6714404: Add UseStringCache switch to enable String caching under AggressiveOpts

Summary: Poke String.stringCacheEnabled during vm initialization
Reviewed-by: never

src/share/vm/classfile/vmSymbols.hpp

@@ -283,6 +283,7 @@
   template(cache_field_name,                          "cache")                                    \
   template(value_name,                                "value")                                    \
   template(frontCacheEnabled_name,                    "frontCacheEnabled")                        \
+  template(stringCacheEnabled_name,                   "stringCacheEnabled")                       \
                                                                                                   \
   /* non-intrinsic name/signature pairs: */                                                       \
   template(register_method_name,                      "register")                                 \

src/share/vm/runtime/globals.hpp

@@ -2246,6 +2246,9 @@ class CommandLineFlags {
   product(bool, AggressiveOpts, false,                                      \
           "Enable aggressive optimizations - see arguments.cpp")            \
                                                                             \
+  product(bool, UseStringCache, false,                                      \
+          "Enable String cache capabilities on String.java")                \
+                                                                            \
   /* statistics */                                                          \
   develop(bool, UseVTune, false,                                            \
           "enable support for Intel's VTune profiler")                      \

src/share/vm/runtime/thread.cpp

@@ -2926,6 +2926,7 @@ jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
     }
 
     if (AggressiveOpts) {
+      {
         // Forcibly initialize java/util/HashMap and mutate the private
         // static final "frontCacheEnabled" field before we start creating instances
 #ifdef ASSERT
@@ -2944,6 +2945,26 @@ jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
         }
       }
 
+      if (UseStringCache) {
+        // Forcibly initialize java/lang/String and mutate the private
+        // static final "stringCacheEnabled" field before we start creating instances
+#ifdef ASSERT
+        klassOop tmp_k = SystemDictionary::find(vmSymbolHandles::java_lang_String(), Handle(), Handle(), CHECK_0);
+        assert(tmp_k == NULL, "java/lang/String should not be loaded yet");
+#endif
+        klassOop k_o = SystemDictionary::resolve_or_null(vmSymbolHandles::java_lang_String(), Handle(), Handle(), CHECK_0);
+        KlassHandle k = KlassHandle(THREAD, k_o);
+        guarantee(k.not_null(), "Must find java/lang/String");
+        instanceKlassHandle ik = instanceKlassHandle(THREAD, k());
+        ik->initialize(CHECK_0);
+        fieldDescriptor fd;
+        // Possible we might not find this field; if so, don't break
+        if (ik->find_local_field(vmSymbols::stringCacheEnabled_name(), vmSymbols::bool_signature(), &fd)) {
+          k()->bool_field_put(fd.offset(), true);
+        }
+      }
+    }
+
     // Initialize java_lang.System (needed before creating the thread)
     if (InitializeJavaLangSystem) {
       initialize_class(vmSymbolHandles::java_lang_System(), CHECK_0);