RFR: 8350118: Simplify the layout access VarHandle

[Maurizio Cimadamore]

On Fri, 21 Feb 2025 00:01:36 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

I wonder if the changes affecting `isAccessModeSupported` should warrant a small CSR?

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PR Comment: https://git.openjdk.org/jdk/pull/23720#issuecomment-2674266390