RFR: 8329997: Add provisions for checking memory segment alignment constraints

[Jorn Vernee]

On Mon, 15 Apr 2024 07:50:24 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to add a new method `MemorySegment::maxByteAlignment` that returns the maximum byte alignment of a segment (both heap and native segments).
> 
> Clients can then use this method to determine if a segment is properly aligned for any given layout (e.g. following a `MemorySegment::reinterpret` operation).

src/java.base/share/classes/jdk/internal/foreign/HeapMemorySegmentImpl.java line 86:

> 84:             alignment = maxAlignMask();
> 85:         }
> 86:         return Math.min(maxAlignMask(), alignment);

Similarly here:

Suggestion:

        return address() == 0
            ? maxAlignMask()
            : Math.min(maxAlignMask(), Long.lowestOneBit(address()));

src/java.base/share/classes/jdk/internal/foreign/NativeMemorySegmentImpl.java line 68:

> 66:                 ? 1L << 62
> 67:                 : alignment;
> 68:     }

This relies on the fact that `lowestOneBit` returns `0` only if `address()` returns `0`, but I suggest making the intent clearer by checking against `address()` directly:

Suggestion:

        return address() == 0
                ? 1L << 62
                : Long.lowestOneBit(address());


(I also suggest introducing a name for the `1L << 62` constant, e.g. `MAX_NATIVE_ALIGN`)

test/jdk/java/foreign/TestMemoryAlignment.java line 194:

> 192:                         new Object[]{MemorySegment.ofArray(new float[]{1}), Float.BYTES},
> 193:                         new Object[]{MemorySegment.ofArray(new double[]{1}), Double.BYTES},
> 194:                         new Object[]{MemorySegment.ofBuffer(ByteBuffer.allocate(8)), Byte.BYTES}

What about the other (heap) buffer types?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/18779#discussion_r1565638655
PR Review Comment: https://git.openjdk.org/jdk/pull/18779#discussion_r1565637521
PR Review Comment: https://git.openjdk.org/jdk/pull/18779#discussion_r1565647024