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

[Maurizio Cimadamore]

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

Marked as reviewed by mcimadamore (Reviewer).

src/java.base/share/classes/jdk/internal/foreign/Utils.java line 130:

> 128:      */
> 129:     public static VarHandle makeRawSegmentViewVarHandle(MemoryLayout enclosing, ValueLayout layout, boolean noStride, long offset) {
> 130:         if (enclosing instanceof ValueLayout direct) {

For now the caching is ok. Moving forrward, I wonder if we shouldn't add more reuse between field var handles in cases like these:


struct Point {
   int x; int y;
}



struct Tuple {
   int u; int v;
}


E.g. if I access the first field in these two structs, then the var handle will be identical? In a way, we use the enclosing layout for two reasons:

* to determine the size `S` of the accessed memory region
* to determine the alignment `A` of the accessed memory region

So, if the static `offset`, accessed `layout`, `S` and `A` are the same for two accesses, we should probably reuse the same var handle instance? E.g. whether we access `x` in a `Point`, or `u` in a `Tuple` makes little difference.

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

PR Review: https://git.openjdk.org/jdk/pull/23720#pullrequestreview-2636794415
PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1967472774