8356255: Add Stable Field Updaters to allow efficient lazy field evaluations #25040

minborg · 2025-05-05T13:41:22Z

This sketch shows how "Stable Updaters" can be used to create stable computations of @Stable fields. Only one updater is needed per class, similar to AtomicIntegerFieldUpdater.

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JDK-8356255: Add Stable Field Updaters to allow efficient lazy field evaluations (Enhancement - P3)

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bridgekeeper · 2025-05-05T13:42:00Z

👋 Welcome back pminborg! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-05-05T13:42:06Z

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openjdk · 2025-05-05T13:42:35Z

@minborg The following labels will be automatically applied to this pull request:

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minborg · 2025-05-05T13:48:17Z

Benchmarks:

Benchmark                        Mode  Cnt  Score   Error  Units
StableUpdatersBenchmark.base     avgt   10  0.715 ± 0.026  ns/op
StableUpdatersBenchmark.updater  avgt   10  0.716 ± 0.079  ns/op

Which shows the updater has the same performance as imperative code which is good.

src/java.base/share/classes/java/lang/reflect/Method.java

minborg · 2025-05-06T09:04:17Z

Updated benchmarks:

Benchmark                              Mode  Cnt  Score   Error  Units
StableUpdatersBenchmark.base           avgt   10  0.838 ± 0.041  ns/op
StableUpdatersBenchmark.baseStatic     avgt   10  0.759 ± 0.088  ns/op
StableUpdatersBenchmark.updater        avgt   10  0.888 ± 0.086  ns/op
StableUpdatersBenchmark.updaterStatic  avgt   10  0.753 ± 0.017  ns/op
StableUpdatersBenchmark.uri            avgt   10  0.781 ± 0.206  ns/op
StableUpdatersBenchmark.uriStatic      avgt   10  0.724 ± 0.048  ns/op

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

test/jdk/java/lang/StableValue/StableFieldUpdaterExampleTest.java

src/java.base/share/classes/java/lang/reflect/Method.java

mlbridge · 2025-05-06T11:48:11Z

Webrevs

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

src/java.base/share/classes/java/net/URI.java

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

david-beaumont

Starting to look nicely idiomatic. If I saw this I would however want to add a thin wrapper specific for hash codes where the "zero replacement" and generic types are more hidden.

HashCoder.forIntField(Foo.class, "hash", ...)
HashCoder.forLongField(...)
I like that there's an annotation on the field now though!

test/jdk/java/lang/StableValue/StableFieldUpdaterExampleTest.java

liach · 2025-05-07T12:54:46Z

The MH+VH form should be much more lightweight: MH and VH are just MemberName references.

I wonder if we can advertise the MH+VH version as an indy BSM:

public static CallSite lazyAccessor(Lookup lookup, String unused, MethodType callType, VarHandle field, MethodHandle computer)

The CallSite would be constant, with StableIntFieldUpdaterVarHandle.applyAsInt bound to the specific updater. This way, we can even defer the creation of the updater.

ExE-Boss

StableFieldUpdater.checkAndAdapt(…) also performs an implicit null‑check:

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

david-beaumont · 2025-05-08T15:22:22Z

test/jdk/java/lang/StableValue/StableFieldUpdaterExampleTest.java

+        private final Baz baz;
+
+        private static final ToIntFunction<LazyFoo> HASH_UPDATER =
+                StableFieldUpdater.ofInt(LazyFoo.class, "hash",


Very clean now.

test/jdk/java/lang/StableValue/StableFieldUpdaterExampleTest.java

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

ExE-Boss · 2025-05-14T15:49:29Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

+    public static CallSite lazyAtMostOnce(MethodHandles.Lookup lookup,
+                                          String unused,
+                                          VarHandle accessor,
+                                          MethodHandle underlying) {


This is not a valid bootstrap method signature, as that needs a MethodType for an invokedynamic or Class<?> for a dynamic constant:

Suggested change

public static CallSite lazyAtMostOnce(MethodHandles.Lookup lookup,

String unused,

VarHandle accessor,

MethodHandle underlying) {

public static CallSite lazyAtMostOnce(MethodHandles.Lookup lookup,

String unused,

MethodType type,

VarHandle accessor,

MethodHandle underlying) {

ExE-Boss · 2025-05-14T15:53:48Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

+     * {@return a function that lazily sets the field accessible via the provided
+     *          {@code accessor} by invoking the provided {@code underlying} function
+     *          if the field has its default value (e.g. zero). Otherwise, the
+     *          returned function returns the set field value}
+     * <p>


The inline form of @return is not indented elsewhere in the JDK:

Suggested change

* {@return a function that lazily sets the field accessible via the provided

* {@code accessor} by invoking the provided {@code underlying} function

* if the field has its default value (e.g. zero). Otherwise, the

* returned function returns the set field value}

* <p>

* {@return a function that lazily sets the field accessible via the provided

* {@code accessor} by invoking the provided {@code underlying} function

* if the field has its default value (e.g. zero). Otherwise, the returned

* function returns the set field value}

ExE-Boss · 2025-05-14T15:55:15Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

+     * @throws IllegalArgumentException if the provided {@code underlying} function does
+     *                                  take exactly one parameter of a reference type.


Suggested change

* @throws IllegalArgumentException if the provided {@code underlying} function does

* take exactly one parameter of a reference type.

* @throws IllegalArgumentException if the provided {@code underlying} function does

* not take the same parameter types as the

* {@code accessor} var handle takes coordinate types.

ExE-Boss · 2025-05-14T16:10:22Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdaterGenerator.java

+            final List<Class<?>> shapeCoordinateTypes = new ArrayList<>(varHandle.coordinateTypes().size());
+            for (Class<?> coordinate: varHandle.coordinateTypes()) {
+                shapeCoordinateTypes.add(toObjectIfReference(coordinate));
+            }
+            return new Shape(toObjectIfReference(varHandle.varType()), shapeCoordinateTypes);


This should construct an unmodifiable list:

Suggested change

final List<Class<?>> shapeCoordinateTypes = new ArrayList<>(varHandle.coordinateTypes().size());

for (Class<?> coordinate: varHandle.coordinateTypes()) {

shapeCoordinateTypes.add(toObjectIfReference(coordinate));

}

return new Shape(toObjectIfReference(varHandle.varType()), shapeCoordinateTypes);

final var coordinateTypes = varHandle.coordinateTypes();

final var shapeCoordinateTypes = new Object[coordinateTypes.size()];

for (int i = 0; i < shapeCoordinateTypes.length; i++) {

shapeCoordinateTypes[i] = toObjectIfReference(coordinateTypes.get(i));

}

return new Shape(toObjectIfReference(varHandle.varType()),

SharedSecrets.getJavaUtilCollectionAccess().listFromTrustedArray(shapeCoordinateTypes));

ExE-Boss · 2025-05-14T16:16:11Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

+        // Allow `invokeExact()` of the `apply(Object)` method
+        final MethodHandle adaptedUnderlying = underlyingType.parameterType(0).equals(Object.class)
+                || underlyingType.parameterType(0).isArray()
+                ? underlying
+                : underlying.asType(underlyingType.changeParameterType(0, Object.class));
+


This procedure is repeated in StableFieldUpdaterGenerator::handle(…):

Suggested change

// Allow `invokeExact()` of the `apply(Object)` method

final MethodHandle adaptedUnderlying = underlyingType.parameterType(0).equals(Object.class)

|| underlyingType.parameterType(0).isArray()

? underlying

: underlying.asType(underlyingType.changeParameterType(0, Object.class));

ExE-Boss · 2025-05-14T16:19:02Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdaterGenerator.java

+                v = (Object) accessor.getAcquire(t);
+                if (v == null) {
+                    try {
+                        v = (int) underlying.invokeExact(t);


Can’t invokeExact here, as the return type of underlying isn’t guaranteed to be Object:

Suggested change

v = (int) underlying.invokeExact(t);

v = (Object) underlying.invoke(t);

ExE-Boss · 2025-05-14T16:26:27Z

src/java.base/share/classes/jdk/internal/lang/stable/StableFieldUpdater.java

+        var handle = MhUtil.findStatic(LOCAL_LOOKUP,
+                "atMostOnce", MethodType.methodType(MethodHandle.class, VarHandle.class, MethodHandle.class));
+        return new ConstantCallSite(MethodHandles.insertArguments(handle, 0, accessor, underlying));


This makes no sense as a bootstrap for a dynamic call site, as that’ll simply be equivalent to a call to atMostOnce.

I presume thou meant something like the following, which would actually create the stable field updater for use at the call site:

Suggested change

var handle = MhUtil.findStatic(LOCAL_LOOKUP,

"atMostOnce", MethodType.methodType(MethodHandle.class, VarHandle.class, MethodHandle.class));

return new ConstantCallSite(MethodHandles.insertArguments(handle, 0, accessor, underlying));

return new ConstantCallSite(atMostOnce(accessor, underlying).asType(type));

or by using the createTargetHook of ConstantCallSite:

Suggested change

var handle = MhUtil.findStatic(LOCAL_LOOKUP,

"atMostOnce", MethodType.methodType(MethodHandle.class, VarHandle.class, MethodHandle.class));

return new ConstantCallSite(MethodHandles.insertArguments(handle, 0, accessor, underlying));

var handle = MhUtil.findStatic(LOCAL_LOOKUP,

"atMostOnce", MethodType.methodType(MethodHandle.class, VarHandle.class, MethodHandle.class));

return new ConstantCallSite(type, MethodHandles.dropArguments(

MethodHandles.insertArguments(handle, 0, accessor, underlying),

0, ConstantCallSite.class));

bridgekeeper · 2025-07-09T19:10:13Z

@minborg This pull request has been inactive for more than 8 weeks and will be automatically closed if another 8 weeks passes without any activity. To avoid this, simply issue a /touch or /keepalive command to the pull request. Feel free to ask for assistance if you need help with progressing this pull request towards integration!

minborg added 3 commits May 5, 2025 15:21

Initial commit

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Add @stable for URI and add benchmark

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minborg changed the title ~~Sketch: StableUpdaters~~ 8356255: Add Stable Field Updaters to allow efficient lazy field evaluations May 6, 2025

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Document field alignment assumption

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liach mentioned this pull request Jun 5, 2025

8358426: Improve lazy computation in Locale #25646

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		* @throws IllegalArgumentException if the provided {@code underlying} function does
		* take exactly one parameter of a reference type.

	v = (int) underlying.invokeExact(t);
	v = (Object) underlying.invoke(t);

8356255: Add Stable Field Updaters to allow efficient lazy field evaluations #25040

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