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

[Chen Liang]

On Mon, 24 Feb 2025 11:34:15 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> 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/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.

I think direct var handles are quite cheap already and don't really need caching. What really worth caching should be the combinators and the VH derived from those combinators.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23720#discussion_r1968262515