Generalize map dependent codes array to allow multiple dependency groups.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 12413123c09c2a922da3f76a7fbe3679a3dd0609..1f6e7cc0878c87a8630b77f4e05bd40f7d89cc18 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4683,24 +4683,52 @@ class Code: public HeapObject {
 
 
 // This class describes the layout of dependent codes array of a map. The
-// first element contains the number of codes as a Smi. The subsequent
-// elements contain code objects. The suffix of the array can be filled with the
-// undefined value if the number of codes is less than the length of the array.
+// array is partitioned into several groups of dependent codes. Each group
+// contains codes with the same dependency on the map. The array has the
+// following layout for n dependency groups:
+//
+// +----+----+-----+----+---------+----------+-----+---------+-----------+
+// | C1 | C2 | ... | Cn | group 1 |  group 2 | ... | group n | undefined |
+// +----+----+-----+----+---------+----------+-----+---------+-----------+
+//
+// The first n elements are Smis, each of them specifies the number of codes
+// in the corresponding group. The subsequent elements contain grouped code
+// objects. The suffix of the array can be filled with the undefined value if
+// the number of codes is less than the length of the array.

[Michael Starzinger, 2013/02/14 10:40:52]

Can we also add a sentence stating that the order of the code objects within a
group is not preserved by this implementation?

[ulan_google, 2013/02/14 11:59:06]

Done.

+// 
+// All code indexes used in the class are counted starting from the first
+// code object of the first group. In other words, code index 0 corresponds
+// to array index n = kCodesStartIndex.
+
 class DependentCodes: public FixedArray {
  public:
-  inline int number_of_codes();
-  inline void set_number_of_codes(int value);
+  enum DependencyGroup {
+    // Group of codes that weakly embed this map and depend on being
+    // deoptimized when the map is garbage collected.
+    kWeaklyEmbeddedGroup,
+    kGroupCount = kWeaklyEmbeddedGroup + 1
+  };
+  // Array for holding the index of the first code object of each group.
+  // The last element stores the total number of code objects.
+  typedef int GroupStartIndexes[kGroupCount + 1];
+  inline int number_of_codes(DependencyGroup group);
+  inline void set_number_of_codes(DependencyGroup group, int value);
   inline Code* code_at(int i);
   inline void set_code_at(int i, Code* value);
   inline Object** code_slot_at(int i);
   inline void clear_code_at(int i);

[Michael Starzinger, 2013/02/14 10:40:52]

As discussed offline: Can we (visually) separate the low-level from the
high-level accessors? The low-level ones should only be used by the GC, adding a
comment in that regard would help to clarify that.

[ulan_google, 2013/02/14 11:59:06]

Done.

-  static Handle<DependentCodes> Append(Handle<DependentCodes> codes,
-                                       Handle<Code> value);
   static inline DependentCodes* cast(Object* object);
-  bool Contains(Code* code);
+  inline void ComputeGroupStartIndexes(GroupStartIndexes starts);
+  bool Contains(DependencyGroup group, Code* code);
+  static Handle<DependentCodes> Insert(Handle<DependentCodes> codes,
+                                       DependencyGroup group,
+                                       Handle<Code> value);
+
  private:
-  static const int kNumberOfCodesIndex = 0;
-  static const int kCodesIndex = 1;
+  // Make a room at the end of the given group by moving out the first
+  // code objects of the subsequent groups.
+  inline void ExtendGroup(DependencyGroup group);
+  static const int kCodesStartIndex = kGroupCount;
 };
 
 
@@ -5151,7 +5179,8 @@ class Map: public HeapObject {
     return instance_type() >= FIRST_JS_OBJECT_TYPE;
   }
 
-  inline void AddDependentCode(Handle<Code> code);
+  inline void AddDependentCode(DependentCodes::DependencyGroup group,
+                               Handle<Code> code);
 
   // Dispatched behavior.
   DECLARE_PRINTER(Map)