8342975: C2: Micro-optimize PhaseIdealLoop::Dominators()

Reviewed-by: dlong, kvn

Author: shipilev

src/hotspot/share/opto/domgraph.cpp

@@ -410,10 +410,9 @@ void PhaseIdealLoop::Dominators() {
   // Setup mappings from my Graph to Tarjan's stuff and back
   // Note: Tarjan uses 1-based arrays
   NTarjan *ntarjan = NEW_RESOURCE_ARRAY(NTarjan,C->unique()+1);
-  // Initialize _control field for fast reference
-  int i;
-  for( i= C->unique()-1; i>=0; i-- )
-    ntarjan[i]._control = nullptr;
+  // Initialize all fields at once for safety and extra performance.
+  // Among other things, this initializes _control field for fast reference.
+  memset(ntarjan, 0, (C->unique() + 1)*sizeof(NTarjan));
 
   // Store the DFS order for the main loop
   const uint fill_value = max_juint;
@@ -429,6 +428,7 @@ void PhaseIdealLoop::Dominators() {
   ntarjan[0]._size = ntarjan[0]._semi = 0;
   ntarjan[0]._label = &ntarjan[0];
 
+  int i;
   for( i = dfsnum-1; i>1; i-- ) {        // For all nodes in reverse DFS order
     NTarjan *w = &ntarjan[i];            // Get Node from DFS
     assert(w->_control != nullptr,"bad DFS walk");