RFR: Enable fast synchronizer root scanning by default

[Aleksey Shipilev]

Hi,

We see good performance improvements, and positive code reviews upstream for
fast synchronizer root scanning. Let's enable it by default:

$ hg diff
diff -r 85c3e4589c8e src/share/vm/gc/shenandoah/shenandoah_globals.hpp
--- a/src/share/vm/gc/shenandoah/shenandoah_globals.hpp	Wed May 10 22:23:37 2017
+0200
+++ b/src/share/vm/gc/shenandoah/shenandoah_globals.hpp	Thu May 11 11:00:49 2017
+0200
@@ -214,7 +214,7 @@
           "If true, only execute storeval barrier when updating "           \
           "references.")                                                    \
                                                                             \
-  experimental(bool, ShenandoahFastSyncRoots, false,                        \
+  experimental(bool, ShenandoahFastSyncRoots, true,                         \
           "Enable fast synchronizer roots scanning")                        \
                                                                             \
   diagnostic(bool, ShenandoahSATBBarrier, true,                             \


Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey