Put live register bits in stackmaps.

runtime/vm/stack_frame.cc

Index: runtime/vm/stack_frame.cc
diff --git a/runtime/vm/stack_frame.cc b/runtime/vm/stack_frame.cc
index 01a67359e704d6a986aeebdec790a61575a57144..3dcdeeaa6e09084645adc60ed06714c218f54b67 100644
--- a/runtime/vm/stack_frame.cc
+++ b/runtime/vm/stack_frame.cc
@@ -67,14 +67,20 @@ void StackFrame::VisitObjectPointers(ObjectPointerVisitor* visitor) {
       // A stack map is present in the code object, use the stack map to visit
       // frame slots which are marked as having objects.
       intptr_t length = map.Length();
-      for (intptr_t bit_index = 0; bit_index < length; ++bit_index) {
-        if (map.IsObject(bit_index)) {
-          visitor->VisitPointer(end_addr - bit_index);
-        }
+      // Spill slots are at the 'bottom' of the frame.

[Vyacheslav Egorov (Google), 2012/08/27 14:54:51]

This only works for calls with no arguments. Consider adding todo and a comment
about that.

[Kevin Millikin (Google), 2012/08/29 12:40:41]

Done.

+      intptr_t spill_slot_count = length - map.RegisterCount();
+      for (intptr_t bit = 0; bit < spill_slot_count; ++bit) {
+        if (map.IsObject(bit)) visitor->VisitPointer(end_addr);
+        --end_addr;
       }
-      // The stack slots that are not spill slots (i.e., outgoing arguments)
-      // are tagged objects.
-      end_addr -= length;
+
+      // The live registers at the 'top' of the frame comprise the rest of the
+      // stack map.
+      for (intptr_t bit = length - 1; bit >= spill_slot_count; --bit) {
+        if (map.IsObject(bit)) visitor->VisitPointer(start_addr);
+        ++start_addr;
+      }
+
       // The end address can be one slot (but not more) past the start
       // address in the case that all slots were covered by the stack map.
       ASSERT((end_addr + 1) >= start_addr);