[GR-64664] Fix JFR epoch id.

PullRequest: graal/20691

Author: christianhaeubl

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/jfr/traceid/JfrTraceIdEpoch.java

@@ -30,6 +30,7 @@
 import org.graalvm.nativeimage.Platforms;
 
 import com.oracle.svm.core.Uninterruptible;
+import com.oracle.svm.core.thread.Target_java_lang_VirtualThread;
 import com.oracle.svm.core.thread.VMOperation;
 
 import jdk.graal.compiler.api.replacements.Fold;
@@ -43,7 +44,12 @@ public class JfrTraceIdEpoch {
     private static final long EPOCH_0_BIT = 0b01;
     private static final long EPOCH_1_BIT = 0b10;
 
-    private long epochId;
+    /**
+     * Start the epoch id at 1, so that we can inject fields into JDK classes that store the epoch
+     * id (see for example {@link Target_java_lang_VirtualThread#jfrEpochId}). This avoids problems
+     * with uninitialized injected fields that have the value 0 by default.
+     */
+    private long epochId = 1;
 
     @Fold
     public static JfrTraceIdEpoch getInstance() {

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/thread/Target_java_lang_VirtualThread.java

@@ -123,8 +123,8 @@ public final class Target_java_lang_VirtualThread {
     // Checkstyle: resume
 
     @Inject //
-    @RecomputeFieldValue(kind = RecomputeFieldValue.Kind.Custom, declClass = ResetToMinusOneTransformer.class) //
-    public long jfrEpochId = -1;
+    @RecomputeFieldValue(kind = RecomputeFieldValue.Kind.Reset) //
+    public long jfrEpochId;
 
     @Alias
     private static native ForkJoinPool createDefaultScheduler();
@@ -602,10 +602,3 @@ static Thread asThread(Object obj) {
     private VirtualThreadHelper() {
     }
 }
-
-final class ResetToMinusOneTransformer implements FieldValueTransformer {
-    @Override
-    public Object transform(Object receiver, Object originalValue) {
-        return -1L;
-    }
-}