7110718: -XX:MarkSweepAlwaysCompactCount=0 crashes the JVM

Summary: Interpret MarkSweepAlwaysCompactCount < 1 as never do full compaction
Reviewed-by: ysr, tonyp, jmasa, johnc

src/share/vm/gc_implementation/parallelScavenge/psMarkSweepDecorator.cpp

 /*
- * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2001, 2011, 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
@@ -96,7 +96,8 @@ void PSMarkSweepDecorator::precompact() {
    * by the MarkSweepAlwaysCompactCount parameter. This is a significant
    * performance improvement!
    */
-  bool skip_dead = ((PSMarkSweep::total_invocations() % MarkSweepAlwaysCompactCount) != 0);
+  bool skip_dead = (MarkSweepAlwaysCompactCount < 1)
+    || ((PSMarkSweep::total_invocations() % MarkSweepAlwaysCompactCount) != 0);
 
   size_t allowed_deadspace = 0;
   if (skip_dead) {

src/share/vm/memory/space.hpp

 /*
- * Copyright (c) 1997, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2011, 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
@@ -533,7 +533,8 @@ protected:
    * by the MarkSweepAlwaysCompactCount parameter.                           \
    */                                                                        \
   int invocations = SharedHeap::heap()->perm_gen()->stat_record()->invocations;\
-  bool skip_dead = ((invocations % MarkSweepAlwaysCompactCount) != 0);       \
+  bool skip_dead = (MarkSweepAlwaysCompactCount < 1)                         \
+    ||((invocations % MarkSweepAlwaysCompactCount) != 0);                    \
                                                                              \
   size_t allowed_deadspace = 0;                                              \
   if (skip_dead) {                                                           \