[foreign-memaccess+abi] RFR: 8264933: Improve stream support in memory segments

[Maurizio Cimadamore]

On Fri, 9 Apr 2021 12:25:52 GMT, Rémi Forax <github.com+828220+forax at openjdk.org> wrote:

> So not using the "bulk" version leads to less performance but because it's a Stream of MemorySegment, you have to know as a user if you use the bulk version or not.

Yes
> 
> I think i would prefer the stream to be a stream of the addresses,a LongStream instead of being a Stream of MemorySegment, so your last example can be written
> 
> ```
>   segment.addresses(C_POINTER)
>        .map(CLinker::toJavaString)
>        .toArray(String[]::new);
> ```
> 
> Here, the element is just there to get its size.
> 
> Having a stream of offsets/addresses will also work better with VarHandles, by example
> 
> ```
>   segment.addresses(AN_INT)
>        .mapToInt(offset -> HANDLE.getInt(segment, offset))
>        .sum();
> ```

I see what you mean: instead of returning slices, return offsets, and let the user deal with it.  Honestly, I'm not sure - it seems like we are, again, prematurely concluding that slicing is gonna be more expensive, while the benchmarks I have show that the performance we get are the same as with Unsafe.

Note that we want each returned segment to be disjoint - an offset only tells me the starting point - doesn't tell me how big it is - which means it is possible/easier for clients to make mistake and write on "somebody else's" slice.

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PR: https://git.openjdk.java.net/panama-foreign/pull/494