Removes undefined shifting behavior from BitVector::Equals.

runtime/vm/bit_vector.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/bit_vector.h"
 
 #include "vm/os.h"
 
 namespace dart {
 
@@ skipping 23 matching lines @@
 
 
 bool BitVector::Equals(const BitVector& other) const {
   if (length_ != other.length_) return false;
   intptr_t i = 0;
   for (; i < data_length_ - 1; i++) {
     if (data_[i] != other.data_[i]) return false;
   }
   if (i < data_length_) {
     // Don't compare bits beyond length_.
-    uword mask =
-        static_cast<uword>(-1) >> (kBitsPerWord - (length_ % kBitsPerWord));
+    const intptr_t shift_size = (kBitsPerWord - length_) & (kBitsPerWord - 1);
+    const uword mask = static_cast<uword>(-1) >> shift_size;
     if ((data_[i] & mask) != (other.data_[i] & mask)) return false;
   }
   return true;
 }
 
 
 bool BitVector::AddAll(const BitVector* from) {
   ASSERT(data_length_ == from->data_length_);
   bool changed = false;
   for (intptr_t i = 0; i < data_length_; i++) {
@@ skipping 46 matching lines @@
 
 void BitVector::Print() const {
   OS::Print("[");
   for (intptr_t i = 0; i < length_; i++) {
     OS::Print(Contains(i) ? "1" : "0");
   }
   OS::Print("]");
 }
 
 }  // namespace dart