Adding fast path for generalizing maps.

Review URL: https://chromiumcodereview.appspot.com/14629005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14523 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -2460,12 +2460,17 @@ MaybeObject* Map::GeneralizeRepresentation(int modify_index,
   // Check the state of the root map.
   DescriptorArray* updated_descriptors = updated->instance_descriptors();
 
+  int valid = updated->NumberOfOwnDescriptors();
+  if (updated_descriptors->IsMoreGeneralThan(
+          verbatim, valid, descriptors, old_descriptors)) {
+    Representation updated_representation =
+        updated_descriptors->GetDetails(modify_index).representation();
+    if (new_representation.fits_into(updated_representation)) return updated;
+  }
+
   DescriptorArray* new_descriptors;
   MaybeObject* maybe_descriptors = updated_descriptors->Merge(
-      verbatim,
-      updated->NumberOfOwnDescriptors(),
-      descriptors,
-      old_descriptors);
+      verbatim, valid, descriptors, old_descriptors);
   if (!maybe_descriptors->To(&new_descriptors)) return maybe_descriptors;
 
   old_reprepresentation =
@@ -2477,9 +2482,9 @@ MaybeObject* Map::GeneralizeRepresentation(int modify_index,
       verbatim, descriptors, new_descriptors);
 
   int split_descriptors = split_map->NumberOfOwnDescriptors();
-  // Check whether |split_map| matches what we were looking for. If so, return
-  // it.
-  if (descriptors == split_descriptors) return split_map;
+  // This is shadowed by |updated_descriptors| being more general than
+  // |old_descriptors|.
+  ASSERT(descriptors != split_descriptors);
 
   int descriptor = split_descriptors;
   split_map->DeprecateTarget(
@@ -7260,7 +7265,7 @@ void DescriptorArray::CopyFrom(int dst_index,
 }
 
 
-// Generalize the |other| descriptor array by merging it with the (at least
+// Generalize the |other| descriptor array by merging it into the (at least
 // partly) updated |this| descriptor array.
 // The method merges two descriptor array in three parts. Both descriptor arrays
 // are identical up to |verbatim|. They also overlap in keys up to |valid|.
@@ -7343,6 +7348,37 @@ MaybeObject* DescriptorArray::Merge(int verbatim,
 }
 
 
+// Checks whether a merge of |other| into |this| would return a copy of |this|.
+bool DescriptorArray::IsMoreGeneralThan(int verbatim,
+                                        int valid,
+                                        int new_size,
+                                        DescriptorArray* other) {
+  ASSERT(verbatim <= valid);
+  ASSERT(valid <= new_size);
+  if (valid != new_size) return false;
+
+  for (int descriptor = verbatim; descriptor < valid; descriptor++) {
+    PropertyDetails details = GetDetails(descriptor);
+    PropertyDetails other_details = other->GetDetails(descriptor);
+    if (details.type() != other_details.type()) {
+      if (details.type() != FIELD ||
+          other_details.type() != CONSTANT_FUNCTION) {
+        return false;
+      }
+    } else if (details.type() == CONSTANT_FUNCTION) {
+      if (GetValue(descriptor) != other->GetValue(descriptor)) {
+        return false;
+      }
+    } else if (!other_details.representation().fits_into(
+                   details.representation())) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+
 // We need the whiteness witness since sort will reshuffle the entries in the
 // descriptor array. If the descriptor array were to be black, the shuffling
 // would move a slot that was already recorded as pointing into an evacuation

src/objects.h

@@ -2809,6 +2809,11 @@ class DescriptorArray: public FixedArray {
                                      int new_size,
                                      DescriptorArray* other);
 
+  bool IsMoreGeneralThan(int verbatim,
+                         int valid,
+                         int new_size,
+                         DescriptorArray* other);
+
   MUST_USE_RESULT MaybeObject* CopyUpTo(int enumeration_index);
 
   // Sort the instance descriptors by the hash codes of their keys.

src/property-details.h

@@ -99,16 +99,20 @@ class Representation {
 
   static Representation FromKind(Kind kind) { return Representation(kind); }
 
-  bool Equals(const Representation& other) {
+  bool Equals(const Representation& other) const {
     return kind_ == other.kind_;
   }
 
-  bool is_more_general_than(const Representation& other) {
+  bool is_more_general_than(const Representation& other) const {
     ASSERT(kind_ != kExternal);
     ASSERT(other.kind_ != kExternal);
     return kind_ > other.kind_;
   }
 
+  bool fits_into(const Representation& other) const {
+    return other.is_more_general_than(*this) || other.Equals(*this);
+  }
+
   Representation generalize(Representation other) {
     if (is_more_general_than(other)) {
       return *this;