Don't flatten every time we call CharCodeAt

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index be4652bed5894264ca48baea2205f8ecf8209078..cc0faaac1b35c05ef41b100a4d8f09b14649f3e6 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -79,13 +79,14 @@
 //         - JSFunctionResultCache
 //         - SerializedScopeInfo
 //       - String
-//         - SeqString
-//           - SeqAsciiString
-//           - SeqTwoByteString
 //         - ConsString
-//         - ExternalString
-//           - ExternalAsciiString
-//           - ExternalTwoByteString
+//         - FlatString
+//           - SeqString
+//             - SeqAsciiString
+//             - SeqTwoByteString
+//           - ExternalString
+//             - ExternalAsciiString
+//             - ExternalTwoByteString
 //       - HeapNumber
 //       - Code
 //       - Map
@@ -681,6 +682,7 @@ class MaybeObject BASE_EMBEDDED {
   V(HeapNumber)                                \
   V(String)                                    \
   V(Symbol)                                    \
+  V(FlatString)                                \
   V(SeqString)                                 \
   V(ExternalString)                            \
   V(ConsString)                                \
@@ -5353,6 +5355,10 @@ class String: public HeapObject {
   // to this method are not efficient unless the string is flat.
   inline uint16_t Get(int index);
 
+  // Try to get a character (smi encoded) from the string.
+  // The method may try to flatten the string, or part of the string.
+  inline MaybeObject* GetMayFlatten(int index);
+
   // Try to flatten the string.  Checks first inline to see if it is
   // necessary.  Does nothing if the string is not a cons string.
   // Flattening allocates a sequential string with the same data as
@@ -5640,10 +5646,22 @@ class String: public HeapObject {
 };
 
 
-// The SeqString abstract class captures sequential string values.
-class SeqString: public String {
+class FlatString: public String {
  public:
+  // Casting.
+  static inline FlatString* cast(Object* obj);
+
+  // Get a character from a flat string.
+  inline uint16_t FlatStringGet(int index);
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(FlatString);
+};
+
 
+// The SeqString abstract class captures sequential string values.
+class SeqString: public FlatString {
+ public:
   // Casting.
   static inline SeqString* cast(Object* obj);
 
@@ -5782,6 +5800,7 @@ class ConsString: public String {
 
   // Dispatched behavior.
   uint16_t ConsStringGet(int index);
+  MaybeObject* ConsStringGetMayFlatten(int index);
 
   // Casting.
   static inline ConsString* cast(Object* obj);
@@ -5819,7 +5838,7 @@ class ConsString: public String {
 //
 // The API expects that all ExternalStrings are created through the
 // API.  Therefore, ExternalStrings should not be used internally.
-class ExternalString: public String {
+class ExternalString: public FlatString {
  public:
   // Casting
   static inline ExternalString* cast(Object* obj);