RFR: 8351565: Implement JEP 502: Stable Values (Preview) [v51]

[Rémi Forax]

On Mon, 7 Apr 2025 16:00:41 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> Implement JEP 502.
>> 
>> The PR passes tier1-tier3 tests.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix typo in return type

Hello,
I do not know if you know but StableValue.map() does not seems to be optimized correctly.

Here is the benchmark i use:

@Warmup(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 2, timeUnit = TimeUnit.SECONDS)
@Fork(value = 1, jvmArgs = { "--enable-preview" })
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@State(Scope.Benchmark)
public class StableValueBenchmarks {
  private static final MemoryLayout LAYOUT = MemoryLayout.structLayout(
      ValueLayout.JAVA_INT.withName("x"),
      ValueLayout.JAVA_INT.withName("y")
  );

  private static final long SIZEOF = LAYOUT.byteSize();
  private static final long OFFSET_X = LAYOUT.byteOffset(groupElement("x"));
  private static final long OFFSET_Y = LAYOUT.byteOffset(groupElement("y"));

  private static final VarHandle VH_X = LAYOUT.arrayElementVarHandle(groupElement("x"))
      .withInvokeExactBehavior();
  private static final VarHandle VH_Y = LAYOUT.arrayElementVarHandle(groupElement("y"))
      .withInvokeExactBehavior();

  private static final Supplier<VarHandle> SV_X = StableValue.supplier(
      () -> LAYOUT.arrayElementVarHandle(groupElement("x")).withInvokeExactBehavior());
  private static final Supplier<VarHandle> SV_Y = StableValue.supplier(
      () -> LAYOUT.arrayElementVarHandle(groupElement("y")).withInvokeExactBehavior());

  private static final Map<String, VarHandle> SMAP = StableValue.map(
      Set.of("x", "y"),
      name -> LAYOUT.arrayElementVarHandle(groupElement(name)).withInvokeExactBehavior());

  private final MemorySegment confined;
  {
    var array = new int[512 * (int) SIZEOF / (int) ValueLayout.JAVA_INT.byteSize()];
    var heap = MemorySegment.ofArray(array);
    for(var i = 0; i < 512; i++) {
      heap.set(ValueLayout.JAVA_INT, i * SIZEOF + OFFSET_X, i);
      heap.set(ValueLayout.JAVA_INT, i * SIZEOF + OFFSET_Y, i);
    }
    confined = Arena.ofConfined().allocate(LAYOUT, 512);
    confined.copyFrom(heap);
  }

  @Benchmark
  public int confinedVarHandleLoop() {
    var sum = 0;
    for(var i = 0; i < 512; i++) {
      var x = (int) VH_X.get(confined, 0L, (long) i);
      var y = (int) VH_Y.get(confined, 0L, (long) i);
      sum += x +y;
    }
    return sum;
  }

  @Benchmark
  public int confinedStableValueLoop() {
    var sum = 0;
    for(var i = 0; i < 512; i++) {
      var x = (int) SV_X.get().get(confined, 0L, (long) i);
      var y = (int) SV_Y.get().get(confined, 0L, (long) i);
      sum += x +y;
    }
    return sum;
  }

  @Benchmark
  public int confinedStableMapLoop() {
    var sum = 0;
    for(var i = 0; i < 512; i++) {
      var x = (int) SMAP.get("x").get(confined, 0L, (long) i);
      var y = (int) SMAP.get("y").get(confined, 0L, (long) i);
      sum += x +y;
    }
    return sum;
  }
}


Results for Stable.map() are way of:

Benchmark                                      Mode  Cnt      Score    Error  Units
StableValueBenchmarks.confinedStableMapLoop    avgt    5  21832,999 ± 50,422  ns/op
StableValueBenchmarks.confinedStableValueLoop  avgt    5    144,546 ±  0,640  ns/op
StableValueBenchmarks.confinedVarHandleLoop    avgt    5    146,320 ±  1,789  ns/op

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

PR Comment: https://git.openjdk.org/jdk/pull/23972#issuecomment-2790451306