JavadocHelperTest.java OOMEs with Parallel GC and ZGC

Either of two runs:
make test TEST=langtools/jdk/internal/shellsupport/doc/JavadocHelperTest.java TEST_VM_OPTS="-XX:+UseZGC -XX:+ZGenerational"
make test TEST=langtools/jdk/internal/shellsupport/doc/JavadocHelperTest.java TEST_VM_OPTS="-XX:+UseZGC"

cause

java.lang.IllegalStateException: java.lang.OutOfMemoryError: Java heap space
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.parse(JavacTaskImpl.java:252)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.findSource(JavadocHelper.java:767)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.getSourceElement(JavadocHelper.java:657)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.getResolvedDocComment(JavadocHelper.java:170)
at JavadocHelperTest.testAllDocs(JavadocHelperTest.java:424)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
at org.testng.internal.TestInvoker.invokeMethod(TestInvoker.java:599)
at org.testng.internal.TestInvoker.invokeTestMethod(TestInvoker.java:174)
at org.testng.internal.MethodRunner.runInSequence(MethodRunner.java:46)
at org.testng.internal.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:822)
at org.testng.internal.TestInvoker.invokeTestMethods(TestInvoker.java:147)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:146)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:128)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1597)
at org.testng.TestRunner.privateRun(TestRunner.java:764)
at org.testng.TestRunner.run(TestRunner.java:585)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:384)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:378)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:337)
at org.testng.SuiteRunner.run(SuiteRunner.java:286)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:53)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:96)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1218)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1140)
at org.testng.TestNG.runSuites(TestNG.java:1069)
at org.testng.TestNG.run(TestNG.java:1037)
at com.sun.javatest.regtest.agent.TestNGRunner.main(TestNGRunner.java:102)
at com.sun.javatest.regtest.agent.TestNGRunner.main(TestNGRunner.java:58)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
at java.base/java.lang.Thread.run(Thread.java:1570)
Caused by: java.lang.OutOfMemoryError: Java heap space
at java.base/java.util.HashMap.resize(HashMap.java:710)
at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1206)
at jdk.compiler/com.sun.tools.javac.util.StringNameTable.fromString(StringNameTable.java:65)
at jdk.compiler/com.sun.tools.javac.util.Names.fromString(Names.java:462)
at jdk.compiler/com.sun.tools.javac.util.Names.<init>(Names.java:246)
at jdk.compiler/com.sun.tools.javac.util.Names.instance(Names.java:44)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.<init>(JavaCompiler.java:391)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.instance(JavaCompiler.java:127)
at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment.<init>(JavacProcessingEnvironment.java:213)
at jdk.compiler/com.sun.tools.javac.processing.JavacProcessingEnvironment.instance(JavacProcessingEnvironment.java:198)
at jdk.compiler/com.sun.tools.javac.api.BasicJavacTask.initPlugins(BasicJavacTask.java:218)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.prepareCompiler(JavacTaskImpl.java:204)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.parseInternal(JavacTaskImpl.java:257)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl$$Lambda/0x0000000361187cd0.call(Unknown Source)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.invocationHelper(JavacTaskImpl.java:152)
at jdk.compiler/com.sun.tools.javac.api.JavacTaskImpl.parse(JavacTaskImpl.java:248)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.findSource(JavadocHelper.java:767)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.getSourceElement(JavadocHelper.java:657)
at jdk.compiler/jdk.internal.shellsupport.doc.JavadocHelper$OnDemandJavadocHelper.getResolvedDocComment(JavadocHelper.java:170)
at JavadocHelperTest.testAllDocs(JavadocHelperTest.java:424)
at java.base/java.lang.invoke.LambdaForm$DMH/0x0000000361038000.invokeVirtual(LambdaForm$DMH)
at java.base/java.lang.invoke.LambdaForm$MH/0x0000000361038800.invoke(LambdaForm$MH)
at java.base/java.lang.invoke.Invokers$Holder.invokeExact_MT(Invokers$Holder)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invokeImpl(DirectMethodHandleAccessor.java:153)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:132)
at org.testng.internal.TestInvoker.invokeMethod(TestInvoker.java:599)
at org.testng.internal.TestInvoker.invokeTestMethod(TestInvoker.java:174)
at org.testng.internal.MethodRunner.runInSequence(MethodRunner.java:46)
at org.testng.internal.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:822)
at org.testng.internal.TestInvoker.invokeTestMethods(TestInvoker.java:147)