RFR: 8334137: Marlin: replace sun.misc.Unsafe memory access methods with FFM

[Kevin Rushforth]

On Mon, 2 Jun 2025 20:11:20 GMT, Laurent Bourgès <lbourges at openjdk.org> wrote:

>> modules/javafx.graphics/src/main/java/com/sun/marlin/OffHeapArray.java line 46:
>> 
>>> 44:     // FFM stuff
>>> 45:     private static final ValueLayout.OfByte BYTE_LAYOUT = ValueLayout.JAVA_BYTE;
>>> 46:     private static final ValueLayout.OfInt INT_LAYOUT = ValueLayout.JAVA_INT.withOrder(ByteOrder.BIG_ENDIAN);
>> 
>> @minborg Is this the best layout to use? I chose it thinking that, since we don't need to access this memory from native code, it might perform better to use Java's big endian byte order. Is this correct?
>
> It may depend on hardward arch ???

Based on some additional checking I did, there is no good reason to set the byte order explicitly, so I'll just use `ValueLayout.JAVA_INT`.

>> modules/javafx.graphics/src/main/java/com/sun/marlin/OffHeapArray.java line 78:
>> 
>>> 76:         // note: may throw OOME:
>>> 77:         // KCR FIXME: Set a MemoryLayout
>>> 78:         this.segment = arena.allocate(len);
>> 
>> I'll choose a memory layout and also specify 4-byte alignment.
>
> Yes 4-bytes min or more like 16 if it helps the compiler to use avx instructions for fill / copy ?

I'll set it to 16 then.

>> modules/javafx.graphics/src/main/java/com/sun/marlin/OffHeapArray.java line 145:
>> 
>>> 143:         if (this.used > 0) {
>>> 144:             MemorySegment.copy(segment, 0, newSegment, 0, Math.min(this.used, len));
>>> 145:         }
>> 
>> @bourgesl I presume that there is never a need to copy more than `used` bytes?
>
> True.

Thanks for confirming.

>> modules/javafx.graphics/src/main/java/com/sun/marlin/Renderer.java line 650:
>> 
>>> 648:         // KCR: Double-check this
>>> 649:         // Clear used bytes in edges array
>>> 650:         edges.setUsed(0);
>> 
>> @bourgesl I presume that clearing `used` is correct here?
>
> Yes. (I skipped that code as edges were left dirty until the next init() call to reset it unconditionally, resize was just a realloc to trim array)

Thanks for confirming.

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

PR Review Comment: https://git.openjdk.org/jfx/pull/1814#discussion_r2132049582
PR Review Comment: https://git.openjdk.org/jfx/pull/1814#discussion_r2132050659
PR Review Comment: https://git.openjdk.org/jfx/pull/1814#discussion_r2132051163
PR Review Comment: https://git.openjdk.org/jfx/pull/1814#discussion_r2132051446