--- a/hotspot/src/share/vm/runtime/arguments.cpp	Wed Oct 01 16:09:01 2014 +0200
+++ b/hotspot/src/share/vm/runtime/arguments.cpp	Thu Oct 02 11:22:02 2014 +0200
@@ -1690,11 +1690,10 @@
 #ifdef COMPILER1
   FastTLABRefill = false;
 #endif
-  FLAG_SET_DEFAULT(ParallelGCThreads,
-                     Abstract_VM_Version::parallel_worker_threads());
+  FLAG_SET_DEFAULT(ParallelGCThreads, Abstract_VM_Version::parallel_worker_threads());
   if (ParallelGCThreads == 0) {
-    FLAG_SET_DEFAULT(ParallelGCThreads,
-                     Abstract_VM_Version::parallel_worker_threads());
+    assert(!FLAG_IS_DEFAULT(ParallelGCThreads), "The default value for ParallelGCThreads should not be 0.");
+    vm_exit_during_initialization("The flag -XX:+UseG1GC can not be combined with -XX:ParallelGCThreads=0", NULL);
   }
 
 #if INCLUDE_ALL_GCS

--- a/hotspot/test/TEST.groups	Wed Oct 01 16:09:01 2014 +0200
+++ b/hotspot/test/TEST.groups	Thu Oct 02 11:22:02 2014 +0200
@@ -229,6 +229,7 @@
   gc/arguments/TestG1HeapSizeFlags.java \
   gc/arguments/TestMaxHeapSizeTools.java \
   gc/arguments/TestMaxNewSize.java \
+  gc/arguments/TestParallelGCThreads.java \
   gc/arguments/TestUseCompressedOopsErgo.java \
   gc/class_unloading/TestG1ClassUnloadingHWM.java \
   gc/g1/ \
@@ -256,6 +257,7 @@
   gc/arguments/TestAlignmentToUseLargePages.java \
   gc/arguments/TestMaxNewSize.java \
   gc/arguments/TestMinInitialErgonomics.java \
+  gc/arguments/TestParallelGCThreads.java \
   gc/arguments/TestUseCompressedOopsErgo.java \
   gc/metaspace/TestMetaspacePerfCounters.java \
   gc/parallelScavenge/ \
@@ -270,6 +272,7 @@
   gc/arguments/TestAlignmentToUseLargePages.java \
   gc/arguments/TestCMSHeapSizeFlags.java \
   gc/arguments/TestMaxNewSize.java \
+  gc/arguments/TestParallelGCThreads.java \
   gc/arguments/TestUseCompressedOopsErgo.java \
   gc/class_unloading/TestCMSClassUnloadingEnabledHWM.java \
   gc/concurrentMarkSweep/ \

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/test/gc/arguments/TestParallelGCThreads.java	Thu Oct 02 11:22:02 2014 +0200
@@ -0,0 +1,64 @@
+/*
+* Copyright (c) 2014, 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.
+*/
+
+/*
+ * @test TestParallelGCThreads
+ * @key gc
+ * @bug 8059527
+ * @summary Tests argument processing for ParallelGCThreads
+ * @library /testlibrary
+ * @run driver TestParallelGCThreads
+ */
+
+import com.oracle.java.testlibrary.*;
+
+public class TestParallelGCThreads {
+
+  public static void main(String args[]) throws Exception {
+
+    // For each parallel collector (G1, Parallel, ParNew/CMS)
+    for (String gc : new String[] {"G1", "Parallel", "ConcMarkSweep"}) {
+
+      // Make sure the VM does not allow ParallelGCThreads set to 0
+      String[] flags = new String[] {"-XX:+Use" + gc + "GC", "-XX:ParallelGCThreads=0", "-XX:+PrintFlagsFinal", "-version"};
+      ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(flags);
+      OutputAnalyzer output = new OutputAnalyzer(pb.start());
+      output.shouldHaveExitValue(1);
+
+      // Do some basic testing to ensure the flag updates the count
+      for (long i = 1; i <= 3; i++) {
+        flags = new String[] {"-XX:+Use" + gc + "GC", "-XX:ParallelGCThreads=" + i, "-XX:+PrintFlagsFinal", "-version"};
+        long count = getParallelGCThreadCount(flags);
+        Asserts.assertEQ(count, i, "Specifying ParallelGCThreads=" + i + " for " + gc + "GC does not set the thread count properly!");
+      }
+    }
+  }
+
+  public static long getParallelGCThreadCount(String flags[]) throws Exception {
+    ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(flags);
+    OutputAnalyzer output = new OutputAnalyzer(pb.start());
+    output.shouldHaveExitValue(0);
+    String stdout = output.getStdout();
+    return FlagsValue.getFlagLongValue("ParallelGCThreads", stdout);
+  }
+}