[foreign-memaccess] [Rev 01] RFR: Move MemoryAddress::copy

[Paul Sandoz]

On Fri, 15 May 2020 14:45:13 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> As discussed, this patch moves `MemoryAddress::copy` as an *instance* method of `MemorySegment`, namely
>> `MemorySegment::copyFrom`. I chose the `copyFrom` suggestion from John, since it allows us more freedom, in the future,
>> to add overloads for different sources for the copy operation.
>> I've rewrorked the documentation a bit to speak about segment offsets rather than addresses.
>> 
>> I believe all use cases touched by this change actually lead to simpler code, which is good.
>> 
>> I've also added a benchmark for some of the bulk operations such as fill and copyFrom - the numbers I've got look good:
>> 
>> Benchmark             Mode  Cnt       Score       Error  Units
>> BulkOps.segment_copy  avgt   30  441675.750 ? 12794.881  ns/op
>> BulkOps.segment_fill  avgt   30  122465.908 ?  1760.711  ns/op
>> BulkOps.unsafe_copy   avgt   30  430153.694 ?  9055.253  ns/op
>> BulkOps.unsafe_fill   avgt   30  120413.079 ?  3799.484  ns/op
>> 
>> Cheers
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed copyright

I like it. I always thought the prior location of `copy` was a little odd, but it took another method for it to really
stand out to me.

src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemorySegment.java line 313:

> 312:     /**
> 313:      * Perform bulk copy from given source segment to this segment. More specifically, the bytes at
> 314:      * offset {@code 0} through {@code src.byteSize() - 1} in the source segment are copied into this segment

"Performs a bulk copy..."

src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemorySegment.java line 322:

> 321:      * The result of a bulk copy is unspecified if, in the uncommon case, the source segment does not overlap with
> 322:      * this segment, but it instead refers to an overlapping regions of the same backing storage using different
> addresses. 323:      * For example, this may occur if the same file is {@link MemorySegment#mapFromPath mapped} to two
> segments.

"The result of a bulk copy is unspecified if, in the uncommon case, the source segment and this segment do not overlap,
but refer to overlapping regions of the same backing storage using different addresses"

test/micro/org/openjdk/bench/jdk/incubator/foreign/BulkOps.java line 72:

> 71:     public void unsafe_fill() {
> 72:         unsafe.setMemory(unsafe_addr, ALLOC_SIZE, (byte)0);
> 73:     }

Out of an abundance of caution you might wanna choose a non-zero value just in case the JIT knows it's zeroing already
zeroed memory (i suspect it does not).

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PR: https://git.openjdk.java.net/panama-foreign/pull/169