Correct overflow check; signed integer overflow is undefined

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 17275 matching lines @@
       }
       default:
         UNIMPLEMENTED();
     }
   }
   if (!IsBigint() && !other.IsBigint()) {
     const int64_t left_value = AsInt64Value();
     const int64_t right_value = other.AsInt64Value();
     switch (operation) {
       case Token::kADD: {
-        if (((left_value < 0) != (right_value < 0)) ||
-            ((left_value + right_value) < 0) == (left_value < 0)) {
+        if (!Utils::WillAddOverflow(left_value, right_value)) {
           return Integer::New(left_value + right_value, space);
         }
         break;
       }
       case Token::kSUB: {
-        // TODO(srdjan): XCode 7 produces different code in -O0 than in -O2 for
-        // following code when 'left_value - right_value' overflows into a
-        // positive number (left negative, right positive):
-        //   if (((left_value < 0) == (right_value < 0)) ||
-        //       ((left_value - right_value) < 0) == (left_value < 0)) {
-        //
-        // Restructuring code using temporary variables is a workaround.
-        const bool both_same_sign = (left_value < 0) == (right_value < 0);
-        const bool result_same_sign_as_left =
-            ((left_value - right_value) < 0) == (left_value < 0);
-        if (both_same_sign || result_same_sign_as_left) {
+        if (!Utils::WillSubOverflow(left_value, right_value)) {
           return Integer::New(left_value - right_value, space);
         }
         break;
       }
       case Token::kMUL: {
         if ((Utils::HighestBit(left_value) +
              Utils::HighestBit(right_value)) < 62) {
           return Integer::New(left_value * right_value, space);
         }
         break;
@@ skipping 4598 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart