[foreign-memaccess] RFR: Add MemorySegment::mismatch

[Chris Hegarty]

On Wed, 20 May 2020 14:16:11 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Hi,
>> 
>> As part of feedback on the Foreign Memory API (when experimenting with its usage internally in the JDK), a small number
>> of potential usability enhancements could be made to the API. This is the fourth such, and last on my current todo
>> list.  This change proposes to add a new method:
>> MemorySegment::mismatch
>> 
>> The mismatch semantic is very useful for building equality and comparison logic on top of segments. I found that I
>> needed such when modeling and comparing native socket address in the JDK implementation. It is possible to write your
>> own, but requires a non-trivial amount of not-trivial code - to do it right! I also think that we can provide a more
>> efficient implementation building on top of the JDK's internal mismatch support.  I still need to do some perf testing
>> and add a micro-benchmake ( Maurizio suggested possibly amending TestBulkOps ). There is also the question about
>> possibly improving the JDK's internal implementation to work on long sizes (which could be done separately). For now, I
>> just want to share the idea, along with the proposed specification and initial implementation.  Comments welcome.
>
> src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemorySegment.java line 347:
> 
>> 346:      * the smaller of the segment sizes, and it follows that the offset is only
>> 347:      * valid for the larger segment. Otherwise, there is no mismatch.
>> 348:      *
> 
> there is no mismatch and the returned value is (I think you say `-1` below, perhaps would be good to state that here
> also).

Correct. Added, Otherwise, there is no mismatch "and {@code  -1} is returned".

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PR: https://git.openjdk.java.net/panama-foreign/pull/180