mean here? (the old names weren't great either, I realize). I think it would be better to decide, once and for all, how to refer to these things, and then use consistent naming throughout. I see we have discussed using "noStride" vs. "fixedOffset". Since the implementation now uses the term "constantOffset", let's just call the other "variableOffset" ?

src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template line 120:

> 118: 
> 119: // This class must be accessed through non-aligned VarHandleSegmentAs$Type$s
> 120: final class VarHandleSegmentAs$Type$sAligned extends VarHandleSegmentAs$Type$s {

I kind of liked the nested classes -- why the change?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1975192481
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1975193951