RFR: ZGC: stricter flag handling

[Stefan Karlsson]

Hi all,

Please review this patch.

1) Check for -XX:ParallelGCThreads=0 and -XX:ConcGCThreads=0 and print 
the same error message as G1.

2) Set ClassUnloading and ClassUnloadingWithConcurrentMark to false.

diff --git a/src/hotspot/share/gc/z/zArguments.cpp 
b/src/hotspot/share/gc/z/zArguments.cpp
--- a/src/hotspot/share/gc/z/zArguments.cpp
+++ b/src/hotspot/share/gc/z/zArguments.cpp
@@ -53,11 +53,20 @@
      FLAG_SET_DEFAULT(ParallelGCThreads, ZWorkers::calculate_nparallel());
    }

+  if (ParallelGCThreads == 0) {
+    vm_exit_during_initialization("The flag -XX:+UseZGC can not be 
combined with -XX:ParallelGCThreads=0");
+  }
+
    // Select number of concurrent threads
    if (FLAG_IS_DEFAULT(ConcGCThreads)) {
      FLAG_SET_DEFAULT(ConcGCThreads, ZWorkers::calculate_nconcurrent());
    }

+  if (ConcGCThreads == 0) {
+    vm_exit_during_initialization("The flag -XX:+UseZGC can not be 
combined with -XX:ConcGCThreads=0");
+  }
+
+
  #ifdef COMPILER2
    // Enable loop strip mining by default
    if (FLAG_IS_DEFAULT(UseCountedLoopSafepoints)) {
@@ -75,6 +84,10 @@
    FLAG_SET_DEFAULT(UseCompressedOops, false);
    FLAG_SET_DEFAULT(UseCompressedClassPointers, false);

+  // ClassUnloading not (yet) supported
+  FLAG_SET_DEFAULT(ClassUnloading, false);
+  FLAG_SET_DEFAULT(ClassUnloadingWithConcurrentMark, false);
+
    // Verification before exit not (yet) supported
    FLAG_SET_DEFAULT(VerifyBeforeExit, false);
  }

Thanks,
StefanK