Add comments for constants which replaced large integer literals

runtime/lib/bigint.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:typed_data' show Uint32List;
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
@@ skipping 39 matching lines @@
   // Bits per digit.
   static const int _DIGIT_BITS = 32;
   static const int _LOG2_DIGIT_BITS = 5;
   static const int _DIGIT_BASE = 1 << _DIGIT_BITS;
   static const int _DIGIT_MASK = (1 << _DIGIT_BITS) - 1;
 
   // Bits per half digit.
   static const int _DIGIT2_BITS = _DIGIT_BITS >> 1;
   static const int _DIGIT2_MASK = (1 << _DIGIT2_BITS) - 1;
 
-  // Bits per 2 digits
+  // Bits per 2 digits. Used to perform modulo 2^64 arithmetic.
+  // Note: in --limit-ints-to-64-bits mode most arithmetic operations are
+  // already modulo 2^64. Still, it is harmless to apply _TWO_DIGITS_MASK:
+  // (1 << _TWO_DIGITS_BITS) is 0 (all bits are shifted out), so
+  // _TWO_DIGITS_MASK is -1 (its bit pattern is 0xffffffffffffffff).
   static const int _TWO_DIGITS_BITS = _DIGIT_BITS << 1;
   static const int _TWO_DIGITS_MASK = (1 << _TWO_DIGITS_BITS) - 1;
 
   // Min and max of non bigint values.
   static const int _MIN_INT64 = (-1) << 63;
   static const int _MAX_INT64 = 0x7fffffffffffffff;
 
   // Bigint constant values.
   // Note: Not declared as final in order to satisfy optimizer, which expects
   // constants to be in canonical form (Smi).
@@ skipping 2048 matching lines @@
     return _reduce(r_digits, r_used);
   }
 
   int _mul(Uint32List x_digits, int x_used, Uint32List y_digits, int y_used,
       Uint32List r_digits) {
     var r_used =
         _Bigint._mulDigits(x_digits, x_used, y_digits, y_used, r_digits);
     return _reduce(r_digits, r_used);
   }
 }