[json] Specialize parsed arrays to most specific kind

src/json-parser.cc

Index: src/json-parser.cc
diff --git a/src/json-parser.cc b/src/json-parser.cc
index ec1329a1c74c53d9c5f293f53b78d9d27f8b5be5..93d305df7ade2c9486b060148c23b7583c519457 100644
--- a/src/json-parser.cc
+++ b/src/json-parser.cc
@@ -488,6 +488,46 @@ void JsonParser<seq_one_byte>::CommitStateToJsonObject(
   }
 }
 
+class ElementKindLattice {
+ private:
+  enum {
+    SMI_ELEMENTS,
+    NUMBER_ELEMENTS,
+    OBJECT_ELEMENTS,
+  };
+
+ public:
+  ElementKindLattice() : value_(SMI_ELEMENTS) {}
+
+  void Update(Handle<Object> o) {
+    if (o->IsSmi()) {
+      return;
+    } else if (o->IsHeapNumber()) {
+      if (value_ < NUMBER_ELEMENTS) value_ = NUMBER_ELEMENTS;
+    } else {
+      DCHECK(!o->IsNumber());
+      value_ = OBJECT_ELEMENTS;
+    }
+  }
+
+  ElementsKind GetElementsKind() const {
+    switch (value_) {
+      case SMI_ELEMENTS:
+        return FAST_SMI_ELEMENTS;
+      case NUMBER_ELEMENTS:
+        return FAST_DOUBLE_ELEMENTS;
+      case OBJECT_ELEMENTS:
+        return FAST_ELEMENTS;
+      default:
+        UNREACHABLE();
+        return FAST_ELEMENTS;
+    }
+  }
+
+ private:
+  int value_;
+};
+
 // Parse a JSON array. Position must be right at '['.
 template <bool seq_one_byte>
 Handle<Object> JsonParser<seq_one_byte>::ParseJsonArray() {
@@ -495,26 +535,49 @@ Handle<Object> JsonParser<seq_one_byte>::ParseJsonArray() {
   ZoneList<Handle<Object> > elements(4, zone());
   DCHECK_EQ(c0_, '[');
 
+  ElementKindLattice lattice;
+
   AdvanceSkipWhitespace();
   if (c0_ != ']') {
     do {
       Handle<Object> element = ParseJsonValue();
       if (element.is_null()) return ReportUnexpectedCharacter();
       elements.Add(element, zone());
+      lattice.Update(element);
     } while (MatchSkipWhiteSpace(','));
     if (c0_ != ']') {
       return ReportUnexpectedCharacter();
     }
   }
   AdvanceSkipWhitespace();
+
   // Allocate a fixed array with all the elements.
-  Handle<FixedArray> fast_elements =
-      factory()->NewFixedArray(elements.length(), pretenure_);
-  for (int i = 0, n = elements.length(); i < n; i++) {
-    fast_elements->set(i, *elements[i]);
+
+  Handle<Object> json_array;
+  const ElementsKind kind = lattice.GetElementsKind();
+
+  switch (kind) {
+    case FAST_ELEMENTS:
+    case FAST_SMI_ELEMENTS: {
+      Handle<FixedArray> elems =
+          factory()->NewFixedArray(elements.length(), pretenure_);
+      for (int i = 0; i < elements.length(); i++) elems->set(i, *elements[i]);
+      json_array = factory()->NewJSArrayWithElements(elems, kind, pretenure_);
+      break;
+    }
+    case FAST_DOUBLE_ELEMENTS: {
+      Handle<FixedDoubleArray> elems = Handle<FixedDoubleArray>::cast(
+          factory()->NewFixedDoubleArray(elements.length(), pretenure_));
+      for (int i = 0; i < elements.length(); i++) {
+        elems->set(i, elements[i]->Number());
+      }
+      json_array = factory()->NewJSArrayWithElements(elems, kind, pretenure_);
+      break;
+    }
+    default:
+      UNREACHABLE();
   }
-  Handle<Object> json_array = factory()->NewJSArrayWithElements(
-      fast_elements, FAST_ELEMENTS, pretenure_);
+
   return scope.CloseAndEscape(json_array);
 }