Add an option to retain JNI IDs after class unloading

In order to measure the impact of keeping jmethodIDs valid for users of
AsyncGetCallTrace, add a command line option to retain the JNI ID pools
on class unload.

New option is `-XX:[+|-]KeepJNIIDs`

Related: #17466

Signed-off-by: Graham Chapman <[email protected]>

Author: gacholio

Diff for: runtime/oti/j9consts.h

@@ -337,8 +337,7 @@ extern "C" {
 #define J9_EXTENDED_RUNTIME_REDUCE_CPU_MONITOR_OVERHEAD 0x80000000
 
 /* constants for J9JavaVM.extendedRuntimeFlags2 */
-/* Bit 0x1 is free to use. */
-#define J9_EXTENDED_RUNTIME2_UNUSED_0x1 0x1
+#define J9_EXTENDED_RUNTIME2_KEEP_JNI_IDS 0x1
 #define J9_EXTENDED_RUNTIME2_COMPRESS_OBJECT_REFERENCES 0x2
 #define J9_EXTENDED_RUNTIME2_ENABLE_PREVIEW 0x4
 #define J9_EXTENDED_RUNTIME2_LOAD_AGENT_MODULE 0x8

Diff for: runtime/oti/jvminit.h

@@ -355,6 +355,8 @@ enum INIT_STAGE {
 #define VMOPT_XXHANDLESIGUSR2 "-XX:+HandleSIGUSR2"
 #define VMOPT_XXHEAPDUMPONOOM "-XX:+HeapDumpOnOutOfMemoryError"
 #define VMOPT_XXNOHEAPDUMPONOOM "-XX:-HeapDumpOnOutOfMemoryError"
+#define VMOPT_XXKEEPJNIIDS "-XX:+KeepJNIIDs"
+#define VMOPT_XXNOKEEPJNIIDS "-XX:-KeepJNIIDs"
 #define VMOPT_XDUMP_EXIT_OUTOFMEMORYERROR "-Xdump:exit:events=systhrow,filter=java/lang/OutOfMemoryError"
 #define VMOPT_XDUMP_EXIT_OUTOFMEMORYERROR_DISABLE "-Xdump:exit:none:events=systhrow,filter=java/lang/OutOfMemoryError"
 

Diff for: runtime/vm/classallocation.c

@@ -376,8 +376,11 @@ freeClassLoader(J9ClassLoader *classLoader, J9JavaVM *javaVM, J9VMThread *vmThre
 		RELEASE_CLASS_LOADER_BLOCKS_MUTEX(javaVM);
 	}
 
-	if (classLoader->jniIDs != NULL) {
-		pool_kill(classLoader->jniIDs);
+	if (NULL != classLoader->jniIDs) {
+		if (J9_ARE_NO_BITS_SET(javaVM->extendedRuntimeFlags2, J9_EXTENDED_RUNTIME2_KEEP_JNI_IDS)) {
+			pool_kill(classLoader->jniIDs);
+		}
+		classLoader->jniIDs = NULL;
 	}
 
 	if (NULL != classLoader->classHashTable) {

Diff for: runtime/vm/jvminit.c

@@ -4074,6 +4074,16 @@ processVMArgsFromFirstToLast(J9JavaVM * vm)
 	}
 #endif /* JAVA_SPEC_VERSION >= 19 */
 
+	{
+		IDATA keepJNIIDs = FIND_AND_CONSUME_VMARG(EXACT_MATCH, VMOPT_XXKEEPJNIIDS, NULL);
+		IDATA noKeepJNIIDs = FIND_AND_CONSUME_VMARG(EXACT_MATCH, VMOPT_XXNOKEEPJNIIDS, NULL);
+		if (keepJNIIDs > noKeepJNIIDs) {
+			vm->extendedRuntimeFlags2 |= J9_EXTENDED_RUNTIME2_KEEP_JNI_IDS;
+		} else if (keepJNIIDs < noKeepJNIIDs) {
+			vm->extendedRuntimeFlags2 &= ~(UDATA)J9_EXTENDED_RUNTIME2_KEEP_JNI_IDS;
+		}
+	}
+
 	return JNI_OK;
 }