Do fewer encoding checks in FlatStringReader used in the JSON stringifier.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/740673002

Cr-Commit-Position: refs/heads/master@{#25416}

src/json-stringifier.h

@@ -641,7 +641,7 @@ void BasicJsonStringifier::SerializeString_(Handle<String> string) {
   } else {
     FlatStringReader reader(isolate_, string);
     for (int i = 0; i < reader.length(); i++) {
-      SrcChar c = static_cast<SrcChar>(reader.Get(i));
+      SrcChar c = reader.Get<SrcChar>(i);
       if (DoNotEscape(c)) {
         builder_.Append<SrcChar, DestChar>(c);
       } else {

src/objects-inl.h

@@ -473,12 +473,24 @@ STATIC_ASSERT((kExternalStringTag | kTwoByteStringTag) ==
 
 STATIC_ASSERT(v8::String::TWO_BYTE_ENCODING == kTwoByteStringTag);
 
+
 uc32 FlatStringReader::Get(int index) {
-  DCHECK(0 <= index && index <= length_);
   if (is_one_byte_) {
-    return static_cast<const byte*>(start_)[index];
+    return Get<uint8_t>(index);
+  } else {
+    return Get<uc16>(index);
+  }
+}
+
+
+template <typename Char>
+Char FlatStringReader::Get(int index) {
+  DCHECK_EQ(is_one_byte_, sizeof(Char) == 1);
+  DCHECK(0 <= index && index <= length_);
+  if (sizeof(Char) == 1) {
+    return static_cast<Char>(static_cast<const uint8_t*>(start_)[index]);
   } else {
-    return static_cast<const uc16*>(start_)[index];
+    return static_cast<Char>(static_cast<const uc16*>(start_)[index]);
   }
 }
 

src/objects.h

@@ -9401,6 +9401,8 @@ class FlatStringReader : public Relocatable {
   FlatStringReader(Isolate* isolate, Vector<const char> input);
   void PostGarbageCollection();
   inline uc32 Get(int index);
+  template <typename Char>
+  inline Char Get(int index);
   int length() { return length_; }
  private:
   String** str_;