RFR: 8350118: Simplify the layout access VarHandle

[Chen Liang]

On Fri, 21 Feb 2025 10:12:45 GMT, Maurizio Cimadamore <mcimadamore 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
>
> src/java.base/share/classes/java/lang/invoke/X-VarHandleSegmentView.java.template line 77:
> 
>> 75:         }
>> 76: 
>> 77:         @ForceInline
> 
> Question: can this and the `offset` method go in the `VarHandleSegmentViewBase` class? They don't seem to depend on anything in these classes - so it seems mostly duplicated code?

Now this will require code update to static import `offset` from `VarHandleSegmentViewBase`. Is that acceptable?

> src/java.base/share/classes/jdk/internal/foreign/layout/ValueLayouts.java line 164:
> 
>> 162:         @ForceInline
>> 163:         public final VarHandle varHandle() {
>> 164:             record VarHandleCache() implements Function<AbstractValueLayout<?>, VarHandle> {
> 
> Can this cache be removed? The var handles created here should be "direct" -- meaning that we don't expect LayoutPath to add any adaptations on top of what's already done in the "raw" var handle returned by `Utils:makeRawSegmentViewVarHandle`?

We have two caches, one in `ValueLayouts` and another in `Utils`. Should we remove both?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1965763775
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1965771047