RFR: 8350118: Simplify the layout access VarHandle [v2]

[Jorn Vernee]

On Fri, 21 Feb 2025 20:14:19 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:
> 
>   Review remarks, dates, some more simplifications

src/java.base/share/classes/jdk/internal/foreign/LayoutPath.java line 216:

> 214:         } else {
> 215:             // simpler adaptation
> 216:             handle = MethodHandles.insertCoordinates(handle, 2, offset);            // (MS, long)

Okay, looking at this code I think I get it: when the offset into the enclosing layout is fixed, we don't have strides (from `sequenceElement()`), so we don't need the extra adaptation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1972024703