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

[Maurizio Cimadamore]

On 29/09/2021 18:03, Lee Rhodes wrote:
> On Tue, 28 Sep 2021 12:59:19 GMT, Julia Boes <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>
>> :
>>
>>> +    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