Check that array length stays a safe integer in Array.prototype.push

src/js/array.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 499 matching lines @@
 function ArrayPush() {
   CHECK_OBJECT_COERCIBLE(this, "Array.prototype.push");
 
   if (%IsObserved(this))
     return ObservedArrayPush.apply(this, arguments);
 
   var array = TO_OBJECT(this);
   var n = TO_LENGTH_OR_UINT32(array.length);
   var m = %_ArgumentsLength();
 
+  // The length of arguments can never be more than 2 ** 32, so

[adamk, 2015/10/26 21:21:02]

It looks to me like it can't be more than 2**16, actually...

[Dan Ehrenberg, 2015/10/26 22:59:31]

We talked about this offline. Apparently it can be more...

+  // we can do the comparison after subtracting that much from
+  // the length in order to avoid integer overflow.

[adamk, 2015/10/26 21:21:03]

Is this math necessary to get the right answer or is it an optimization? If the
latter, did this actually show up in some benchmark? It's definitely a little
funny-looking for a the "slow" path.

[Dan Ehrenberg, 2015/10/26 22:59:31]

It's necessary for the answer. Without that change, it was just not detecting a
single element push.

+  if ((n - (1 << 32)) + m > kMaxSafeInteger - (1 << 32)) {
+   throw MakeTypeError(kPushPastSafeLength, m, n);

[adamk, 2015/10/26 21:21:02]

Nit: indentation off, needs one more leading space.

[Dan Ehrenberg, 2015/10/26 22:59:31]

Fixed

+  }
+
   for (var i = 0; i < m; i++) {
     array[i+n] = %_Arguments(i);
   }
 
   var new_length = n + m;
   array.length = new_length;
   return new_length;
 }
 
 
@@ skipping 1142 matching lines @@
 %InstallToContext([
   "array_pop", ArrayPop,
   "array_push", ArrayPush,
   "array_shift", ArrayShift,
   "array_splice", ArraySplice,
   "array_slice", ArraySlice,
   "array_unshift", ArrayUnshift,
 ]);
 
 });