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

[Phil Race]

On Thu, 25 Jul 2024 23:32:02 GMT, Jorn Vernee <jvernee 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 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`.

Ah. I was using the copy method that uses a MemorySegment wrapped array then my microbenchmark showed the unwrapped one was  faster, so I made a late switch (although then did a full retest) but missed that.

I think testing is still fine because offset is always zero in our code .. 

I will fix that, and think about the other comments.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20339#discussion_r1692409459