Optimize "smi or old" filter in scavenger.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
===================================================================
--- runtime/vm/raw_object.h	(revision 39523)
+++ runtime/vm/raw_object.h	(working copy)
@@ -292,19 +292,31 @@
     uword value = reinterpret_cast<uword>(this);
     return (value & kSmiTagMask) == kHeapObjectTag;
   }
-
+  // Assumes this is a heap object.
   bool IsNewObject() const {
     ASSERT(IsHeapObject());
     uword addr = reinterpret_cast<uword>(this);
     return (addr & kNewObjectAlignmentOffset) == kNewObjectAlignmentOffset;
   }
+  // Assumes this is a heap object.
   bool IsOldObject() const {
     ASSERT(IsHeapObject());
     uword addr = reinterpret_cast<uword>(this);
     return (addr & kNewObjectAlignmentOffset) == kOldObjectAlignmentOffset;
   }
+  // Assumes this is a heap object.
   bool IsVMHeapObject() const;
 
+  // Like !IsHeapObject() || IsOldObject(), but compiles to a single branch.
+  bool IsSmiOrOldObject() const {
+    COMPILE_ASSERT(kHeapObjectTag == 1);
+    COMPILE_ASSERT(kNewObjectAlignmentOffset == kWordSize);
+    static const uword kNewObjectBits =
+        (kNewObjectAlignmentOffset | kHeapObjectTag);
+    const uword addr = reinterpret_cast<uword>(this);
+    return (addr & kNewObjectBits) != kNewObjectBits;
+  }
+
   // Support for GC marking bit.
   bool IsMarked() const {
     return MarkBit::decode(ptr()->tags_);