8159370: Add FlagGuard for easier modification of flags for unit tests

Reviewed-by: kbarrett, jwilhelm

hotspot/src/share/vm/runtime/globals.hpp

@@ -463,6 +463,29 @@ class SizeTFlagSetting {
   ~SizeTFlagSetting()                           { *flag = val; }
 };
 
+// Helper class for temporarily saving the value of a flag during a scope.
+template <size_t SIZE>
+class FlagGuard {
+  unsigned char _value[SIZE];
+  void* const _addr;
+
+  // Hide operator new, this class should only be allocated on the stack.
+  // NOTE: Cannot include memory/allocation.hpp here due to circular
+  //       dependencies.
+  void* operator new(size_t size) throw();
+  void* operator new [](size_t size) throw();
+
+ public:
+  FlagGuard(void* flag_addr) : _addr(flag_addr) {
+    memcpy(_value, _addr, SIZE);
+  }
+
+  ~FlagGuard() {
+    memcpy(_addr, _value, SIZE);
+  }
+};
+
+#define FLAG_GUARD(f) FlagGuard<sizeof(f)> f ## _guard(&f)
 
 class CommandLineFlags {
 public:

hotspot/test/native/runtime/test_globals.cpp

@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "runtime/globals.hpp"
+#include "unittest.hpp"
+
+#define TEST_FLAG(f, type, value)                     \
+  do {                                                \
+    ASSERT_TRUE(Flag::find_flag(#f)->is_ ## type());  \
+    type original_value = f;                          \
+    {                                                 \
+      FLAG_GUARD(f);                                  \
+      f = value;                                      \
+    }                                                 \
+    ASSERT_EQ(original_value, f);                     \
+  } while (0)
+
+TEST_VM(FlagGuard, bool_flag) {
+  TEST_FLAG(AlwaysActAsServerClassMachine, bool, true);
+}
+
+TEST_VM(FlagGuard, int_flag) {
+  TEST_FLAG(ParGCArrayScanChunk, int, 1337);
+}
+
+TEST_VM(FlagGuard, intx_flag) {
+  TEST_FLAG(RefDiscoveryPolicy, intx, 1337);
+}
+
+TEST_VM(FlagGuard, uint_flag) {
+  TEST_FLAG(ConcGCThreads, uint, 1337);
+}
+
+TEST_VM(FlagGuard, uintx_flag) {
+  TEST_FLAG(GCTaskTimeStampEntries, uintx, 1337);
+}
+
+TEST_VM(FlagGuard, size_t_flag) {
+  TEST_FLAG(HeapSizePerGCThread, size_t, 1337);
+}
+
+TEST_VM(FlagGuard, uint64_t_flag) {
+  TEST_FLAG(MaxRAM, uint64_t, 1337);
+}
+
+TEST_VM(FlagGuard, double_flag) {
+  TEST_FLAG(CompileThresholdScaling, double, 3.141569);
+}
+
+TEST_VM(FlagGuard, ccstr_flag) {
+  TEST_FLAG(PerfDataSaveFile, ccstr, "/a/random/path");
+}