8085965: VM hangs in C2Compiler

Summary: CMSClassUnloadingEnabled and ExplicitGCInvokesConcurrentAndUnloadsClasses should be disabled when -Xnoclassgc is specified
Reviewed-by: jmasa, kbarrett

src/share/vm/memory/genMarkSweep.cpp

@@ -211,7 +211,7 @@ void GenMarkSweep::mark_sweep_phase1(int level,
                          false, // Younger gens are not roots.
                          true,  // activate StrongRootsScope
                          GenCollectedHeap::SO_None,
-                         GenCollectedHeap::StrongRootsOnly,
+                         ClassUnloading,
                          &follow_root_closure,
                          &follow_root_closure,
                          &follow_cld_closure);

src/share/vm/runtime/arguments.cpp

@@ -1372,6 +1372,12 @@ void Arguments::set_cms_and_parnew_gc_flags() {
   if (!FLAG_IS_DEFAULT(CMSParPromoteBlocksToClaim) || !FLAG_IS_DEFAULT(OldPLABWeight)) {
     CFLS_LAB::modify_initialization(OldPLABSize, OldPLABWeight);
   }
+
+  if (!ClassUnloading) {
+    FLAG_SET_CMDLINE(bool, CMSClassUnloadingEnabled, false);
+    FLAG_SET_CMDLINE(bool, ExplicitGCInvokesConcurrentAndUnloadsClasses, false);
+  }
+
   if (PrintGCDetails && Verbose) {
     tty->print_cr("MarkStackSize: %uk  MarkStackSizeMax: %uk",
       (unsigned int) (MarkStackSize / K), (uint) (MarkStackSizeMax / K));