RFR: 8337237: Use FFM instead of Unsafe for Java 2D RenderBuffer class

[Jorn Vernee]

On Thu, 25 Jul 2024 22:53:36 GMT, Phil Race <prr at openjdk.org> wrote:

> Migrate from using Unsafe to FFM's MemorySegment API for allocating and setting native memory.
> This code is used by Metal, OpenGL and D3D, so I manually tested SwingSet2 and J2Demo as well as running all the usual tests.
> I also did some micro-benchmarking on the performance of Unsafe vs MemorySegment.
> The performance of either is more than sufficient for us .. ie they could be 10x slower and we wouldn't even notice.
> But they are in the same ballpark, and if one or the other is clearly faster it is the new FFM code.

src/java.desktop/share/classes/sun/java2d/pipe/RenderBuffer.java line 73:

> 71:     private final long baseAddress;
> 72:     private long curOffset;
> 73:     private final int capacity;

You could perhaps drop the `capacity` field, and use `segment.byteSize()` in it's place.

src/java.desktop/share/classes/sun/java2d/pipe/RenderBuffer.java line 175:

> 173:             int offsetInBytes = offset * SIZEOF_SHORT;
> 174:             int lengthInBytes = length * SIZEOF_SHORT;
> 175:             MemorySegment.copy(x, offsetInBytes, segment, JAVA_SHORT, curOffset, length);

This doesn't look right. When copying from an array like this, the source offset is an array index, not an offset in bytes. So I believe the second argument here should just be `offset`.

src/java.desktop/share/classes/sun/java2d/pipe/RenderBuffer.java line 176:

> 174:             int lengthInBytes = length * SIZEOF_SHORT;
> 175:             MemorySegment.copy(x, offsetInBytes, segment, JAVA_SHORT, curOffset, length);
> 176:             position(position() + lengthInBytes);

If you want, you could use `MemoryLayout::scale` here, i.e.
Suggestion:

            position(JAVA_SHORT.scale(position(), length));

(up to preference though)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20339#discussion_r1692278576
PR Review Comment: https://git.openjdk.org/jdk/pull/20339#discussion_r1692274831
PR Review Comment: https://git.openjdk.org/jdk/pull/20339#discussion_r1692286004