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

make/modules/java.base/gensrc/GensrcScopedMemoryAccess.gmk line 45:

> 43:   $1_Type := $2
> 44: 
> 45:   ifeq ($$($1_Type), Boolean)

These changes in ScopedMemoryAccess are not strictly required -- but I imagine they simplify your implementation of the var handle impl methods. So, it's ok to leave them in.

make/modules/java.base/gensrc/GensrcVarHandles.gmk line 171:

> 169: # Param 1 - Variable declaration prefix
> 170: # Param 2 - Type with first letter capitalized
> 171: # TODO decide if CAS/Bitwise/AtomicAdd should be enabled for subints

Exactly - let's keep this out for now (they seem to be currently enabled -- and I'd prefer to do that separately, after some discussion)

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1965204113
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1965202095