RFR: Verifier should avoid pushing on

[Aleksey Shipilev]

This little change cuts down testing time for about 30%:

$ hg diff
diff -r b514d97e3d24 src/share/vm/gc/shenandoah/shenandoahVerifier.cpp
--- a/src/share/vm/gc/shenandoah/shenandoahVerifier.cpp	Tue Aug 01 17:33:56 2017 +0200
+++ b/src/share/vm/gc/shenandoah/shenandoahVerifier.cpp	Tue Aug 01 19:26:00 2017 +0200
@@ -607,12 +607,15 @@
     oop obj = oop(addr);
     cl.verify_oop_standalone(obj);

-    // Verify everything reachable from that object too:
-    stack.push(obj);
+    // Verify everything reachable from that object too, hopefully realizing
+    // everything was already marked, and never touching further:
+    cl.verify_oops_from(obj);
+    (*processed)++;
+
     while (!stack.is_empty()) {
-      (*processed)++;
       ShenandoahVerifierTask task = stack.pop();
       cl.verify_oops_from(task.obj());
+      (*processed)++;
     }
   }
 };

The reason is, doing the initial Stack.push() is costly when stack is empty, which it almost always is.

Thanks,
-Aleksey