7133038: G1: Some small profile based optimizations

Some minor profile based optimizations. Reduce the number of branches and branch mispredicts by removing some virtual calls, through closure specalization, and refactoring some conditional statements.

Reviewed-by: brutisso, tonyp

hotspot/src/share/vm/gc_implementation/g1/heapRegion.cpp

@@ -659,7 +659,7 @@ oops_on_card_seq_iterate_careful(MemRegion mr,
   // If we're within a stop-world GC, then we might look at a card in a
   // GC alloc region that extends onto a GC LAB, which may not be
   // parseable.  Stop such at the "saved_mark" of the region.
-  if (G1CollectedHeap::heap()->is_gc_active()) {
+  if (g1h->is_gc_active()) {
     mr = mr.intersection(used_region_at_save_marks());
   } else {
     mr = mr.intersection(used_region());
@@ -688,53 +688,63 @@ oops_on_card_seq_iterate_careful(MemRegion mr,
     OrderAccess::storeload();
   }
 
+  // Cache the boundaries of the memory region in some const locals
+  HeapWord* const start = mr.start();
+  HeapWord* const end = mr.end();
+
   // We used to use "block_start_careful" here.  But we're actually happy
   // to update the BOT while we do this...
-  HeapWord* cur = block_start(mr.start());
-  assert(cur <= mr.start(), "Postcondition");
+  HeapWord* cur = block_start(start);
+  assert(cur <= start, "Postcondition");
 
-  while (cur <= mr.start()) {
-    if (oop(cur)->klass_or_null() == NULL) {
+  oop obj;
+
+  HeapWord* next = cur;
+  while (next <= start) {
+    cur = next;
+    obj = oop(cur);
+    if (obj->klass_or_null() == NULL) {
       // Ran into an unparseable point.
       return cur;
     }
     // Otherwise...
-    int sz = oop(cur)->size();
-    if (cur + sz > mr.start()) break;
-    // Otherwise, go on.
-    cur = cur + sz;
+    next = (cur + obj->size());
   }
-  oop obj;
-  obj = oop(cur);
-  // If we finish this loop...
-  assert(cur <= mr.start()
-         && obj->klass_or_null() != NULL
-         && cur + obj->size() > mr.start(),
+
+  // If we finish the above loop...We have a parseable object that
+  // begins on or before the start of the memory region, and ends
+  // inside or spans the entire region.
+
+  assert(obj == oop(cur), "sanity");
+  assert(cur <= start &&
+         obj->klass_or_null() != NULL &&
+         (cur + obj->size()) > start,
          "Loop postcondition");
+
   if (!g1h->is_obj_dead(obj)) {
     obj->oop_iterate(cl, mr);
   }
 
-  HeapWord* next;
-  while (cur < mr.end()) {
+  while (cur < end) {
     obj = oop(cur);
     if (obj->klass_or_null() == NULL) {
       // Ran into an unparseable point.
       return cur;
     };
+
     // Otherwise:
     next = (cur + obj->size());
+
     if (!g1h->is_obj_dead(obj)) {
-      if (next < mr.end()) {
+      if (next < end || !obj->is_objArray()) {
+        // This object either does not span the MemRegion
+        // boundary, or if it does it's not an array.
+        // Apply closure to whole object.
         obj->oop_iterate(cl);
       } else {
-        // this obj spans the boundary.  If it's an array, stop at the
-        // boundary.
-        if (obj->is_objArray()) {
-          obj->oop_iterate(cl, mr);
-        } else {
-          obj->oop_iterate(cl);
-        }
+        // This obj is an array that spans the boundary.
+        // Stop at the boundary.
+        obj->oop_iterate(cl, mr);
       }
     }
     cur = next;