Optimize creation of ASCII strings in UTF-8 JSON parser.

runtime/lib/convert_patch.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:_internal" show POWERS_OF_TEN;
 
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
   _BuildJsonListener listener;
@@ skipping 198 matching lines @@
   // Grows to the exact size asked for.
   void ensureCapacity(int newCapacity) {
     Uint8List list = this.list;
     if (newCapacity <= list.length) return;
     Uint8List newList = new Uint8List(newCapacity);
     newList.setRange(0, list.length, list, 0);
     this.list = newList;
   }
 
   String getString() {
-    var list = this.list;
-    if (length < list.length) {
-      list = new Uint8List.view(list.buffer, 0, length);
-    }
-    String result = new String.fromCharCodes(list);
+    String result = new String.fromCharCodes(list, 0, length);
     return result;
   }
 
   // TODO(lrn): See if parsing of numbers can be abstracted to something
   // not only working on strings, but also on char-code lists, without lossing
   // performance.
   int parseInt() => int.parse(getString());
   double parseDouble() => double.parse(getString());
 }
 
@@ skipping 307 matching lines @@
 
   /**
    * Extracts a literal string from a slice of the current chunk.
    *
    * No interpretation of the content is performed, except for converting
    * the source format to string.
    * This can be implemented more or less efficiently depending on the
    * underlying source.
    *
    * This is used for string literals that contain no escapes.
+   *
+   * The [bits] integer is an upper bound on the code point in the range
+   * from `start` to `end`.
+   * Usually found by doing bitwise or of all the values.
+   * The function may choose to optimize depending on the value.
    */
-  String getString(int start, int end);
+  String getString(int start, int end, int bits);
 
   /**
    * Parse a slice of the current chunk as an integer.
    *
    * The format is expected to be correct.
    */
   int parseInt(int start, int end) {
-    return int.parse(getString(start, end));
+    const int asciiBits = 0x7f;  // Integer literals are ASCII only.
+    return int.parse(getString(start, end, asciiBits));
   }
 
   /**
    * Parse a slice of the current chunk as a double.
    *
    * The format is expected to be correct.
    * This is used by [parseNumber] when the double value cannot be
    * built exactly during parsing.
    */
   double parseDouble(int start, int end) {
-    return double.parse(getString(start, end));
+    const int asciiBits = 0x7f;  // Double literals are ASCII only.
+    return double.parse(getString(start, end, asciiBits));
   }
 
   /**
    * Create a _NumberBuffer containing the digits from [start] to [chunkEnd].
    *
    * This creates a number buffer and initializes it with the part of the
    * number literal ending the current chunk
    */
   void createNumberBuffer(int start) {
     assert(start >= 0);
@@ skipping 382 matching lines @@
    * Parses a string value.
    *
    * Initial [position] is right after the initial quote.
    * Returned position right after the final quote.
    */
   int parseString(int position) {
     // Format: '"'([^\x00-\x1f\\\"]|'\\'[bfnrt/\\"])*'"'
     // Initial position is right after first '"'.
     int start = position;
     int end = chunkEnd;
+    int bits = 0;
     while (position < end) {
       int char = getChar(position++);
+      bits |= char;  // Includes final '"', but that never matters.
       // BACKSLASH is larger than QUOTE and SPACE.
       if (char > BACKSLASH) {
         continue;
       }
       if (char == BACKSLASH) {
         beginString();
         int sliceEnd = position - 1;
         if (start < sliceEnd) addSliceToString(start, sliceEnd);
-        return parseStringToBuffer(position - 1);
+        return parseStringToBuffer(sliceEnd);
       }
       if (char == QUOTE) {
-        listener.handleString(getString(start, position - 1));
+        listener.handleString(getString(start, position - 1, bits));
         return position;
       }
       if (char < SPACE) {
         fail(position - 1, "Control character in string");
       }
     }
     beginString();
     if (start < end) addSliceToString(start, end);
     return chunkString(STR_PLAIN);
   }
@@ skipping 317 matching lines @@
  * Chunked JSON parser that parses [String] chunks.
  */
 class _JsonStringParser extends _ChunkedJsonParser {
   String chunk;
   int chunkEnd;
 
   _JsonStringParser(_JsonListener listener) : super(listener);
 
   int getChar(int position) => chunk.codeUnitAt(position);
 
-  String getString(int start, int end) {
+  String getString(int start, int end, int bits) {
     return chunk.substring(start, end);
   }
 
   void beginString() {
     this.buffer = new StringBuffer();
   }
 
   void addSliceToString(int start, int end) {
     StringBuffer buffer = this.buffer;
     buffer.write(chunk.substring(start, end));
@@ skipping 325 matching lines @@
 class _JsonUtf8Parser extends _ChunkedJsonParser {
   final bool allowMalformed;
   List<int> chunk;
   int chunkEnd;
 
   _JsonUtf8Parser(_JsonListener listener, this.allowMalformed)
       : super(listener);
 
   int getChar(int position) => chunk[position];
 
-  String getString(int start, int end) {
+  String getString(int start, int end, int bits) {
+    const int maxAsciiChar = 0x7f;
+    if (bits <= maxAsciiChar) {
+      return new String.fromCharCodes(chunk, start, end);
+    }
     beginString();
     if (start < end) addSliceToString(start, end);
     String result = endString();
     return result;
   }
 
   void beginString() {
     this.buffer = new _Utf8StringBuffer(allowMalformed);
   }
 
@@ skipping 12 matching lines @@
     this.buffer = null;
     return buffer.toString();
   }
 
   void copyCharsToList(int start, int end, List target, int offset) {
     int length = end - start;
     target.setRange(offset, offset + length, chunk, start);
   }
 
   double parseDouble(int start, int end) {
-    String string = getString(start, end);
+    String string = getString(start, end, 0x7f);
     return _parseDouble(string, 0, string.length);
   }
 }
 
 double _parseDouble(String source, int start, int end)
     native "Double_parse";
 
 /**
  * Implements the chunked conversion from a UTF-8 encoding of JSON
  * to its corresponding object.
@@ skipping 30 matching lines @@
     _parser.parse(start);
   }
 
   void close() {
     _parser.close();
     var decoded = _parser.result;
     _sink.add(decoded);
     _sink.close();
   }
 }