[bigint] Increase maximum length.

In a benchmark that I'm looking at, the numbers quickly overflow our
current limit.

This patch increases kMaxLength to the greatest value that's possible
without requiring further code changes.

Bug: v8:6791
Change-Id: I7a0d126dcd566d536375a294fa4dcf10b8823ed7
Reviewed-on: https://chromium-review.googlesource.com/833876
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50240}

src/heap/spaces.cc

@@ -3495,13 +3495,14 @@ void LargeObjectSpace::Verify() {
     // We have only code, sequential strings, external strings (sequential
     // strings that have been morphed into external strings), thin strings
     // (sequential strings that have been morphed into thin strings), fixed
-    // arrays, fixed double arrays, byte arrays, feedback vectors and free space
-    // (right after allocation) in the large object space.
+    // arrays, fixed double arrays, byte arrays, feedback vectors, bigints and
+    // free space (right after allocation) in the large object space.
     CHECK(object->IsAbstractCode() || object->IsSeqString() ||
           object->IsExternalString() || object->IsThinString() ||
           object->IsFixedArray() || object->IsFixedDoubleArray() ||
           object->IsPropertyArray() || object->IsByteArray() ||
-          object->IsFeedbackVector() || object->IsFreeSpace());
+          object->IsFeedbackVector() || object->IsBigInt() ||
+          object->IsFreeSpace());
 
     // The object itself should look OK.
     object->ObjectVerify();

src/objects/bigint.h

@@ -24,14 +24,11 @@ class BigIntBase : public HeapObject {
     return LengthBits::decode(static_cast<uint32_t>(bitfield));
   }
 
-  // The maximum kMaxLength that the current implementation supports would be
-  // kMaxInt / kDigitBits. However, we use a lower limit for now, because
-  // raising it later is easier than lowering it.
-  // Support up to 1 million bits.
-  static const int kMaxLengthBits = 1024 * 1024;
+  // Increasing kMaxLength will require code changes.
+  static const int kMaxLengthBits = kMaxInt - kPointerSize * kBitsPerByte - 1;
   static const int kMaxLength = kMaxLengthBits / (kPointerSize * kBitsPerByte);
 
-  static const int kLengthFieldBits = 20;
+  static const int kLengthFieldBits = 30;
   STATIC_ASSERT(kMaxLength <= ((1 << kLengthFieldBits) - 1));
   class LengthBits : public BitField<int, 0, kLengthFieldBits> {};
   class SignBits : public BitField<bool, LengthBits::kNext, 1> {};

test/mjsunit/harmony/bigint/exp.js

@@ -40,4 +40,4 @@ assertEquals(81n, (-3n) ** 4n);
 assertEquals(-243n, (-3n) ** 5n);
 
 assertThrows(() => 3n ** -2n, RangeError);  // Negative exponent.
-assertThrows(() => 2n ** (1024n ** 3n), RangeError);  // Too big.
+assertThrows(() => 2n ** (1024n ** 4n), RangeError);  // Too big.

test/mjsunit/harmony/bigint/too-big-literal.js

-// Copyright 2017 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.
-//
-// Flags: --harmony-bigint
-
-const MAX_BIGINT_BITS = 1024 * 1024;  // Matches BigInt::kMaxLengthBits
-const MAX_BIGINT_CHARS = MAX_BIGINT_BITS / 4;
-
-const TOO_MANY_ONES = Array(MAX_BIGINT_CHARS + 2).join("1") + "n";
-
-const tooBigHex = "0x" + TOO_MANY_ONES;
-
-assertThrows(tooBigHex, SyntaxError);