From 52c595d6499031d4f34ca21e118affb8d6283c03 Mon Sep 17 00:00:00 2001
From: Peter Hofer <peter.hofer@oracle.com>
Date: Thu, 16 Feb 2023 10:27:39 +0100
Subject: [PATCH] Never burden @ValueBased objects with a monitor field,
 including boxed primitives.

---
 .../svm/hosted/HostedConfiguration.java       | 20 +++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/HostedConfiguration.java b/substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/HostedConfiguration.java
index 9d17937e9641..1a0ab027517f 100644
--- a/substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/HostedConfiguration.java
+++ b/substratevm/src/com.oracle.svm.hosted/src/com/oracle/svm/hosted/HostedConfiguration.java
@@ -24,6 +24,7 @@
  */
 package com.oracle.svm.hosted;
 
+import java.lang.annotation.Annotation;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.HashSet;
@@ -74,12 +75,15 @@
 import com.oracle.svm.hosted.meta.HostedMetaAccess;
 import com.oracle.svm.hosted.meta.HostedUniverse;
 import com.oracle.svm.hosted.substitute.UnsafeAutomaticSubstitutionProcessor;
+import com.oracle.svm.util.ReflectionUtil;
 
 import jdk.vm.ci.meta.JavaKind;
 import jdk.vm.ci.meta.MetaAccessProvider;
 import jdk.vm.ci.meta.ResolvedJavaField;
 
 public class HostedConfiguration {
+    @SuppressWarnings("unchecked") private static final Class<? extends Annotation> VALUE_BASED_ANNOTATION = //
+                    (Class<? extends Annotation>) ReflectionUtil.lookupClass(false, "jdk.internal.ValueBased");
 
     public HostedConfiguration() {
     }
@@ -230,7 +234,10 @@ public AbstractAnalysisResultsBuilder createStaticAnalysisResultsBuilder(Inflati
 
     public void collectMonitorFieldInfo(BigBang bb, HostedUniverse hUniverse, Set<AnalysisType> immutableTypes) {
         /* First set the monitor field for types that always need it. */
-        getForceMonitorSlotTypes(bb).forEach(type -> setMonitorField(hUniverse, type));
+        for (AnalysisType type : getForceMonitorSlotTypes(bb)) {
+            assert !immutableTypes.contains(type);
+            setMonitorField(hUniverse, type);
+        }
 
         /* Then decide what other types may need it. */
         processedSynchronizedTypes(bb, hUniverse, immutableTypes);
@@ -253,11 +260,16 @@ protected void processedSynchronizedTypes(BigBang bb, HostedUniverse hUniverse,
     }
 
     /**
-     * Monitor fields on arrays would increase the array header too much. Also, types that must be
-     * immutable cannot have a monitor field.
+     * Monitor fields on arrays would enlarge the array header too much.
+     *
+     * Also, never burden @ValueBased classes with a monitor field, which in particular reduces
+     * sizes of boxed primitives. Using monitors with those types is discouraged (instances are
+     * often cached) and not guaranteed to work in the future.
+     *
+     * Types that must be immutable cannot have a monitor field.
      */
     protected static void maybeSetMonitorField(HostedUniverse hUniverse, Set<AnalysisType> immutableTypes, AnalysisType type) {
-        if (!type.isArray() && !immutableTypes.contains(type)) {
+        if (!type.isArray() && !immutableTypes.contains(type) && !type.isAnnotationPresent(VALUE_BASED_ANNOTATION)) {
             setMonitorField(hUniverse, type);
         }
     }