[foreign-memaccess+abi] RFR: 8274157: java.foreign: Add method MemorySegment::isOverlapping

[Maurizio Cimadamore]

On 29/09/2021 22:19, leerho wrote:
> Nothing ��!  You are correct for this particular use case. But 
> /isOverlapping(segment)/ is such a basic bit of information about two 
> segments there might be other use cases that would find this valuable.

Ok - point taken.

And I think that, since we dropped MemoryAddress::segmentOffset (out of 
necessity), having some other way to find the offset of a segment 
relative to each other might come in handy anyway.

Maurizio

>
>
>
> On Wed, Sep 29, 2021 at 10:57 AM Maurizio Cimadamore 
> <maurizio.cimadamore at oracle.com 
> <mailto:maurizio.cimadamore at oracle.com>> wrote:
>
>
>     On 29/09/2021 18:03, Lee Rhodes wrote:
>     > On Tue, 28 Sep 2021 12:59:19 GMT, Julia Boes <jboes at openjdk.org
>     <mailto:jboes at openjdk.org>> wrote:
>     >
>     >> This change adds a method to the MemorySegment API that detects
>     if two segments are overlapping.
>     > Doing byte swapping (i.e, adjusting for endianness) is not
>     required.   This
>     > is for copying structs from one segment to another where all the
>     structs
>     > are the same, but arbitrary size.  If we have to restrict these
>     sizes to
>     > multiples of 2,4, or 8 bytes that would be OK.  But allowing
>     these structs
>     > to be larger than 8 bytes is critical, otherwise we would use
>     existing copy
>     > methods.
>
>     If you don't need byte swapping - then isn't this:
>
>     ```
>     static void copy(MemorySegment srcSegment, long srcOffset,
>     MemorySegment
>     dstSegment, long dstOffset, long bytes)
>     ```
>
>     enough for your needs? What goes wrong if I use the above copy
>     method on
>     one of your struct arrays?
>
>     Maurizio
>
>     >
>     > On Wed, Sep 29, 2021 at 9:31 AM Radoslaw Smogura ***@***.***>
>     > wrote:
>     >
>     >> ***@***.**** commented on this pull request.
>     >> ------------------------------
>     >>
>     >> In
>     >>
>     src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/AbstractMemorySegmentImpl.java
>     >>
>     <https://github.com/openjdk/panama-foreign/pull/585#discussion_r718684769
>     <https://urldefense.com/v3/__https://github.com/openjdk/panama-foreign/pull/585*discussion_r718684769__;Iw!!ACWV5N9M2RV99hQ!YR8HDJD74V1BfaqWbh75wuy6g0jfIjp8pChFyiDl_oEP910hwCtOuZdedUpK-vUC74L_JBU$>>
>     >> :
>     >>
>     >>> +    public boolean isOverlapping(MemorySegment other) {
>     >> +        AbstractMemorySegmentImpl that =
>     (AbstractMemorySegmentImpl)Objects.requireNonNull(other);
>     >> +        if (base() == that.base()) {  // both are either
>     native or heap
>     >> +            this.checkAccess(0, this.length, true);
>     >> +            that.checkAccess(0, that.length, true);
>     >> +            this.checkValidState();
>     >> +            that.checkValidState();
>     >> +
>     >> +            final long thisStart = this.min();
>     >> +            final long thatStart = that.min();
>     >> +            if (thisStart == thatStart) {
>     >> +                return true;
>     >> +            }
>     >> +            final long thisEnd = thisStart + this.byteSize() - 1L;
>     >> +            final long thatEnd = thatStart + that.byteSize() - 1L;
>     >> +            if (thisStart < thatStart && thisEnd >= thatStart) {
>     >>
>     >> Not sure if this is relevant, would it make sense to replace
>     this checks by
>     >> thisStart <= thatEnd && thisEnd >= thatStart
>     >> I think it can be more performant.
>     >>
>     >> Maybe we will be able to drop subtract of 1 and use strong equality
>     >>
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>     > -------------
>     >
>     > PR: https://git.openjdk.java.net/panama-foreign/pull/585
>     <https://git.openjdk.java.net/panama-foreign/pull/585>
>