[bigint] Make kMaxLength platform-independent.

Taking kSystemPointerSize into account when determining the maximum
allowed BigInt size accidentally made the limit platform-specific.
This patch chooses a platform-independent constant (1<<30) instead.

Bug: chromium:909614
Change-Id: I4717969bc56e6dd5f1eed70b7e60e621989d0719
Reviewed-on: https://chromium-review.googlesource.com/c/1355625
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57983}

src/objects/bigint.h

@@ -38,9 +38,12 @@ class BigIntBase : public HeapObjectPtr {
   static inline BigIntBase unchecked_cast(ObjectPtr o) {
     return bit_cast<BigIntBase>(o);
   }
-  // Increasing kMaxLength will require code changes.
-  static const int kMaxLengthBits =
-      kMaxInt - kSystemPointerSize * kBitsPerByte - 1;
+
+  // The maximum kMaxLengthBits that the current implementation supports
+  // would be kMaxInt - kSystemPointerSize * kBitsPerByte - 1.
+  // Since we want a platform independent limit, choose a nice round number
+  // somewhere below that maximum.
+  static const int kMaxLengthBits = 1 << 30;  // ~1 billion.
   static const int kMaxLength =
       kMaxLengthBits / (kSystemPointerSize * kBitsPerByte);
 

test/mjsunit/harmony/bigint/regressions.js

@@ -19,8 +19,8 @@ assertTrue(0n === 0n);
 
 // crbug.com/818277: Must throw without DCHECK failures.
 // In order to run acceptably fast in Debug mode, this test assumes that
-// we allow at least 2 billion bits in a BigInt.
-var close_to_limit = 2n ** 2000000000n;
+// we allow at least 1 billion bits in a BigInt.
+var close_to_limit = 2n ** 1000000000n;
 assertThrows(() => close_to_limit ** 100n, RangeError);
 
 // Check boundary conditions of the power-of-two fast path.

test/mjsunit/regress/regress-crbug-909614.js

+// Copyright 2018 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.
+
+let just_under = 2n ** 30n - 1n;
+let just_above = 2n ** 30n;
+
+assertDoesNotThrow(() => { var dummy = 2n ** just_under; });
+assertThrows(() => { var dummy = 2n ** just_above; });