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

[Maurizio Cimadamore]

On 20/05/2020 16:36, Paul Sandoz wrote:
> On Wed, 20 May 2020 14:06:48 GMT, Chris Hegarty <chegar 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.
> Very nice.
>
> Improving the JDK implementation is good. In fact i think you could so that now. In `ArraysSupport`, with strip mining:
>
>      public static int vectorizedMismatchLarge(
>                                           Object a, long aOffset,
>                                           Object b, long bOffset,
>                                           long length,
>                                           int log2ArrayIndexScale)
>
> Then you can specialize mismatch of memory segments for length threshold and type (the threshold only really makes
> sense for the first check, once you go over `Integer.MAX_VALUE` the cost of another round with a small length part is
> really low overall).

To be clear - are you suggesting to add vectorizedMismatchLarge inside 
ArraySupport (but not add intrinsic support for that, for now) - and 
then to have the memory segment implementation to call either that or 
the standard version based on whether the segment is small or not?

Maurizio

>
> -------------
>
> Marked as reviewed by psandoz (Committer).
>
> PR: https://git.openjdk.java.net/panama-foreign/pull/180