Issue #1240 - Part 3c - Fast-forward to the V8 version of BigIntType.

Disabling some sections temporarily since the dependencies are not there yet.
Based on the following: https://bugzilla.mozilla.org/show_bug.cgi?id=1502797
https://bugzilla.mozilla.org/show_bug.cgi?id=1471134
https://bugzilla.mozilla.org/show_bug.cgi?id=1441098 Part 3 & 4
Add structured clone support for BigInt and Enable BigInt wrapping from DOM bindings.
https://bugzilla.mozilla.org/show_bug.cgi?id=1522738

11 files changed, 3556 insertions, 124 deletions

diff --git a/js/src/builtin/BigInt.cpp b/js/src/builtin/BigInt.cpp
index 2790a20ccc..72dcb3be56 100644
--- a/js/src/builtin/BigInt.cpp
+++ b/js/src/builtin/BigInt.cpp
@@ -1,5 +1,4 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
- * vim: set ts=8 sts=4 et sw=4 tw=99:
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
diff --git a/js/src/builtin/BigInt.h b/js/src/builtin/BigInt.h
index 75bf99867e..daa9fafac7 100644
--- a/js/src/builtin/BigInt.h
+++ b/js/src/builtin/BigInt.h
@@ -1,5 +1,4 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
- * vim: set ts=8 sts=4 et sw=4 tw=99:
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
diff --git a/js/src/gdb/tests/test-jsval.cpp b/js/src/gdb/tests/test-jsval.cpp
index 3bd197aa77..2007464cbe 100644
--- a/js/src/gdb/tests/test-jsval.cpp
+++ b/js/src/gdb/tests/test-jsval.cpp
@@ -19,7 +19,7 @@ FRAGMENT(jsval, simple) {
   RootedString hello(cx, JS_NewStringCopyZ(cx, "Hello!"));
   RootedValue friendly_string(cx, StringValue(hello));
   RootedValue symbol(cx, SymbolValue(GetSymbolFor(cx, hello)));
-  RootedValue bi(cx, BigIntValue(BigInt::create(cx)));
+  RootedValue bi(cx, BigIntValue(BigInt::zero(cx)));
 
   RootedValue global(cx);
   global.setObject(*CurrentGlobalOrNull(cx));
diff --git a/js/src/js.msg b/js/src/js.msg
index 7e6ddd81bc..1cb571d189 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -623,6 +623,7 @@ MSG_DEF(JSMSG_GET_ASYNC_ITER_RETURNED_PRIMITIVE, 0, JSEXN_TYPEERR, "[Symbol.asyn
 // BigInt
 MSG_DEF(JSMSG_BIGINT_TO_NUMBER, 0, JSEXN_TYPEERR, "can't convert BigInt to number")
 MSG_DEF(JSMSG_NUMBER_TO_BIGINT, 0, JSEXN_RANGEERR, "can't convert non-finite number to BigInt")
+MSG_DEF(JSMSG_BIGINT_TOO_LARGE, 0, JSEXN_RANGEERR, "BigInt is too large to allocate")
 MSG_DEF(JSMSG_BIGINT_DIVISION_BY_ZERO, 0, JSEXN_RANGEERR, "BigInt division by zero")
 MSG_DEF(JSMSG_BIGINT_NEGATIVE_EXPONENT, 0, JSEXN_RANGEERR, "BigInt negative exponent")
 MSG_DEF(JSMSG_BIGINT_INVALID_SYNTAX, 0, JSEXN_SYNTAXERR, "invalid BigInt syntax")
diff --git a/js/src/jsatom.cpp b/js/src/jsatom.cpp
index 89e9f71aae..2a72ac38a3 100644
--- a/js/src/jsatom.cpp
+++ b/js/src/jsatom.cpp
@@ -487,7 +487,8 @@ ToAtomSlow(ExclusiveContext* cx, typename MaybeRooted<Value, allowGC>::HandleTyp
         return nullptr;
     }
     if (v.isBigInt()) {
-        JSAtom* atom = BigIntToAtom(cx, v.toBigInt());
+        RootedBigInt i(cx, v.toBigInt());
+        JSAtom* atom = BigIntToAtom(cx, i);
         if (!allowGC && !atom)
             cx->recoverFromOutOfMemory();
         return atom;
diff --git a/js/src/jsbool.cpp b/js/src/jsbool.cpp
index ee24f76987..0a70fe49f2 100644
--- a/js/src/jsbool.cpp
+++ b/js/src/jsbool.cpp
@@ -172,7 +172,7 @@ js::ToBooleanSlow(HandleValue v)
     if (v.isString())
         return v.toString()->length() != 0;
     if (v.isBigInt())
-        return v.toBigInt()->toBoolean();
+        return !v.toBigInt()->isZero();
 
     MOZ_ASSERT(v.isObject());
     return !EmulatesUndefined(&v.toObject());
diff --git a/js/src/jsstr.cpp b/js/src/jsstr.cpp
index 3b5461b441..593cf4d708 100644
--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -3737,7 +3737,8 @@ js::ToStringSlow(ExclusiveContext* cx, typename MaybeRooted<Value, allowGC>::Han
     } else if (v.isBigInt()) {
         if (!allowGC)
             return nullptr;
-        str = BigInt::toString(cx, v.toBigInt(), 10);
+        RootedBigInt i(cx, v.toBigInt());
+        str = BigInt::toString(cx, i, 10);
     } else {
         MOZ_ASSERT(v.isUndefined());
         str = cx->names().undefined;
diff --git a/js/src/vm/BigIntType.cpp b/js/src/vm/BigIntType.cpp
index 50f92bce49..c2910c6c19 100644
--- a/js/src/vm/BigIntType.cpp
+++ b/js/src/vm/BigIntType.cpp
@@ -1,148 +1,3201 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+/*
+ * Portions of this code taken from WebKit, whose copyright is as follows:
+ *
+ *   Copyright (C) 2017 Caio Lima <ticaiolima@gmail.com>
+ *   Copyright (C) 2017-2018 Apple Inc. All rights reserved.
+ *
+ *   Redistribution and use in source and binary forms, with or without
+ *   modification, are permitted provided that the following conditions
+ *   are met:
+ *   1. Redistributions of source code must retain the above copyright
+ *      notice, this list of conditions and the following disclaimer.
+ *   2. Redistributions in binary form must reproduce the above copyright
+ *      notice, this list of conditions and the following disclaimer in the
+ *      documentation and/or other materials provided with the distribution.
+ *
+ *   THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ *   EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ *   PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ *   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ *   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ *   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ *   OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ *   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ *   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Portions of this code taken from V8, whose copyright notice is as follows:
+ *
+ *   Copyright 2017 the V8 project authors. All rights reserved.
+ *   Redistribution and use in source and binary forms, with or without
+ *   modification, are permitted provided that the following conditions are
+ *   met:
+ *       * Redistributions of source code must retain the above copyright
+ *         notice, this list of conditions and the following disclaimer.
+ *       * Redistributions in binary form must reproduce the above
+ *         copyright notice, this list of conditions and the following
+ *         disclaimer in the documentation and/or other materials provided
+ *         with the distribution.
+ *       * Neither the name of Google Inc. nor the names of its
+ *         contributors may be used to endorse or promote products derived
+ *         from this software without specific prior written permission.
+ *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ *   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ *   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ *   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ *   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ *   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ *   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ *   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ *   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ *   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ *   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Portions of this code taken from Dart, whose copyright notice is as follows:
+ *
+ *   Copyright (c) 2014 the Dart project authors.  Please see the AUTHORS file
+ * [1] 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 [2].
+ *
+ *   [1] https://github.com/dart-lang/sdk/blob/master/AUTHORS
+ *   [2] https://github.com/dart-lang/sdk/blob/master/LICENSE
+ *
+ * Portions of this code taken from Go, whose copyright notice is as follows:
+ *
+ *   Copyright 2009 The Go Authors. All rights reserved.
+ *   Use of this source code is governed by a BSD-style
+ *   license that can be found in the LICENSE file [3].
+ *
+ *   [3] https://golang.org/LICENSE
+ */
+
 #include "vm/BigIntType.h"
 
+#include "mozilla/Casting.h"
 #include "mozilla/FloatingPoint.h"
 #include "mozilla/HashFunctions.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Range.h"
+#include "mozilla/RangedPtr.h"
+#include "mozilla/WrappingOperations.h"
+
+#include <functional>
+#include <math.h>
+#include <memory>
 
 #include "jsapi.h"
+#include "jsnum.h"
 #include "jscntxt.h"
 
 #include "builtin/BigInt.h"
 #include "gc/Allocator.h"
-#include "gc/Tracer.h"
+#include "js/Initialization.h"
+#include "js/Utility.h"
 #include "vm/SelfHosting.h"
 
+#include "vm/String.h"
+
 using namespace js;
 
-BigInt*
-BigInt::create(ExclusiveContext* cx)
-{
-    BigInt* x = Allocate<BigInt>(cx);
-    if (!x)
+using mozilla::Abs;
+using mozilla::AssertedCast;
+using mozilla::BitwiseCast;
+using mozilla::IsFinite;
+using mozilla::Maybe;
+using mozilla::NegativeInfinity;
+using mozilla::Nothing;
+using mozilla::PositiveInfinity;
+using mozilla::Range;
+using mozilla::RangedPtr;
+using mozilla::Some;
+using mozilla::WrapToSigned;
+
+static inline unsigned DigitLeadingZeroes(BigInt::Digit x) {
+  return sizeof(x) == 4 ? mozilla::CountLeadingZeroes32(x)
+                        : mozilla::CountLeadingZeroes64(x);
+}
+
+BigInt* BigInt::createUninitialized(ExclusiveContext* cx, size_t length,
+                                    bool isNegative) {
+  if (length > MaxDigitLength) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                    JSMSG_BIGINT_TOO_LARGE);
+    }
+    return nullptr;
+  }
+
+  UniquePtr<Digit[], JS::FreePolicy> heapDigits;
+  if (length > InlineDigitsLength) {
+    heapDigits = cx->make_pod_array<Digit>(length);
+    if (!heapDigits) {
+      return nullptr;
+    }
+  } else {
+    heapDigits = nullptr;
+  }
+
+  BigInt* x = Allocate<BigInt>(cx);
+  if (!x) {
+    return nullptr;
+  }
+
+  x->lengthSignAndReservedBits_ =
+      (length << LengthShift) | (isNegative ? SignBit : 0);
+  MOZ_ASSERT(x->digitLength() == length);
+  MOZ_ASSERT(x->isNegative() == isNegative);
+
+  if (heapDigits) {
+    x->heapDigits_ = heapDigits.release();
+  }
+
+  return x;
+}
+
+void BigInt::initializeDigitsToZero() {
+  auto digs = digits();
+  std::uninitialized_fill_n(digs.begin(), digs.Length(), 0);
+}
+
+void BigInt::finalize(js::FreeOp* fop) {
+  if (hasHeapDigits()) {
+    fop->free_(heapDigits_);
+  }
+}
+
+js::HashNumber BigInt::hash() {
+  js::HashNumber h =
+      mozilla::HashBytes(digits().data(), digitLength() * sizeof(Digit));
+  return mozilla::AddToHash(h, isNegative());
+}
+
+size_t BigInt::sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+  return hasInlineDigits() ? 0 : mallocSizeOf(heapDigits_);
+}
+
+BigInt* BigInt::zero(ExclusiveContext* cx) {
+  return createUninitialized(cx, 0, false);
+}
+
+BigInt* BigInt::one(ExclusiveContext* cx) {
+  BigInt* ret = createUninitialized(cx, 1, false);
+
+  if (!ret) {
+    return nullptr;
+  }
+
+  ret->setDigit(0, 1);
+
+  return ret;
+}
+
+BigInt* BigInt::neg(ExclusiveContext* cx, HandleBigInt x) {
+  if (x->isZero()) {
+    return x;
+  }
+
+  BigInt* result = copy(cx, x);
+  if (!result) {
+    return nullptr;
+  }
+  result->lengthSignAndReservedBits_ ^= SignBit;
+  return result;
+}
+
+#if !defined(JS_64BIT)
+#define HAVE_TWO_DIGIT 1
+using TwoDigit = uint64_t;
+#elif defined(HAVE_INT128_SUPPORT)
+#define HAVE_TWO_DIGIT 1
+using TwoDigit = __uint128_t;
+#endif
+
+inline BigInt::Digit BigInt::digitMul(Digit a, Digit b, Digit* high) {
+#if defined(HAVE_TWO_DIGIT)
+  TwoDigit result = static_cast<TwoDigit>(a) * static_cast<TwoDigit>(b);
+  *high = result >> DigitBits;
+
+  return static_cast<Digit>(result);
+#else
+  // Multiply in half-pointer-sized chunks.
+  // For inputs [AH AL]*[BH BL], the result is:
+  //
+  //            [AL*BL]  // rLow
+  //    +    [AL*BH]     // rMid1
+  //    +    [AH*BL]     // rMid2
+  //    + [AH*BH]        // rHigh
+  //    = [R4 R3 R2 R1]  // high = [R4 R3], low = [R2 R1]
+  //
+  // Where of course we must be careful with carries between the columns.
+  Digit aLow = a & HalfDigitMask;
+  Digit aHigh = a >> HalfDigitBits;
+  Digit bLow = b & HalfDigitMask;
+  Digit bHigh = b >> HalfDigitBits;
+
+  Digit rLow = aLow * bLow;
+  Digit rMid1 = aLow * bHigh;
+  Digit rMid2 = aHigh * bLow;
+  Digit rHigh = aHigh * bHigh;
+
+  Digit carry = 0;
+  Digit low = digitAdd(rLow, rMid1 << HalfDigitBits, &carry);
+  low = digitAdd(low, rMid2 << HalfDigitBits, &carry);
+
+  *high = (rMid1 >> HalfDigitBits) + (rMid2 >> HalfDigitBits) + rHigh + carry;
+
+  return low;
+#endif
+}
+
+BigInt::Digit BigInt::digitDiv(Digit high, Digit low, Digit divisor,
+                               Digit* remainder) {
+  MOZ_ASSERT(high < divisor, "division must not overflow");
+#if defined(__x86_64__)
+  Digit quotient;
+  Digit rem;
+  __asm__("divq  %[divisor]"
+          // Outputs: `quotient` will be in rax, `rem` in rdx.
+          : "=a"(quotient), "=d"(rem)
+          // Inputs: put `high` into rdx, `low` into rax, and `divisor` into
+          // any register or stack slot.
+          : "d"(high), "a"(low), [divisor] "rm"(divisor));
+  *remainder = rem;
+  return quotient;
+#elif defined(__i386__)
+  Digit quotient;
+  Digit rem;
+  __asm__("divl  %[divisor]"
+          // Outputs: `quotient` will be in eax, `rem` in edx.
+          : "=a"(quotient), "=d"(rem)
+          // Inputs: put `high` into edx, `low` into eax, and `divisor` into
+          // any register or stack slot.
+          : "d"(high), "a"(low), [divisor] "rm"(divisor));
+  *remainder = rem;
+  return quotient;
+#else
+  static constexpr Digit HalfDigitBase = 1ull << HalfDigitBits;
+  // Adapted from Warren, Hacker's Delight, p. 152.
+  unsigned s = DigitLeadingZeroes(divisor);
+  // If `s` is DigitBits here, it causes an undefined behavior.
+  // But `s` is never DigitBits since `divisor` is never zero here.
+  MOZ_ASSERT(s != DigitBits);
+  divisor <<= s;
+
+  Digit vn1 = divisor >> HalfDigitBits;
+  Digit vn0 = divisor & HalfDigitMask;
+
+  // `sZeroMask` which is 0 if s == 0 and all 1-bits otherwise.
+  //
+  // `s` can be 0. If `s` is 0, performing "low >> (DigitBits - s)" must not
+  // be done since it causes an undefined behavior since `>> DigitBits` is
+  // undefined in C++. Quoted from C++ spec, "The type of the result is that of
+  // the promoted left operand.
+  //
+  // The behavior is undefined if the right operand is negative, or greater
+  // than or equal to the length in bits of the promoted left operand". We
+  // mask the right operand of the shift by `shiftMask` (`DigitBits - 1`),
+  // which makes `DigitBits - 0` zero.
+  //
+  // This shifting produces a value which covers 0 < `s` <= (DigitBits - 1)
+  // cases. `s` == DigitBits never happen as we asserted.  Since `sZeroMask`
+  // clears the value in the case of `s` == 0, `s` == 0 case is also covered.
+  static_assert(sizeof(intptr_t) == sizeof(Digit),
+                "unexpected size of BigInt::Digit");
+  Digit sZeroMask =
+      static_cast<Digit>((-static_cast<intptr_t>(s)) >> (DigitBits - 1));
+  static constexpr unsigned shiftMask = DigitBits - 1;
+  Digit un32 =
+      (high << s) | ((low >> ((DigitBits - s) & shiftMask)) & sZeroMask);
+
+  Digit un10 = low << s;
+  Digit un1 = un10 >> HalfDigitBits;
+  Digit un0 = un10 & HalfDigitMask;
+  Digit q1 = un32 / vn1;
+  Digit rhat = un32 - q1 * vn1;
+
+  while (q1 >= HalfDigitBase || q1 * vn0 > rhat * HalfDigitBase + un1) {
+    q1--;
+    rhat += vn1;
+    if (rhat >= HalfDigitBase) {
+      break;
+    }
+  }
+
+  Digit un21 = un32 * HalfDigitBase + un1 - q1 * divisor;
+  Digit q0 = un21 / vn1;
+  rhat = un21 - q0 * vn1;
+
+  while (q0 >= HalfDigitBase || q0 * vn0 > rhat * HalfDigitBase + un0) {
+    q0--;
+    rhat += vn1;
+    if (rhat >= HalfDigitBase) {
+      break;
+    }
+  }
+
+  *remainder = (un21 * HalfDigitBase + un0 - q0 * divisor) >> s;
+  return q1 * HalfDigitBase + q0;
+#endif
+}
+
+// Multiplies `source` with `factor` and adds `summand` to the result.
+// `result` and `source` may be the same BigInt for inplace modification.
+void BigInt::internalMultiplyAdd(BigInt* source, Digit factor, Digit summand,
+                                 unsigned n, BigInt* result) {
+  MOZ_ASSERT(source->digitLength() >= n);
+  MOZ_ASSERT(result->digitLength() >= n);
+
+  Digit carry = summand;
+  Digit high = 0;
+  for (unsigned i = 0; i < n; i++) {
+    Digit current = source->digit(i);
+    Digit newCarry = 0;
+
+    // Compute this round's multiplication.
+    Digit newHigh = 0;
+    current = digitMul(current, factor, &newHigh);
+
+    // Add last round's carryovers.
+    current = digitAdd(current, high, &newCarry);
+    current = digitAdd(current, carry, &newCarry);
+
+    // Store result and prepare for next round.
+    result->setDigit(i, current);
+    carry = newCarry;
+    high = newHigh;
+  }
+
+  if (result->digitLength() > n) {
+    result->setDigit(n++, carry + high);
+
+    // Current callers don't pass in such large results, but let's be robust.
+    while (n < result->digitLength()) {
+      result->setDigit(n++, 0);
+    }
+  } else {
+    MOZ_ASSERT(!(carry + high));
+  }
+}
+
+// Multiplies `this` with `factor` and adds `summand` to the result.
+void BigInt::inplaceMultiplyAdd(Digit factor, Digit summand) {
+  internalMultiplyAdd(this, factor, summand, digitLength(), this);
+}
+
+// Multiplies `multiplicand` with `multiplier` and adds the result to
+// `accumulator`, starting at `accumulatorIndex` for the least-significant
+// digit.  Callers must ensure that `accumulator`'s digitLength and
+// corresponding digit storage is long enough to hold the result.
+void BigInt::multiplyAccumulate(BigInt* multiplicand, Digit multiplier,
+                                BigInt* accumulator,
+                                unsigned accumulatorIndex) {
+  MOZ_ASSERT(accumulator->digitLength() >
+             multiplicand->digitLength() + accumulatorIndex);
+  if (!multiplier) {
+    return;
+  }
+
+  Digit carry = 0;
+  Digit high = 0;
+  for (unsigned i = 0; i < multiplicand->digitLength();
+       i++, accumulatorIndex++) {
+    Digit acc = accumulator->digit(accumulatorIndex);
+    Digit newCarry = 0;
+
+    // Add last round's carryovers.
+    acc = digitAdd(acc, high, &newCarry);
+    acc = digitAdd(acc, carry, &newCarry);
+
+    // Compute this round's multiplication.
+    Digit multiplicandDigit = multiplicand->digit(i);
+    Digit low = digitMul(multiplier, multiplicandDigit, &high);
+    acc = digitAdd(acc, low, &newCarry);
+
+    // Store result and prepare for next round.
+    accumulator->setDigit(accumulatorIndex, acc);
+    carry = newCarry;
+  }
+
+  while (carry || high) {
+    MOZ_ASSERT(accumulatorIndex < accumulator->digitLength());
+    Digit acc = accumulator->digit(accumulatorIndex);
+    Digit newCarry = 0;
+    acc = digitAdd(acc, high, &newCarry);
+    high = 0;
+    acc = digitAdd(acc, carry, &newCarry);
+    accumulator->setDigit(accumulatorIndex, acc);
+    carry = newCarry;
+    accumulatorIndex++;
+  }
+}
+
+inline int8_t BigInt::absoluteCompare(BigInt* x, BigInt* y) {
+  MOZ_ASSERT(!x->digitLength() || x->digit(x->digitLength() - 1));
+  MOZ_ASSERT(!y->digitLength() || y->digit(y->digitLength() - 1));
+
+  // Sanity checks to catch negative zeroes escaping to the wild.
+  MOZ_ASSERT(!x->isNegative() || !x->isZero());
+  MOZ_ASSERT(!y->isNegative() || !y->isZero());
+
+  int diff = x->digitLength() - y->digitLength();
+  if (diff) {
+    return diff < 0 ? -1 : 1;
+  }
+
+  int i = x->digitLength() - 1;
+  while (i >= 0 && x->digit(i) == y->digit(i)) {
+    i--;
+  }
+
+  if (i < 0) {
+    return 0;
+  }
+
+  return x->digit(i) > y->digit(i) ? 1 : -1;
+}
+
+BigInt* BigInt::absoluteAdd(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y,
+                            bool resultNegative) {
+  bool swap = x->digitLength() < y->digitLength();
+  // Ensure `left` has at least as many digits as `right`.
+  HandleBigInt& left = swap ? y : x;
+  HandleBigInt& right = swap ? x : y;
+
+  if (left->isZero()) {
+    MOZ_ASSERT(right->isZero());
+    return left;
+  }
+
+  if (right->isZero()) {
+    return resultNegative == left->isNegative() ? left : neg(cx, left);
+  }
+
+  RootedBigInt result(
+      cx, createUninitialized(cx, left->digitLength() + 1, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+  Digit carry = 0;
+  unsigned i = 0;
+  for (; i < right->digitLength(); i++) {
+    Digit newCarry = 0;
+    Digit sum = digitAdd(left->digit(i), right->digit(i), &newCarry);
+    sum = digitAdd(sum, carry, &newCarry);
+    result->setDigit(i, sum);
+    carry = newCarry;
+  }
+
+  for (; i < left->digitLength(); i++) {
+    Digit newCarry = 0;
+    Digit sum = digitAdd(left->digit(i), carry, &newCarry);
+    result->setDigit(i, sum);
+    carry = newCarry;
+  }
+
+  result->setDigit(i, carry);
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+BigInt* BigInt::absoluteSub(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y,
+                            bool resultNegative) {
+  MOZ_ASSERT(x->digitLength() >= y->digitLength());
+
+  if (x->isZero()) {
+    MOZ_ASSERT(y->isZero());
+    return x;
+  }
+
+  if (y->isZero()) {
+    return resultNegative == x->isNegative() ? x : neg(cx, x);
+  }
+
+  int8_t comparisonResult = absoluteCompare(x, y);
+  MOZ_ASSERT(comparisonResult >= 0);
+  if (comparisonResult == 0) {
+    return zero(cx);
+  }
+
+  RootedBigInt result(
+      cx, createUninitialized(cx, x->digitLength(), resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+  Digit borrow = 0;
+  unsigned i = 0;
+  for (; i < y->digitLength(); i++) {
+    Digit newBorrow = 0;
+    Digit difference = digitSub(x->digit(i), y->digit(i), &newBorrow);
+    difference = digitSub(difference, borrow, &newBorrow);
+    result->setDigit(i, difference);
+    borrow = newBorrow;
+  }
+
+  for (; i < x->digitLength(); i++) {
+    Digit newBorrow = 0;
+    Digit difference = digitSub(x->digit(i), borrow, &newBorrow);
+    result->setDigit(i, difference);
+    borrow = newBorrow;
+  }
+
+  MOZ_ASSERT(!borrow);
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// Divides `x` by `divisor`, returning the result in `quotient` and `remainder`.
+// Mathematically, the contract is:
+//
+//   quotient = (x - remainder) / divisor, with 0 <= remainder < divisor.
+//
+// If `quotient` is an empty handle, an appropriately sized BigInt will be
+// allocated for it; otherwise the caller must ensure that it is big enough.
+// `quotient` can be the same as `x` for an in-place division. `quotient` can
+// also be `Nothing()` if the caller is only interested in the remainder.
+//
+// This function returns false if `quotient` is an empty handle, but allocating
+// the quotient failed.  Otherwise it returns true, indicating success.
+bool BigInt::absoluteDivWithDigitDivisor(ExclusiveContext* cx, HandleBigInt x,
+                                         Digit divisor,
+                                         const Maybe<MutableHandleBigInt>& quotient,
+                                         Digit* remainder,
+                                         bool quotientNegative) {
+  MOZ_ASSERT(divisor);
+
+  MOZ_ASSERT(!x->isZero());
+  *remainder = 0;
+  if (divisor == 1) {
+    if (quotient) {
+      BigInt* q;
+      if (x->isNegative() == quotientNegative) {
+        q = x;
+      } else {
+        q = neg(cx, x);
+        if (!q) {
+          return false;
+        }
+      }
+      quotient.value().set(q);
+    }
+    return true;
+  }
+
+  unsigned length = x->digitLength();
+  if (quotient) {
+    if (!quotient.value()) {
+      BigInt* q = createUninitialized(cx, length, quotientNegative);
+      if (!q) {
+        return false;
+      }
+      quotient.value().set(q);
+    }
+
+    for (int i = length - 1; i >= 0; i--) {
+      Digit q = digitDiv(*remainder, x->digit(i), divisor, remainder);
+      quotient.value()->setDigit(i, q);
+    }
+  } else {
+    for (int i = length - 1; i >= 0; i--) {
+      digitDiv(*remainder, x->digit(i), divisor, remainder);
+    }
+  }
+
+  return true;
+}
+
+// Adds `summand` onto `this`, starting with `summand`'s 0th digit
+// at `this`'s `startIndex`'th digit. Returns the "carry" (0 or 1).
+BigInt::Digit BigInt::absoluteInplaceAdd(BigInt* summand, unsigned startIndex) {
+  Digit carry = 0;
+  unsigned n = summand->digitLength();
+  MOZ_ASSERT(digitLength() > startIndex,
+             "must start adding at an in-range digit");
+  MOZ_ASSERT(digitLength() - startIndex >= n,
+             "digits being added to must not extend above the digits in "
+             "this (except for the returned carry digit)");
+  for (unsigned i = 0; i < n; i++) {
+    Digit newCarry = 0;
+    Digit sum = digitAdd(digit(startIndex + i), summand->digit(i), &newCarry);
+    sum = digitAdd(sum, carry, &newCarry);
+    setDigit(startIndex + i, sum);
+    carry = newCarry;
+  }
+
+  return carry;
+}
+
+// Subtracts `subtrahend` from this, starting with `subtrahend`'s 0th digit
+// at `this`'s `startIndex`-th digit. Returns the "borrow" (0 or 1).
+BigInt::Digit BigInt::absoluteInplaceSub(BigInt* subtrahend,
+                                         unsigned startIndex) {
+  Digit borrow = 0;
+  unsigned n = subtrahend->digitLength();
+  MOZ_ASSERT(digitLength() > startIndex,
+             "must start subtracting from an in-range digit");
+  MOZ_ASSERT(digitLength() - startIndex >= n,
+             "digits being subtracted from must not extend above the "
+             "digits in this (except for the returned borrow digit)");
+  for (unsigned i = 0; i < n; i++) {
+    Digit newBorrow = 0;
+    Digit difference =
+        digitSub(digit(startIndex + i), subtrahend->digit(i), &newBorrow);
+    difference = digitSub(difference, borrow, &newBorrow);
+    setDigit(startIndex + i, difference);
+    borrow = newBorrow;
+  }
+
+  return borrow;
+}
+
+// Returns whether (factor1 * factor2) > (high << kDigitBits) + low.
+inline bool BigInt::productGreaterThan(Digit factor1, Digit factor2, Digit high,
+                                       Digit low) {
+  Digit resultHigh;
+  Digit resultLow = digitMul(factor1, factor2, &resultHigh);
+  return resultHigh > high || (resultHigh == high && resultLow > low);
+}
+
+void BigInt::inplaceRightShiftLowZeroBits(unsigned shift) {
+  MOZ_ASSERT(shift < DigitBits);
+  MOZ_ASSERT(!(digit(0) & ((static_cast<Digit>(1) << shift) - 1)),
+             "should only be shifting away zeroes");
+
+  if (!shift) {
+    return;
+  }
+
+  Digit carry = digit(0) >> shift;
+  unsigned last = digitLength() - 1;
+  for (unsigned i = 0; i < last; i++) {
+    Digit d = digit(i + 1);
+    setDigit(i, (d << (DigitBits - shift)) | carry);
+    carry = d >> shift;
+  }
+  setDigit(last, carry);
+}
+
+// Always copies the input, even when `shift` == 0.
+BigInt* BigInt::absoluteLeftShiftAlwaysCopy(ExclusiveContext* cx, HandleBigInt x,
+                                            unsigned shift,
+                                            LeftShiftMode mode) {
+  MOZ_ASSERT(shift < DigitBits);
+  MOZ_ASSERT(!x->isZero());
+
+  unsigned n = x->digitLength();
+  unsigned resultLength = mode == LeftShiftMode::AlwaysAddOneDigit ? n + 1 : n;
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, x->isNegative()));
+  if (!result) {
+    return nullptr;
+  }
+
+  if (!shift) {
+    for (unsigned i = 0; i < n; i++) {
+      result->setDigit(i, x->digit(i));
+    }
+    if (mode == LeftShiftMode::AlwaysAddOneDigit) {
+      result->setDigit(n, 0);
+    }
+
+    return result;
+  }
+
+  Digit carry = 0;
+  for (unsigned i = 0; i < n; i++) {
+    Digit d = x->digit(i);
+    result->setDigit(i, (d << shift) | carry);
+    carry = d >> (DigitBits - shift);
+  }
+
+  if (mode == LeftShiftMode::AlwaysAddOneDigit) {
+    result->setDigit(n, carry);
+  } else {
+    MOZ_ASSERT(mode == LeftShiftMode::SameSizeResult);
+    MOZ_ASSERT(!carry);
+  }
+
+  return result;
+}
+
+// Divides `dividend` by `divisor`, returning the result in `quotient` and
+// `remainder`. Mathematically, the contract is:
+//
+//   quotient = (dividend - remainder) / divisor, with 0 <= remainder < divisor.
+//
+// Both `quotient` and `remainder` are optional, for callers that are only
+// interested in one of them.  See Knuth, Volume 2, section 4.3.1, Algorithm D.
+// Also see the overview of the algorithm by Jan Marthedal Rasmussen over at
+// https://janmr.com/blog/2014/04/basic-multiple-precision-long-division/.
+bool BigInt::absoluteDivWithBigIntDivisor(ExclusiveContext* cx, HandleBigInt dividend,
+                                          HandleBigInt divisor,
+                                          const Maybe<MutableHandleBigInt>& quotient,
+                                          const Maybe<MutableHandleBigInt>& remainder,
+                                          bool isNegative) {
+  MOZ_ASSERT(divisor->digitLength() >= 2);
+  MOZ_ASSERT(dividend->digitLength() >= divisor->digitLength());
+
+  // Any early error return is detectable by checking the quotient and/or
+  // remainder output values.
+  MOZ_ASSERT(!quotient || !quotient.value());
+  MOZ_ASSERT(!remainder || !remainder.value());
+
+  // The unusual variable names inside this function are consistent with
+  // Knuth's book, as well as with Go's implementation of this algorithm.
+  // Maintaining this consistency is probably more useful than trying to
+  // come up with more descriptive names for them.
+  const unsigned n = divisor->digitLength();
+  const unsigned m = dividend->digitLength() - n;
+
+  // The quotient to be computed.
+  RootedBigInt q(cx);
+  if (quotient) {
+    q = createUninitialized(cx, m + 1, isNegative);
+    if (!q) {
+      return false;
+    }
+  }
+
+  // In each iteration, `qhatv` holds `divisor` * `current quotient digit`.
+  // "v" is the book's name for `divisor`, `qhat` the current quotient digit.
+  RootedBigInt qhatv(cx, createUninitialized(cx, n + 1, isNegative));
+  if (!qhatv) {
+    return false;
+  }
+
+  // D1.
+  // Left-shift inputs so that the divisor's MSB is set. This is necessary to
+  // prevent the digit-wise divisions (see digitDiv call below) from
+  // overflowing (they take a two digits wide input, and return a one digit
+  // result).
+  Digit lastDigit = divisor->digit(n - 1);
+  unsigned shift = DigitLeadingZeroes(lastDigit);
+
+  RootedBigInt shiftedDivisor(cx);
+  if (shift > 0) {
+    shiftedDivisor = absoluteLeftShiftAlwaysCopy(cx, divisor, shift,
+                                                 LeftShiftMode::SameSizeResult);
+    if (!shiftedDivisor) {
+      return false;
+    }
+  } else {
+    shiftedDivisor = divisor;
+  }
+
+  // Holds the (continuously updated) remaining part of the dividend, which
+  // eventually becomes the remainder.
+  RootedBigInt u(cx,
+                 absoluteLeftShiftAlwaysCopy(cx, dividend, shift,
+                                             LeftShiftMode::AlwaysAddOneDigit));
+  if (!u) {
+    return false;
+  }
+
+  // D2.
+  // Iterate over the dividend's digit (like the "grade school" algorithm).
+  // `vn1` is the divisor's most significant digit.
+  Digit vn1 = shiftedDivisor->digit(n - 1);
+  for (int j = m; j >= 0; j--) {
+    // D3.
+    // Estimate the current iteration's quotient digit (see Knuth for details).
+    // `qhat` is the current quotient digit.
+    Digit qhat = std::numeric_limits<Digit>::max();
+
+    // `ujn` is the dividend's most significant remaining digit.
+    Digit ujn = u->digit(j + n);
+    if (ujn != vn1) {
+      // `rhat` is the current iteration's remainder.
+      Digit rhat = 0;
+      // Estimate the current quotient digit by dividing the most significant
+      // digits of dividend and divisor. The result will not be too small,
+      // but could be a bit too large.
+      qhat = digitDiv(ujn, u->digit(j + n - 1), vn1, &rhat);
+
+      // Decrement the quotient estimate as needed by looking at the next
+      // digit, i.e. by testing whether
+      // qhat * v_{n-2} > (rhat << DigitBits) + u_{j+n-2}.
+      Digit vn2 = shiftedDivisor->digit(n - 2);
+      Digit ujn2 = u->digit(j + n - 2);
+      while (productGreaterThan(qhat, vn2, rhat, ujn2)) {
+        qhat--;
+        Digit prevRhat = rhat;
+        rhat += vn1;
+        // v[n-1] >= 0, so this tests for overflow.
+        if (rhat < prevRhat) {
+          break;
+        }
+      }
+    }
+
+    // D4.
+    // Multiply the divisor with the current quotient digit, and subtract
+    // it from the dividend. If there was "borrow", then the quotient digit
+    // was one too high, so we must correct it and undo one subtraction of
+    // the (shifted) divisor.
+    internalMultiplyAdd(shiftedDivisor, qhat, 0, n, qhatv);
+    Digit c = u->absoluteInplaceSub(qhatv, j);
+    if (c) {
+      c = u->absoluteInplaceAdd(shiftedDivisor, j);
+      u->setDigit(j + n, u->digit(j + n) + c);
+      qhat--;
+    }
+
+    if (quotient) {
+      q->setDigit(j, qhat);
+    }
+  }
+
+  if (quotient) {
+    BigInt* bi = destructivelyTrimHighZeroDigits(cx, q);
+    if (!bi) {
+      return false;
+    }
+    quotient.value().set(q);
+  }
+
+  if (remainder) {
+    u->inplaceRightShiftLowZeroBits(shift);
+    remainder.value().set(u);
+  }
+
+  return true;
+}
+
+// Helper for Absolute{And,AndNot,Or,Xor}.
+// Performs the given binary `op` on digit pairs of `x` and `y`; when the
+// end of the shorter of the two is reached, `kind` configures how
+// remaining digits are handled.
+// Example:
+//       y:             [ y2 ][ y1 ][ y0 ]
+//       x:       [ x3 ][ x2 ][ x1 ][ x0 ]
+//                   |     |     |     |
+//                (Fill)  (op)  (op)  (op)
+//                   |     |     |     |
+//                   v     v     v     v
+//  result: [  0 ][ x3 ][ r2 ][ r1 ][ r0 ]
+template <BigInt::BitwiseOpKind kind, typename BitwiseOp>
+inline BigInt* BigInt::absoluteBitwiseOp(ExclusiveContext* cx, HandleBigInt x,
+                                         HandleBigInt y, BitwiseOp&& op) {
+  unsigned xLength = x->digitLength();
+  unsigned yLength = y->digitLength();
+  unsigned numPairs = std::min(xLength, yLength);
+  unsigned resultLength;
+  if (kind == BitwiseOpKind::SymmetricTrim) {
+    resultLength = numPairs;
+  } else if (kind == BitwiseOpKind::SymmetricFill) {
+    resultLength = std::max(xLength, yLength);
+  } else {
+    MOZ_ASSERT(kind == BitwiseOpKind::AsymmetricFill);
+    resultLength = xLength;
+  }
+  bool resultNegative = false;
+
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+
+  unsigned i = 0;
+  for (; i < numPairs; i++) {
+    result->setDigit(i, op(x->digit(i), y->digit(i)));
+  }
+
+  if (kind != BitwiseOpKind::SymmetricTrim) {
+    HandleBigInt& source =
+        kind == BitwiseOpKind::AsymmetricFill ? x : xLength == i ? y : x;
+    for (; i < resultLength; i++) {
+      result->setDigit(i, source->digit(i));
+    }
+  }
+
+  MOZ_ASSERT(i == resultLength);
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+BigInt* BigInt::absoluteAnd(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  return absoluteBitwiseOp<BitwiseOpKind::SymmetricTrim>(cx, x, y,
+                                                         std::bit_and<Digit>());
+}
+
+BigInt* BigInt::absoluteOr(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  return absoluteBitwiseOp<BitwiseOpKind::SymmetricFill>(cx, x, y,
+                                                         std::bit_or<Digit>());
+}
+
+BigInt* BigInt::absoluteAndNot(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  auto digitOperation = [](Digit a, Digit b) { return a & ~b; };
+  return absoluteBitwiseOp<BitwiseOpKind::AsymmetricFill>(cx, x, y,
+                                                          digitOperation);
+}
+
+BigInt* BigInt::absoluteXor(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  return absoluteBitwiseOp<BitwiseOpKind::SymmetricFill>(cx, x, y,
+                                                         std::bit_xor<Digit>());
+}
+
+BigInt* BigInt::absoluteAddOne(ExclusiveContext* cx, HandleBigInt x,
+                               bool resultNegative) {
+  unsigned inputLength = x->digitLength();
+  // The addition will overflow into a new digit if all existing digits are
+  // at maximum.
+  bool willOverflow = true;
+  for (unsigned i = 0; i < inputLength; i++) {
+    if (std::numeric_limits<Digit>::max() != x->digit(i)) {
+      willOverflow = false;
+      break;
+    }
+  }
+
+  unsigned resultLength = inputLength + willOverflow;
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+
+  Digit carry = 1;
+  for (unsigned i = 0; i < inputLength; i++) {
+    Digit newCarry = 0;
+    result->setDigit(i, digitAdd(x->digit(i), carry, &newCarry));
+    carry = newCarry;
+  }
+  if (resultLength > inputLength) {
+    MOZ_ASSERT(carry == 1);
+    result->setDigit(inputLength, 1);
+  } else {
+    MOZ_ASSERT(!carry);
+  }
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// Like the above, but you can specify that the allocated result should have
+// length `resultLength`, which must be at least as large as `x->digitLength()`.
+// The result will be unsigned.
+BigInt* BigInt::absoluteSubOne(ExclusiveContext* cx, HandleBigInt x,
+                               unsigned resultLength) {
+  MOZ_ASSERT(!x->isZero());
+  MOZ_ASSERT(resultLength >= x->digitLength());
+  bool resultNegative = false;
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+
+  unsigned length = x->digitLength();
+  Digit borrow = 1;
+  for (unsigned i = 0; i < length; i++) {
+    Digit newBorrow = 0;
+    result->setDigit(i, digitSub(x->digit(i), borrow, &newBorrow));
+    borrow = newBorrow;
+  }
+  MOZ_ASSERT(!borrow);
+  for (unsigned i = length; i < resultLength; i++) {
+    result->setDigit(i, 0);
+  }
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// Lookup table for the maximum number of bits required per character of a
+// base-N string representation of a number. To increase accuracy, the array
+// value is the actual value multiplied by 32. To generate this table:
+// for (var i = 0; i <= 36; i++) { print(Math.ceil(Math.log2(i) * 32) + ","); }
+static constexpr uint8_t maxBitsPerCharTable[] = {
+    0,   0,   32,  51,  64,  75,  83,  90,  96,  // 0..8
+    102, 107, 111, 115, 119, 122, 126, 128,      // 9..16
+    131, 134, 136, 139, 141, 143, 145, 147,      // 17..24
+    149, 151, 153, 154, 156, 158, 159, 160,      // 25..32
+    162, 163, 165, 166,                          // 33..36
+};
+
+static constexpr unsigned bitsPerCharTableShift = 5;
+static constexpr size_t bitsPerCharTableMultiplier = 1u
+                                                     << bitsPerCharTableShift;
+static constexpr char radixDigits[] = "0123456789abcdefghijklmnopqrstuvwxyz";
+
+static inline uint64_t CeilDiv(uint64_t numerator, uint64_t denominator) {
+  MOZ_ASSERT(numerator != 0);
+  return 1 + (numerator - 1) / denominator;
+};
+
+// Compute (an overapproximation of) the length of the string representation of
+// a BigInt.  In base B an X-digit number has maximum value:
+//
+//   B**X - 1
+//
+// We're trying to find N for an N-digit number in base |radix| full
+// representing a |bitLength|-digit number in base 2, so we have:
+//
+//   radix**N - 1 ≥ 2**bitLength - 1
+//   radix**N ≥ 2**bitLength
+//   N ≥ log2(2**bitLength) / log2(radix)
+//   N ≥ bitLength / log2(radix)
+//
+// so the smallest N is:
+//
+//   N = ⌈bitLength / log2(radix)⌉
+//
+// We want to avoid floating-point computations and precompute the logarithm, so
+// we multiply both sides of the division by |bitsPerCharTableMultiplier|:
+//
+//   N = ⌈(bPCTM * bitLength) / (bPCTM * log2(radix))⌉
+//
+// and then because |maxBitsPerChar| representing the denominator may have been
+// rounded *up* -- which could produce an overall under-computation -- we reduce
+// by one to undo any rounding and conservatively compute:
+//
+//   N ≥ ⌈(bPCTM * bitLength) / (maxBitsPerChar - 1)⌉
+//
+size_t BigInt::calculateMaximumCharactersRequired(HandleBigInt x,
+                                                  unsigned radix) {
+  MOZ_ASSERT(!x->isZero());
+  MOZ_ASSERT(radix >= 2 && radix <= 36);
+
+  size_t length = x->digitLength();
+  Digit lastDigit = x->digit(length - 1);
+  size_t bitLength = length * DigitBits - DigitLeadingZeroes(lastDigit);
+
+  uint8_t maxBitsPerChar = maxBitsPerCharTable[radix];
+  uint64_t maximumCharactersRequired =
+      CeilDiv(static_cast<uint64_t>(bitsPerCharTableMultiplier) * bitLength,
+              maxBitsPerChar - 1);
+  maximumCharactersRequired += x->isNegative();
+
+  return AssertedCast<size_t>(maximumCharactersRequired);
+}
+
+JSLinearString* BigInt::toStringBasePowerOfTwo(ExclusiveContext* cx, HandleBigInt x,
+                                               unsigned radix) {
+  MOZ_ASSERT(mozilla::IsPowerOfTwo(radix));
+  MOZ_ASSERT(radix >= 2 && radix <= 32);
+  MOZ_ASSERT(!x->isZero());
+
+  const unsigned length = x->digitLength();
+  const bool sign = x->isNegative();
+  const unsigned bitsPerChar = mozilla::CountTrailingZeroes32(radix);
+  const unsigned charMask = radix - 1;
+  // Compute the length of the resulting string: divide the bit length of the
+  // BigInt by the number of bits representable per character (rounding up).
+  const Digit msd = x->digit(length - 1);
+
+  const size_t bitLength = length * DigitBits - DigitLeadingZeroes(msd);
+  const size_t charsRequired = CeilDiv(bitLength, bitsPerChar) + sign;
+
+  if (charsRequired > JSString::MAX_LENGTH) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  auto resultChars = cx->make_pod_array<char>(charsRequired);
+  if (!resultChars) {
+    return nullptr;
+  }
+
+  Digit digit = 0;
+  // Keeps track of how many unprocessed bits there are in |digit|.
+  unsigned availableBits = 0;
+  size_t pos = charsRequired;
+  for (unsigned i = 0; i < length - 1; i++) {
+    Digit newDigit = x->digit(i);
+    // Take any leftover bits from the last iteration into account.
+    unsigned current = (digit | (newDigit << availableBits)) & charMask;
+    MOZ_ASSERT(pos);
+    resultChars[--pos] = radixDigits[current];
+    unsigned consumedBits = bitsPerChar - availableBits;
+    digit = newDigit >> consumedBits;
+    availableBits = DigitBits - consumedBits;
+    while (availableBits >= bitsPerChar) {
+      MOZ_ASSERT(pos);
+      resultChars[--pos] = radixDigits[digit & charMask];
+      digit >>= bitsPerChar;
+      availableBits -= bitsPerChar;
+    }
+  }
+
+  // Write out the character containing the lowest-order bit of |msd|.
+  //
+  // This character may include leftover bits from the Digit below |msd|.  For
+  // example, if |x === 2n**64n| and |radix == 32|: the preceding loop writes
+  // twelve zeroes for low-order bits 0-59 in |x->digit(0)| (and |x->digit(1)|
+  // on 32-bit); then the highest 4 bits of of |x->digit(0)| (or |x->digit(1)|
+  // on 32-bit) and bit 0 of |x->digit(1)| (|x->digit(2)| on 32-bit) will
+  // comprise the |current == 0b1'0000| computed below for the high-order 'g'
+  // character.
+  unsigned current = (digit | (msd << availableBits)) & charMask;
+  MOZ_ASSERT(pos);
+  resultChars[--pos] = radixDigits[current];
+
+  // Write out remaining characters represented by |msd|.  (There may be none,
+  // as in the example above.)
+  digit = msd >> (bitsPerChar - availableBits);
+  while (digit != 0) {
+    MOZ_ASSERT(pos);
+    resultChars[--pos] = radixDigits[digit & charMask];
+    digit >>= bitsPerChar;
+  }
+
+  if (sign) {
+    MOZ_ASSERT(pos);
+    resultChars[--pos] = '-';
+  }
+
+  MOZ_ASSERT(pos == 0);
+  return NewStringCopyN<CanGC>(cx, resultChars.get(), charsRequired);
+}
+
+static constexpr BigInt::Digit MaxPowerInDigit(uint8_t radix) {
+  BigInt::Digit result = 1;
+  while (result < BigInt::Digit(-1) / radix) {
+    result *= radix;
+  }
+  return result;
+}
+
+static constexpr uint8_t MaxExponentInDigit(uint8_t radix) {
+  uint8_t exp = 0;
+  BigInt::Digit result = 1;
+  while (result < BigInt::Digit(-1) / radix) {
+    result *= radix;
+    exp += 1;
+  }
+  return exp;
+}
+
+struct RadixInfo {
+  BigInt::Digit maxPowerInDigit;
+  uint8_t maxExponentInDigit;
+
+  constexpr RadixInfo(BigInt::Digit maxPower, uint8_t maxExponent)
+      : maxPowerInDigit(maxPower), maxExponentInDigit(maxExponent) {}
+
+  explicit constexpr RadixInfo(uint8_t radix)
+      : RadixInfo(MaxPowerInDigit(radix), MaxExponentInDigit(radix)) {}
+};
+
+static constexpr const RadixInfo toStringInfo[37] = {
+    {0, 0},        {0, 0},        RadixInfo(2),  RadixInfo(3),  RadixInfo(4),
+    RadixInfo(5),  RadixInfo(6),  RadixInfo(7),  RadixInfo(8),  RadixInfo(9),
+    RadixInfo(10), RadixInfo(11), RadixInfo(12), RadixInfo(13), RadixInfo(14),
+    RadixInfo(15), RadixInfo(16), RadixInfo(17), RadixInfo(18), RadixInfo(19),
+    RadixInfo(20), RadixInfo(21), RadixInfo(22), RadixInfo(23), RadixInfo(24),
+    RadixInfo(25), RadixInfo(26), RadixInfo(27), RadixInfo(28), RadixInfo(29),
+    RadixInfo(30), RadixInfo(31), RadixInfo(32), RadixInfo(33), RadixInfo(34),
+    RadixInfo(35), RadixInfo(36),
+};
+
+JSLinearString* BigInt::toStringGeneric(ExclusiveContext* cx, HandleBigInt x,
+                                        unsigned radix) {
+  MOZ_ASSERT(radix >= 2 && radix <= 36);
+  MOZ_ASSERT(!x->isZero());
+
+  size_t maximumCharactersRequired =
+      calculateMaximumCharactersRequired(x, radix);
+  if (maximumCharactersRequired > JSString::MAX_LENGTH) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  UniqueChars resultString(js_pod_malloc<char>(maximumCharactersRequired));
+  if (!resultString) {
+    ReportOutOfMemory(cx);
+    return nullptr;
+  }
+
+  size_t writePos = maximumCharactersRequired;
+  unsigned length = x->digitLength();
+  Digit lastDigit;
+  if (length == 1) {
+    lastDigit = x->digit(0);
+  } else {
+    unsigned chunkChars = toStringInfo[radix].maxExponentInDigit;
+    Digit chunkDivisor = toStringInfo[radix].maxPowerInDigit;
+
+    unsigned nonZeroDigit = length - 1;
+    MOZ_ASSERT(x->digit(nonZeroDigit) != 0);
+
+    // `rest` holds the part of the BigInt that we haven't looked at yet.
+    // Not to be confused with "remainder"!
+    RootedBigInt rest(cx);
+
+    // In the first round, divide the input, allocating a new BigInt for
+    // the result == rest; from then on divide the rest in-place.
+    //
+    // FIXME: absoluteDivWithDigitDivisor doesn't
+    // destructivelyTrimHighZeroDigits for in-place divisions, leading to
+    // worse constant factors.  See
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1510213.
+    RootedBigInt dividend(cx, x);
+    do {
+      Digit chunk;
+      if (!absoluteDivWithDigitDivisor(cx, dividend, chunkDivisor, Some(&rest),
+                                       &chunk, dividend->isNegative())) {
         return nullptr;
+      }
+
+      dividend = rest;
+      for (unsigned i = 0; i < chunkChars; i++) {
+        MOZ_ASSERT(writePos > 0);
+        resultString[--writePos] = radixDigits[chunk % radix];
+        chunk /= radix;
+      }
+      MOZ_ASSERT(!chunk);
+
+      if (!rest->digit(nonZeroDigit)) {
+        nonZeroDigit--;
+      }
+
+      MOZ_ASSERT(rest->digit(nonZeroDigit) != 0,
+                 "division by a single digit can't remove more than one "
+                 "digit from a number");
+    } while (nonZeroDigit > 0);
+
+    lastDigit = rest->digit(0);
+  }
+
+  do {
+    MOZ_ASSERT(writePos > 0);
+    resultString[--writePos] = radixDigits[lastDigit % radix];
+    lastDigit /= radix;
+  } while (lastDigit > 0);
+  MOZ_ASSERT(writePos < maximumCharactersRequired);
+  MOZ_ASSERT(maximumCharactersRequired - writePos <=
+             static_cast<size_t>(maximumCharactersRequired));
+
+  // Remove leading zeroes.
+  while (writePos + 1 < maximumCharactersRequired &&
+         resultString[writePos] == '0') {
+    writePos++;
+  }
+
+  if (x->isNegative()) {
+    MOZ_ASSERT(writePos > 0);
+    resultString[--writePos] = '-';
+  }
+
+  MOZ_ASSERT(writePos < maximumCharactersRequired);
+  // Would be better to somehow adopt resultString directly.
+  return NewStringCopyN<CanGC>(cx, resultString.get() + writePos,
+                               maximumCharactersRequired - writePos);
+}
+
+BigInt* BigInt::trimHighZeroDigits(ExclusiveContext* cx, HandleBigInt x) {
+  if (x->isZero()) {
+    MOZ_ASSERT(!x->isNegative());
+    return x;
+  }
+  MOZ_ASSERT(x->digitLength());
+
+  int nonZeroIndex = x->digitLength() - 1;
+  while (nonZeroIndex >= 0 && x->digit(nonZeroIndex) == 0) {
+    nonZeroIndex--;
+  }
+
+  if (nonZeroIndex < 0) {
+    return zero(cx);
+  }
+
+  if (nonZeroIndex == static_cast<int>(x->digitLength() - 1)) {
     return x;
+  }
+
+  unsigned newLength = nonZeroIndex + 1;
+  BigInt* trimmedBigInt = createUninitialized(cx, newLength, x->isNegative());
+  if (!trimmedBigInt) {
+    return nullptr;
+  }
+  for (unsigned i = 0; i < newLength; i++) {
+    trimmedBigInt->setDigit(i, x->digit(i));
+  }
+
+  return trimmedBigInt;
+}
+
+BigInt* BigInt::destructivelyTrimHighZeroDigits(ExclusiveContext* cx, HandleBigInt x) {
+  // TODO: Modify in place instead of allocating.
+  return trimHighZeroDigits(cx, x);
+}
+
+// The maximum value `radix**charCount - 1` must be represented as a max number
+// `2**(N * DigitBits) - 1` for `N` digits, so
+//
+//   2**(N * DigitBits) - 1 ≥ radix**charcount - 1
+//   2**(N * DigitBits) ≥ radix**charcount
+//   N * DigitBits ≥ log2(radix**charcount)
+//   N * DigitBits ≥ charcount * log2(radix)
+//   N ≥ ⌈charcount * log2(radix) / DigitBits⌉ (conservatively)
+//
+// or in the code's terms (all numbers promoted to exact mathematical values),
+//
+//   N ≥ ⌈charcount * bitsPerChar / (DigitBits * bitsPerCharTableMultiplier)⌉
+//
+// Note that `N` is computed even more conservatively here because `bitsPerChar`
+// is rounded up.
+bool BigInt::calculateMaximumDigitsRequired(ExclusiveContext* cx, uint8_t radix,
+                                            size_t charcount, size_t* result) {
+  MOZ_ASSERT(2 <= radix && radix <= 36);
+
+  size_t bitsPerChar = maxBitsPerCharTable[radix];
+
+  MOZ_ASSERT(charcount > 0);
+  MOZ_ASSERT(charcount <= std::numeric_limits<size_t>::max() / bitsPerChar);
+  uint64_t n =
+      CeilDiv(charcount * bitsPerChar, DigitBits * bitsPerCharTableMultiplier);
+  if (n > MaxDigitLength) {
+    ReportAllocationOverflow(cx);
+    return false;
+  }
+
+  *result = n;
+  return true;
 }
 
-BigInt*
-BigInt::create(ExclusiveContext* cx, double d)
-{
+template <typename CharT>
+BigInt* BigInt::parseLiteralDigits(ExclusiveContext* cx,
+                                   const Range<const CharT> chars,
+                                   unsigned radix, bool isNegative,
+                                   bool* haveParseError) {
+  MOZ_ASSERT(chars.length());
+
+  RangedPtr<const CharT> start = chars.begin();
+  RangedPtr<const CharT> end = chars.end();
+
+  // Skipping leading zeroes.
+  while (start[0] == '0') {
+    start++;
+    if (start == end) {
+      return zero(cx);
+    }
+  }
+
+  unsigned limit0 = '0' + std::min(radix, 10u);
+  unsigned limita = 'a' + (radix - 10);
+  unsigned limitA = 'A' + (radix - 10);
+
+  size_t length;
+  if (!calculateMaximumDigitsRequired(cx, radix, end - start, &length)) {
+    return nullptr;
+  }
+  RootedBigInt result(cx, createUninitialized(cx, length, isNegative));
+  if (!result) {
     return nullptr;
+  }
+
+  result->initializeDigitsToZero();
+
+  for (; start < end; start++) {
+    uint32_t digit;
+    CharT c = *start;
+    if (c >= '0' && c < limit0) {
+      digit = c - '0';
+    } else if (c >= 'a' && c < limita) {
+      digit = c - 'a' + 10;
+    } else if (c >= 'A' && c < limitA) {
+      digit = c - 'A' + 10;
+    } else {
+      *haveParseError = true;
+      return nullptr;
+    }
+
+    result->inplaceMultiplyAdd(static_cast<Digit>(radix),
+                               static_cast<Digit>(digit));
+  }
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// BigInt proposal section 7.2
+template <typename CharT>
+BigInt* BigInt::parseLiteral(ExclusiveContext* cx, const Range<const CharT> chars,
+                             bool* haveParseError) {
+  RangedPtr<const CharT> start = chars.begin();
+  const RangedPtr<const CharT> end = chars.end();
+  bool isNegative = false;
+
+  MOZ_ASSERT(chars.length());
+
+  if (end - start > 2 && start[0] == '0') {
+    if (start[1] == 'b' || start[1] == 'B') {
+      // StringNumericLiteral ::: BinaryIntegerLiteral
+      return parseLiteralDigits(cx, Range<const CharT>(start + 2, end), 2,
+                                isNegative, haveParseError);
+    }
+    if (start[1] == 'x' || start[1] == 'X') {
+      // StringNumericLiteral ::: HexIntegerLiteral
+      return parseLiteralDigits(cx, Range<const CharT>(start + 2, end), 16,
+                                isNegative, haveParseError);
+    }
+    if (start[1] == 'o' || start[1] == 'O') {
+      // StringNumericLiteral ::: OctalIntegerLiteral
+      return parseLiteralDigits(cx, Range<const CharT>(start + 2, end), 8,
+                                isNegative, haveParseError);
+    }
+  }
+
+  return parseLiteralDigits(cx, Range<const CharT>(start, end), 10, isNegative,
+                            haveParseError);
 }
 
 // BigInt proposal section 5.1.1
-static bool
-IsInteger(double d)
-{
-    // Step 1 is an assertion checked by the caller.
-    // Step 2.
-    if (!mozilla::IsFinite(d))
-        return false;
+static bool IsInteger(double d) {
+  // Step 1 is an assertion checked by the caller.
+  // Step 2.
+  if (!mozilla::IsFinite(d)) {
+    return false;
+  }
 
-    // Step 3.
-    double i = JS::ToInteger(d);
+  // Step 3.
+  double i = JS::ToInteger(d);
 
-    // Step 4.
-    if (i != d)
-        return false;
+  // Step 4.
+  if (i != d) {
+    return false;
+  }
 
-    // Step 5.
-    return true;
+  // Step 5.
+  return true;
+}
+
+BigInt* BigInt::createFromDouble(ExclusiveContext* cx, double d) {
+  MOZ_ASSERT(::IsInteger(d),
+             "Only integer-valued doubles can convert to BigInt");
+
+  if (d == 0) {
+    return zero(cx);
+  }
+
+  int exponent = mozilla::ExponentComponent(d);
+  MOZ_ASSERT(exponent >= 0);
+  int length = exponent / DigitBits + 1;
+  BigInt* result = createUninitialized(cx, length, d < 0);
+  if (!result) {
+    return nullptr;
+  }
+
+  // We construct a BigInt from the double `d` by shifting its mantissa
+  // according to its exponent and mapping the bit pattern onto digits.
+  //
+  //               <----------- bitlength = exponent + 1 ----------->
+  //                <----- 52 ------> <------ trailing zeroes ------>
+  // mantissa:     1yyyyyyyyyyyyyyyyy 0000000000000000000000000000000
+  // digits:    0001xxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
+  //                <-->          <------>
+  //          msdTopBits          DigitBits
+  //
+  using Double = mozilla::FloatingPoint<double>;
+  uint64_t mantissa =
+      mozilla::BitwiseCast<uint64_t>(d) & Double::kSignificandBits;
+  // Add implicit high bit.
+  mantissa |= 1ull << Double::kSignificandWidth;
+
+  const int mantissaTopBit = Double::kSignificandWidth;  // 0-indexed.
+
+  // 0-indexed position of `d`'s most significant bit within the `msd`.
+  int msdTopBit = exponent % DigitBits;
+
+  // Next digit under construction.
+  Digit digit;
+
+  // First, build the MSD by shifting the mantissa appropriately.
+  if (msdTopBit < mantissaTopBit) {
+    int remainingMantissaBits = mantissaTopBit - msdTopBit;
+    digit = mantissa >> remainingMantissaBits;
+    mantissa = mantissa << (64 - remainingMantissaBits);
+  } else {
+    MOZ_ASSERT(msdTopBit >= mantissaTopBit);
+    digit = mantissa << (msdTopBit - mantissaTopBit);
+    mantissa = 0;
+  }
+  result->setDigit(--length, digit);
+
+  // Fill in digits containing mantissa contributions.
+  while (mantissa) {
+    MOZ_ASSERT(length > 0,
+               "double bits were all non-fractional, so there must be "
+               "digits present to hold them");
+
+    if (DigitBits == 64) {
+      result->setDigit(--length, mantissa);
+      break;
+    }
+
+    MOZ_ASSERT(DigitBits == 32);
+    Digit current = mantissa >> 32;
+    mantissa = mantissa << 32;
+    result->setDigit(--length, current);
+  }
+
+  // Fill in low-order zeroes.
+  for (int i = length - 1; i >= 0; i--) {
+    result->setDigit(i, 0);
+  }
+
+  return result;
+}
+
+BigInt* BigInt::createFromUint64(ExclusiveContext* cx, uint64_t n) {
+  if (n == 0) {
+    return zero(cx);
+  }
+
+  const bool isNegative = false;
+
+  if (DigitBits == 32) {
+    Digit low = n;
+    Digit high = n >> 32;
+    size_t length = high ? 2 : 1;
+
+    BigInt* res = createUninitialized(cx, length, isNegative);
+    if (!res) {
+      return nullptr;
+    }
+    res->setDigit(0, low);
+    if (high) {
+      res->setDigit(1, high);
+    }
+    return res;
+  }
+
+  BigInt* res = createUninitialized(cx, 1, isNegative);
+  if (!res) {
+    return nullptr;
+  }
+
+  res->setDigit(0, n);
+  return res;
+}
+
+BigInt* BigInt::createFromInt64(ExclusiveContext* cx, int64_t n) {
+  BigInt* res = createFromUint64(cx, Abs(n));
+  if (!res) {
+    return nullptr;
+  }
+
+  if (n < 0) {
+    res->lengthSignAndReservedBits_ |= SignBit;
+  }
+  MOZ_ASSERT(res->isNegative() == (n < 0));
+
+  return res;
 }
 
 // BigInt proposal section 5.1.2
-BigInt*
-js::NumberToBigInt(ExclusiveContext* cx, double d)
-{
-    // Step 1 is an assertion checked by the caller.
-    // Step 2.
-    if (!IsInteger(d)) {
-        if(cx->isJSContext()) {
-            JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
-                                      JSMSG_NUMBER_TO_BIGINT);
+BigInt* js::NumberToBigInt(ExclusiveContext* cx, double d) {
+  // Step 1 is an assertion checked by the caller.
+  // Step 2.
+  if (!::IsInteger(d)) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_NUMBER_TO_BIGINT);
+    }
+    return nullptr;
+  }
+
+  // Step 3.
+  return BigInt::createFromDouble(cx, d);
+}
+
+BigInt* BigInt::copy(ExclusiveContext* cx, HandleBigInt x) {
+  if (x->isZero()) {
+    return zero(cx);
+  }
+
+  BigInt* result = createUninitialized(cx, x->digitLength(), x->isNegative());
+  if (!result) {
+    return nullptr;
+  }
+  for (size_t i = 0; i < x->digitLength(); i++) {
+    result->setDigit(i, x->digit(i));
+  }
+  return result;
+}
+
+// BigInt proposal section 1.1.7
+BigInt* BigInt::add(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  bool xNegative = x->isNegative();
+
+  // x + y == x + y
+  // -x + -y == -(x + y)
+  if (xNegative == y->isNegative()) {
+    return absoluteAdd(cx, x, y, xNegative);
+  }
+
+  // x + -y == x - y == -(y - x)
+  // -x + y == y - x == -(x - y)
+  if (absoluteCompare(x, y) >= 0) {
+    return absoluteSub(cx, x, y, xNegative);
+  }
+
+  return absoluteSub(cx, y, x, !xNegative);
+}
+
+// BigInt proposal section 1.1.8
+BigInt* BigInt::sub(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  bool xNegative = x->isNegative();
+  if (xNegative != y->isNegative()) {
+    // x - (-y) == x + y
+    // (-x) - y == -(x + y)
+    return absoluteAdd(cx, x, y, xNegative);
+  }
+  // x - y == -(y - x)
+  // (-x) - (-y) == y - x == -(x - y)
+  if (absoluteCompare(x, y) >= 0) {
+    return absoluteSub(cx, x, y, xNegative);
+  }
+
+  return absoluteSub(cx, y, x, !xNegative);
+}
+
+// BigInt proposal section 1.1.4
+BigInt* BigInt::mul(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero()) {
+    return x;
+  }
+  if (y->isZero()) {
+    return y;
+  }
+
+  unsigned resultLength = x->digitLength() + y->digitLength();
+  bool resultNegative = x->isNegative() != y->isNegative();
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+  result->initializeDigitsToZero();
+
+  for (size_t i = 0; i < x->digitLength(); i++) {
+    multiplyAccumulate(y, x->digit(i), result, i);
+  }
+
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// BigInt proposal section 1.1.5
+BigInt* BigInt::div(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  // 1. If y is 0n, throw a RangeError exception.
+  if (y->isZero()) {
+      if (cx->isJSContext()) {
+        JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                  JSMSG_BIGINT_DIVISION_BY_ZERO);
+    }
+    return nullptr;
+  }
+
+  // 2. Let quotient be the mathematical value of x divided by y.
+  // 3. Return a BigInt representing quotient rounded towards 0 to the next
+  //    integral value.
+  if (x->isZero()) {
+    return x;
+  }
+
+  if (absoluteCompare(x, y) < 0) {
+    return zero(cx);
+  }
+
+  RootedBigInt quotient(cx);
+  bool resultNegative = x->isNegative() != y->isNegative();
+  if (y->digitLength() == 1) {
+    Digit divisor = y->digit(0);
+    if (divisor == 1) {
+      return resultNegative == x->isNegative() ? x : neg(cx, x);
+    }
+
+    Digit remainder;
+    if (!absoluteDivWithDigitDivisor(cx, x, divisor, Some(&quotient),
+                                     &remainder, resultNegative)) {
+      return nullptr;
+    }
+  } else {
+    if (!absoluteDivWithBigIntDivisor(cx, x, y, Some(&quotient), Nothing(),
+                                      resultNegative)) {
+      return nullptr;
+    }
+  }
+
+  return destructivelyTrimHighZeroDigits(cx, quotient);
+}
+
+// BigInt proposal section 1.1.6
+BigInt* BigInt::mod(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  // 1. If y is 0n, throw a RangeError exception.
+  if (y->isZero()) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_DIVISION_BY_ZERO);
+    }
+    return nullptr;
+  }
+
+  // 2. If x is 0n, return x.
+  if (x->isZero()) {
+    return x;
+  }
+  // 3. Let r be the BigInt defined by the mathematical relation r = x - (y ×
+  // q) where q is a BigInt that is negative only if x/y is negative and
+  // positive only if x/y is positive, and whose magnitude is as large as
+  // possible without exceeding the magnitude of the true mathematical
+  // quotient of x and y.
+  if (absoluteCompare(x, y) < 0) {
+    return x;
+  }
+
+  if (y->digitLength() == 1) {
+    Digit divisor = y->digit(0);
+    if (divisor == 1) {
+      return zero(cx);
+    }
+
+    Digit remainderDigit;
+    bool unusedQuotientNegative = false;
+    if (!absoluteDivWithDigitDivisor(cx, x, divisor, Nothing(), &remainderDigit,
+                                     unusedQuotientNegative)) {
+      MOZ_CRASH("BigInt div by digit failed unexpectedly");
+    }
+
+    if (!remainderDigit) {
+      return zero(cx);
+    }
+
+    BigInt* remainder = createUninitialized(cx, 1, x->isNegative());
+    if (!remainder) {
+      return nullptr;
+    }
+    remainder->setDigit(0, remainderDigit);
+    return remainder;
+  } else {
+    RootedBigInt remainder(cx);
+    if (!absoluteDivWithBigIntDivisor(cx, x, y, Nothing(), Some(&remainder),
+                                      x->isNegative())) {
+      return nullptr;
+    }
+    MOZ_ASSERT(remainder);
+    return remainder;
+  }
+}
+
+// BigInt proposal section 1.1.3
+BigInt* BigInt::pow(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  // 1. If exponent is < 0, throw a RangeError exception.
+  if (y->isNegative()) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_NEGATIVE_EXPONENT);
+    }
+    return nullptr;
+  }
+
+  // 2. If base is 0n and exponent is 0n, return 1n.
+  if (y->isZero()) {
+    return one(cx);
+  }
+
+  if (x->isZero()) {
+    return x;
+  }
+
+  // 3. Return a BigInt representing the mathematical value of base raised
+  //    to the power exponent.
+  if (x->digitLength() == 1 && x->digit(0) == 1) {
+    // (-1) ** even_number == 1.
+    if (x->isNegative() && (y->digit(0) & 1) == 0) {
+      return neg(cx, x);
+    }
+    // (-1) ** odd_number == -1; 1 ** anything == 1.
+    return x;
+  }
+
+  // For all bases >= 2, very large exponents would lead to unrepresentable
+  // results.
+  static_assert(MaxBitLength < std::numeric_limits<Digit>::max(),
+                "unexpectedly large MaxBitLength");
+  if (y->digitLength() > 1) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_TOO_LARGE);
+    }
+    return nullptr;
+  }
+  Digit exponent = y->digit(0);
+  if (exponent == 1) {
+    return x;
+  }
+  if (exponent >= MaxBitLength) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_TOO_LARGE);
+    }
+    return nullptr;
+  }
+
+  static_assert(MaxBitLength <= std::numeric_limits<int>::max(),
+                "unexpectedly large MaxBitLength");
+  int n = static_cast<int>(exponent);
+  if (x->digitLength() == 1 && x->digit(0) == 2) {
+    // Fast path for 2^n.
+    int length = 1 + (n / DigitBits);
+    // Result is negative for odd powers of -2n.
+    bool resultNegative = x->isNegative() && (n & 1);
+    RootedBigInt result(cx, createUninitialized(cx, length, resultNegative));
+    if (!result) {
+      return nullptr;
+    }
+    result->initializeDigitsToZero();
+    result->setDigit(length - 1, static_cast<Digit>(1) << (n % DigitBits));
+    return result;
+  }
+
+  // This implicitly sets the result's sign correctly.
+  RootedBigInt result(cx, (n & 1) ? x : nullptr);
+  RootedBigInt runningSquare(cx, x);
+  for (n /= 2; n; n /= 2) {
+    runningSquare = mul(cx, runningSquare, runningSquare);
+    if (!runningSquare) {
+      return nullptr;
+    }
+    if (n & 1) {
+      if (!result) {
+        result = runningSquare;
+      } else {
+        result = mul(cx, result, runningSquare);
+        if (!result) {
+          return nullptr;
         }
-        return nullptr;
+      }
+    }
+  }
+  return result;
+}
+
+BigInt* BigInt::lshByAbsolute(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero() || y->isZero()) {
+    return x;
+  }
+
+  if (y->digitLength() > 1 || y->digit(0) > MaxBitLength) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_TOO_LARGE);
     }
+    return nullptr;
+  }
+  Digit shift = y->digit(0);
+  int digitShift = static_cast<int>(shift / DigitBits);
+  int bitsShift = static_cast<int>(shift % DigitBits);
+  int length = x->digitLength();
+  bool grow = bitsShift && (x->digit(length - 1) >> (DigitBits - bitsShift));
+  int resultLength = length + digitShift + grow;
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, x->isNegative()));
+  if (!result) {
+    return nullptr;
+  }
+
+  int i = 0;
+  for (; i < digitShift; i++) {
+    result->setDigit(i, 0);
+  }
 
-    // Step 3.
-    return BigInt::create(cx, d);
+  if (bitsShift == 0) {
+    for (int j = 0; i < resultLength; i++, j++) {
+      result->setDigit(i, x->digit(j));
+    }
+  } else {
+    Digit carry = 0;
+    for (int j = 0; j < length; i++, j++) {
+      Digit d = x->digit(j);
+      result->setDigit(i, (d << bitsShift) | carry);
+      carry = d >> (DigitBits - bitsShift);
+    }
+    if (grow) {
+      result->setDigit(i, carry);
+    } else {
+      MOZ_ASSERT(!carry);
+    }
+  }
+  return result;
 }
 
-BigInt*
-BigInt::copy(ExclusiveContext* cx, HandleBigInt x)
-{
-    BigInt* bi = create(cx);
-    if (!bi)
-        return nullptr;
-    return bi;
+BigInt* BigInt::rshByMaximum(ExclusiveContext* cx, bool isNegative) {
+  if (isNegative) {
+    RootedBigInt negativeOne(cx, createUninitialized(cx, 1, isNegative));
+    if (!negativeOne) {
+      return nullptr;
+    }
+    negativeOne->setDigit(0, 1);
+    return negativeOne;
+  }
+  return zero(cx);
 }
 
-// BigInt proposal section 7.3
-BigInt*
-js::ToBigInt(ExclusiveContext* cx, HandleValue val)
-{
-    RootedValue v(cx, val);
-
-    if(cx->isJSContext()) {
-        // Step 1.
-        if (!ToPrimitive(cx->asJSContext(), JSTYPE_NUMBER, &v))
-        return nullptr;
+BigInt* BigInt::rshByAbsolute(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero() || y->isZero()) {
+    return x;
+  }
+
+  if (y->digitLength() > 1 || y->digit(0) >= MaxBitLength) {
+    return rshByMaximum(cx, x->isNegative());
+  }
+  Digit shift = y->digit(0);
+  int length = x->digitLength();
+  int digitShift = static_cast<int>(shift / DigitBits);
+  int bitsShift = static_cast<int>(shift % DigitBits);
+  int resultLength = length - digitShift;
+  if (resultLength <= 0) {
+    return rshByMaximum(cx, x->isNegative());
+  }
+  // For negative numbers, round down if any bit was shifted out (so that e.g.
+  // -5n >> 1n == -3n and not -2n). Check now whether this will happen and
+  // whether it can cause overflow into a new digit. If we allocate the result
+  // large enough up front, it avoids having to do a second allocation later.
+  bool mustRoundDown = false;
+  if (x->isNegative()) {
+    const Digit mask = (static_cast<Digit>(1) << bitsShift) - 1;
+    if ((x->digit(digitShift) & mask)) {
+      mustRoundDown = true;
+    } else {
+      for (int i = 0; i < digitShift; i++) {
+        if (x->digit(i)) {
+          mustRoundDown = true;
+          break;
+        }
+      }
+    }
+  }
+  // If bits_shift is non-zero, it frees up bits, preventing overflow.
+  if (mustRoundDown && bitsShift == 0) {
+    // Overflow cannot happen if the most significant digit has unset bits.
+    Digit msd = x->digit(length - 1);
+    bool roundingCanOverflow = msd == std::numeric_limits<Digit>::max();
+    if (roundingCanOverflow) {
+      resultLength++;
+    }
+  }
+
+  MOZ_ASSERT(resultLength <= length);
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, x->isNegative()));
+  if (!result) {
+    return nullptr;
+  }
+  if (!bitsShift) {
+    for (int i = digitShift; i < length; i++) {
+      result->setDigit(i - digitShift, x->digit(i));
+    }
+  } else {
+    Digit carry = x->digit(digitShift) >> bitsShift;
+    int last = length - digitShift - 1;
+    for (int i = 0; i < last; i++) {
+      Digit d = x->digit(i + digitShift + 1);
+      result->setDigit(i, (d << (DigitBits - bitsShift)) | carry);
+      carry = d >> bitsShift;
+    }
+    result->setDigit(last, carry);
+  }
+
+  if (mustRoundDown) {
+    MOZ_ASSERT(x->isNegative());
+    // Since the result is negative, rounding down means adding one to
+    // its absolute value. This cannot overflow.  TODO: modify the result in
+    // place.
+    return absoluteAddOne(cx, result, x->isNegative());
+  }
+  return destructivelyTrimHighZeroDigits(cx, result);
+}
+
+// BigInt proposal section 1.1.9. BigInt::leftShift ( x, y )
+BigInt* BigInt::lsh(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (y->isNegative()) {
+    return rshByAbsolute(cx, x, y);
+  }
+  return lshByAbsolute(cx, x, y);
+}
+
+// BigInt proposal section 1.1.10. BigInt::signedRightShift ( x, y )
+BigInt* BigInt::rsh(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (y->isNegative()) {
+    return lshByAbsolute(cx, x, y);
+  }
+  return rshByAbsolute(cx, x, y);
+}
+
+// BigInt proposal section 1.1.17. BigInt::bitwiseAND ( x, y )
+BigInt* BigInt::bitAnd(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero()) {
+    return x;
+  }
+
+  if (y->isZero()) {
+    return y;
+  }
+
+  if (!x->isNegative() && !y->isNegative()) {
+    return absoluteAnd(cx, x, y);
+  }
+
+  if (x->isNegative() && y->isNegative()) {
+    int resultLength = std::max(x->digitLength(), y->digitLength()) + 1;
+    // (-x) & (-y) == ~(x-1) & ~(y-1) == ~((x-1) | (y-1))
+    // == -(((x-1) | (y-1)) + 1)
+    RootedBigInt x1(cx, absoluteSubOne(cx, x, resultLength));
+    if (!x1) {
+      return nullptr;
+    }
+    RootedBigInt y1(cx, absoluteSubOne(cx, y, y->digitLength()));
+    if (!y1) {
+      return nullptr;
+    }
+    RootedBigInt result(cx, absoluteOr(cx, x1, y1));
+    if (!result) {
+      return nullptr;
+    }
+    bool resultNegative = true;
+    return absoluteAddOne(cx, result, resultNegative);
+  }
 
-        // Step 2.
-        // Boolean and string conversions are not yet supported.
-        if (v.isBigInt())
-            return v.toBigInt();
+  MOZ_ASSERT(x->isNegative() != y->isNegative());
+  HandleBigInt& pos = x->isNegative() ? y : x;
+  HandleBigInt& neg = x->isNegative() ? x : y;
 
-        JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr, JSMSG_NOT_BIGINT);
-    }   
+  RootedBigInt neg1(cx, absoluteSubOne(cx, neg, neg->digitLength()));
+  if (!neg1) {
     return nullptr;
+  }
+
+  // x & (-y) == x & ~(y-1) == x & ~(y-1)
+  return absoluteAndNot(cx, pos, neg1);
 }
 
-JSLinearString*
-BigInt::toString(ExclusiveContext* cx, BigInt* x, uint8_t radix)
-{
+// BigInt proposal section 1.1.18. BigInt::bitwiseXOR ( x, y )
+BigInt* BigInt::bitXor(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero()) {
+    return y;
+  }
+
+  if (y->isZero()) {
+    return x;
+  }
+
+  if (!x->isNegative() && !y->isNegative()) {
+    return absoluteXor(cx, x, y);
+  }
+
+  if (x->isNegative() && y->isNegative()) {
+    int resultLength = std::max(x->digitLength(), y->digitLength());
+
+    // (-x) ^ (-y) == ~(x-1) ^ ~(y-1) == (x-1) ^ (y-1)
+    RootedBigInt x1(cx, absoluteSubOne(cx, x, resultLength));
+    if (!x1) {
+      return nullptr;
+    }
+    RootedBigInt y1(cx, absoluteSubOne(cx, y, y->digitLength()));
+    if (!y1) {
+      return nullptr;
+    }
+    return absoluteXor(cx, x1, y1);
+  }
+  MOZ_ASSERT(x->isNegative() != y->isNegative());
+  int resultLength = std::max(x->digitLength(), y->digitLength()) + 1;
+
+  HandleBigInt& pos = x->isNegative() ? y : x;
+  HandleBigInt& neg = x->isNegative() ? x : y;
+
+  // x ^ (-y) == x ^ ~(y-1) == ~(x ^ (y-1)) == -((x ^ (y-1)) + 1)
+  RootedBigInt result(cx, absoluteSubOne(cx, neg, resultLength));
+  if (!result) {
+    return nullptr;
+  }
+  result = absoluteXor(cx, result, pos);
+  if (!result) {
     return nullptr;
+  }
+  bool resultNegative = true;
+  return absoluteAddOne(cx, result, resultNegative);
 }
 
-void
-BigInt::finalize(js::FreeOp* fop)
-{
-    return;
+// BigInt proposal section 1.1.19. BigInt::bitwiseOR ( x, y )
+BigInt* BigInt::bitOr(ExclusiveContext* cx, HandleBigInt x, HandleBigInt y) {
+  if (x->isZero()) {
+    return y;
+  }
+
+  if (y->isZero()) {
+    return x;
+  }
+
+  unsigned resultLength = std::max(x->digitLength(), y->digitLength());
+  bool resultNegative = x->isNegative() || y->isNegative();
+
+  if (!resultNegative) {
+    return absoluteOr(cx, x, y);
+  }
+
+  if (x->isNegative() && y->isNegative()) {
+    // (-x) | (-y) == ~(x-1) | ~(y-1) == ~((x-1) & (y-1))
+    // == -(((x-1) & (y-1)) + 1)
+    RootedBigInt result(cx, absoluteSubOne(cx, x, resultLength));
+    if (!result) {
+      return nullptr;
+    }
+    RootedBigInt y1(cx, absoluteSubOne(cx, y, y->digitLength()));
+    if (!y1) {
+      return nullptr;
+    }
+    result = absoluteAnd(cx, result, y1);
+    if (!result) {
+      return nullptr;
+    }
+    return absoluteAddOne(cx, result, resultNegative);
+  }
+
+  MOZ_ASSERT(x->isNegative() != y->isNegative());
+  HandleBigInt& pos = x->isNegative() ? y : x;
+  HandleBigInt& neg = x->isNegative() ? x : y;
+
+  // x | (-y) == x | ~(y-1) == ~((y-1) &~ x) == -(((y-1) &~ x) + 1)
+  RootedBigInt result(cx, absoluteSubOne(cx, neg, resultLength));
+  if (!result) {
+    return nullptr;
+  }
+  result = absoluteAndNot(cx, result, pos);
+  if (!result) {
+    return nullptr;
+  }
+  return absoluteAddOne(cx, result, resultNegative);
+}
+
+// BigInt proposal section 1.1.2. BigInt::bitwiseNOT ( x )
+BigInt* BigInt::bitNot(ExclusiveContext* cx, HandleBigInt x) {
+  if (x->isNegative()) {
+    // ~(-x) == ~(~(x-1)) == x-1
+    return absoluteSubOne(cx, x, x->digitLength());
+  } else {
+    // ~x == -x-1 == -(x+1)
+    bool resultNegative = true;
+    return absoluteAddOne(cx, x, resultNegative);
+  }
+}
+
+int64_t BigInt::toInt64(BigInt* x) { return WrapToSigned(toUint64(x)); }
+
+uint64_t BigInt::toUint64(BigInt* x) {
+  if (x->isZero()) {
+    return 0;
+  }
+
+  uint64_t digit = x->digit(0);
+
+  if (DigitBits == 32 && x->digitLength() >= 1) {
+    digit |= static_cast<uint64_t>(x->digit(1)) << 32;
+  }
+
+  // Return the two's complement if x is negative.
+  if (x->isNegative()) {
+    return ~(digit - 1);
+  }
+
+  return digit;
+}
+
+// Compute `2**bits - (x & (2**bits - 1))`.  Used when treating BigInt values as
+// arbitrary-precision two's complement signed integers.
+BigInt* BigInt::truncateAndSubFromPowerOfTwo(ExclusiveContext* cx, HandleBigInt x,
+                                             uint64_t bits,
+                                             bool resultNegative) {
+  MOZ_ASSERT(bits != 0);
+  MOZ_ASSERT(!x->isZero());
+
+  size_t resultLength = CeilDiv(bits, DigitBits);
+  RootedBigInt result(cx,
+                      createUninitialized(cx, resultLength, resultNegative));
+  if (!result) {
+    return nullptr;
+  }
+
+  // Process all digits except the MSD.
+  size_t xLength = x->digitLength();
+  Digit borrow = 0;
+  // Take digits from `x` until its length is exhausted.
+  for (size_t i = 0; i < std::min(resultLength - 1, xLength); i++) {
+    Digit newBorrow = 0;
+    Digit difference = digitSub(0, x->digit(i), &newBorrow);
+    difference = digitSub(difference, borrow, &newBorrow);
+    result->setDigit(i, difference);
+    borrow = newBorrow;
+  }
+  // Then simulate leading zeroes in `x` as needed.
+  for (size_t i = xLength; i < resultLength - 1; i++) {
+    Digit newBorrow = 0;
+    Digit difference = digitSub(0, borrow, &newBorrow);
+    result->setDigit(i, difference);
+    borrow = newBorrow;
+  }
+
+  // The MSD might contain extra bits that we don't want.
+  Digit xMSD = resultLength <= xLength ? x->digit(resultLength - 1) : 0;
+  Digit resultMSD;
+  if (bits % DigitBits == 0) {
+    Digit newBorrow = 0;
+    resultMSD = digitSub(0, xMSD, &newBorrow);
+    resultMSD = digitSub(resultMSD, borrow, &newBorrow);
+  } else {
+    size_t drop = DigitBits - (bits % DigitBits);
+    xMSD = (xMSD << drop) >> drop;
+    Digit minuendMSD = Digit(1) << (DigitBits - drop);
+    Digit newBorrow = 0;
+    resultMSD = digitSub(minuendMSD, xMSD, &newBorrow);
+    resultMSD = digitSub(resultMSD, borrow, &newBorrow);
+    MOZ_ASSERT(newBorrow == 0, "result < 2^bits");
+    // If all subtracted bits were zero, we have to get rid of the
+    // materialized minuendMSD again.
+    resultMSD &= (minuendMSD - 1);
+  }
+  result->setDigit(resultLength - 1, resultMSD);
+
+  return trimHighZeroDigits(cx, result);
+}
+
+BigInt* BigInt::asUintN(ExclusiveContext* cx, HandleBigInt x, uint64_t bits) {
+  if (x->isZero()) {
+    return x;
+  }
+
+  if (bits == 0) {
+    return zero(cx);
+  }
+
+  // When truncating a negative number, simulate two's complement.
+  if (x->isNegative()) {
+    bool resultNegative = false;
+    return truncateAndSubFromPowerOfTwo(cx, x, bits, resultNegative);
+  }
+
+  if (bits <= 64) {
+    uint64_t u64 = toUint64(x);
+    uint64_t mask = uint64_t(-1) >> (64 - bits);
+    return createFromUint64(cx, u64 & mask);
+  }
+
+  if (bits >= MaxBitLength) {
+    return x;
+  }
+
+  Digit msd = x->digit(x->digitLength() - 1);
+  size_t msdBits = DigitBits - DigitLeadingZeroes(msd);
+  size_t bitLength = msdBits + (x->digitLength() - 1) * DigitBits;
+
+  if (bits >= bitLength) {
+    return x;
+  }
+
+  size_t length = CeilDiv(bits, DigitBits);
+  bool isNegative = false;
+
+  BigInt* res = createUninitialized(cx, length, isNegative);
+  if (!res) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(length >= 2, "single-digit cases should be handled above");
+  MOZ_ASSERT(length <= x->digitLength());
+  for (size_t i = 0; i < length - 1; i++) {
+    res->setDigit(i, x->digit(i));
+  }
+
+  Digit mask = Digit(-1) >> (DigitBits - (bits % DigitBits));
+  res->setDigit(length - 1, x->digit(length - 1) & mask);
+
+  return res;
+}
+
+BigInt* BigInt::asIntN(ExclusiveContext* cx, HandleBigInt x, uint64_t bits) {
+  if (x->isZero()) {
+    return x;
+  }
+
+  if (bits == 0) {
+    return zero(cx);
+  }
+
+  if (bits == 64) {
+    return createFromInt64(cx, toInt64(x));
+  }
+
+  if (bits > MaxBitLength) {
+    return x;
+  }
+
+  Digit msd = x->digit(x->digitLength() - 1);
+  size_t msdBits = DigitBits - DigitLeadingZeroes(msd);
+  size_t bitLength = msdBits + (x->digitLength() - 1) * DigitBits;
+
+  if (bits > bitLength) {
+    return x;
+  }
+
+  Digit signBit = Digit(1) << ((bits - 1) % DigitBits);
+  if (bits == bitLength && msd < signBit) {
+    return x;
+  }
+
+  // All the cases above were the trivial cases: truncating zero, or to zero
+  // bits, or to more bits than are in `x` (so we return `x` directly), or we
+  // already have the 64-bit fast path.  If we get here, follow the textbook
+  // algorithm from the specification.
+
+  // BigInt.asIntN step 3:  Let `mod` be `x` modulo `2**bits`.
+  RootedBigInt mod(cx, asUintN(cx, x, bits));
+  if (!mod) {
+    return nullptr;
+  }
+
+  // Step 4: If `mod >= 2**(bits - 1)`, return `mod - 2**bits`; otherwise,
+  // return `mod`.
+  if (mod->digitLength() == CeilDiv(bits, DigitBits) &&
+      (mod->digit(mod->digitLength() - 1) & signBit) != 0) {
+    bool resultNegative = true;
+    return truncateAndSubFromPowerOfTwo(cx, mod, bits, resultNegative);
+  }
+
+  return mod;
+}
+
+static bool ValidBigIntOperands(ExclusiveContext* cx, HandleValue lhs,
+                                HandleValue rhs) {
+  MOZ_ASSERT(lhs.isBigInt() || rhs.isBigInt());
+
+  if (!lhs.isBigInt() || !rhs.isBigInt()) {
+    if (cx->isJSContext()) {
+      JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                JSMSG_BIGINT_TO_NUMBER);
+    }
+    return false;
+  }
+
+  return true;
+}
+
+bool BigInt::add(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::add(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::sub(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::sub(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::mul(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::mul(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::div(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::div(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::mod(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::mod(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::pow(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::pow(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::neg(ExclusiveContext* cx, HandleValue operand, MutableHandleValue res) {
+  MOZ_ASSERT(operand.isBigInt());
+
+  RootedBigInt operandBigInt(cx, operand.toBigInt());
+  BigInt* resBigInt = BigInt::neg(cx, operandBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::lsh(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::lsh(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::rsh(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                 MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::rsh(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::bitAnd(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                    MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::bitAnd(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
 }
 
-JSAtom*
-js::BigIntToAtom(ExclusiveContext* cx, BigInt* bi)
-{
-    JSString* str = BigInt::toString(cx, bi, 10);
-    if (!str)
+bool BigInt::bitXor(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                    MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::bitXor(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::bitOr(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                   MutableHandleValue res) {
+  if (!ValidBigIntOperands(cx, lhs, rhs)) {
+    return false;
+  }
+
+  RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+  RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+  BigInt* resBigInt = BigInt::bitOr(cx, lhsBigInt, rhsBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+bool BigInt::bitNot(ExclusiveContext* cx, HandleValue operand,
+                    MutableHandleValue res) {
+  MOZ_ASSERT(operand.isBigInt());
+
+  RootedBigInt operandBigInt(cx, operand.toBigInt());
+  BigInt* resBigInt = BigInt::bitNot(cx, operandBigInt);
+  if (!resBigInt) {
+    return false;
+  }
+  res.setBigInt(resBigInt);
+  return true;
+}
+
+// BigInt proposal section 7.3
+BigInt* js::ToBigInt(ExclusiveContext* cx, HandleValue val) {
+  RootedValue v(cx, val);
+
+  if(cx->isJSContext()) {
+    // Step 1.
+    if (!ToPrimitive(cx->asJSContext(), JSTYPE_NUMBER, &v)) {
+      return nullptr;
+    }
+
+    // Step 2.
+    if (v.isBigInt()) {
+      return v.toBigInt();
+    }
+
+    if (v.isBoolean()) {
+      return v.toBoolean() ? BigInt::one(cx) : BigInt::zero(cx);
+    }
+
+    if (v.isString()) {
+      BigInt* bi = nullptr;
+      RootedString str(cx, v.toString());
+      JS_TRY_VAR_OR_RETURN_NULL(cx, bi, StringToBigInt(cx, str));
+      if (!bi) {
+        JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr,
+                                  JSMSG_BIGINT_INVALID_SYNTAX);
         return nullptr;
-    return AtomizeString(cx, str);
+      }
+      return bi;
+    }
+
+    JS_ReportErrorNumberASCII(cx->asJSContext(), GetErrorMessage, nullptr, JSMSG_NOT_BIGINT);
+  }
+  return nullptr;
+}
+
+double BigInt::numberValue(BigInt* x) {
+  if (x->isZero()) {
+    return 0.0;
+  }
+
+  using Double = mozilla::FloatingPoint<double>;
+  constexpr uint8_t ExponentShift = Double::kExponentShift;
+  constexpr uint8_t SignificandWidth = Double::kSignificandWidth;
+  constexpr unsigned ExponentBias = Double::kExponentBias;
+  constexpr uint8_t SignShift = Double::kExponentWidth + SignificandWidth;
+
+  size_t length = x->digitLength();
+  MOZ_ASSERT(length != 0);
+
+  // Fast path for the likely-common case of up to a uint64_t of magnitude
+  // that doesn't exceed integral precision in IEEE-754.
+  if (length <= 64 / DigitBits) {
+    uint64_t magnitude = x->digit(0);
+    if (DigitBits == 32 && length > 1) {
+      magnitude |= uint64_t(x->digit(1)) << 32;
+    }
+    const uint64_t MaxIntegralPrecisionDouble = uint64_t(1)
+                                                << (SignificandWidth + 1);
+    if (magnitude <= MaxIntegralPrecisionDouble) {
+      return x->isNegative() ? -double(magnitude) : +double(magnitude);
+    }
+  }
+
+  Digit msd = x->digit(length - 1);
+  uint8_t msdLeadingZeroes = DigitLeadingZeroes(msd);
+
+  // `2**ExponentBias` is the largest power of two in a finite IEEE-754
+  // double.  If this bigint has a greater power of two, it'll round to
+  // infinity.
+  uint64_t exponent = length * DigitBits - msdLeadingZeroes - 1;
+  if (exponent > ExponentBias) {
+    return x->isNegative() ? mozilla::NegativeInfinity<double>()
+                           : mozilla::PositiveInfinity<double>();
+  }
+
+  // Otherwise munge the most significant bits of the number into proper
+  // position in an IEEE-754 double and go to town.
+
+  // Omit the most significant bit: the IEEE-754 format includes this bit
+  // implicitly for all double-precision integers.
+  const uint8_t msdIgnoredBits = msdLeadingZeroes + 1;
+  const uint8_t msdIncludedBits = DigitBits - msdIgnoredBits;
+
+  uint8_t bitsFilled = msdIncludedBits;
+
+  // Shift `msd`'s contributed bits upward to remove high-order zeroes and
+  // the highest set bit (which is implicit in IEEE-754 integral values so
+  // must be removed) and to add low-order zeroes.
+  uint64_t shiftedMantissa =
+      msdIncludedBits == 0 ? 0 : uint64_t(msd) << (64 - msdIncludedBits);
+
+  // Add in bits from the next one or two digits if `msd` didn't contain all
+  // bits necessary to define the result.  (The extra bit allows us to
+  // properly round an inexact overall result.)  Any lower bits that are
+  // uselessly set will be shifted away when `shiftedMantissa` is converted to
+  // a real mantissa.
+  if (bitsFilled < SignificandWidth + 1) {
+    MOZ_ASSERT(length >= 2,
+               "single-Digit numbers with this few bits should have been "
+               "handled by the fast-path above");
+
+    Digit second = x->digit(length - 2);
+    if (DigitBits == 32) {
+      shiftedMantissa |= uint64_t(second) << msdIgnoredBits;
+      bitsFilled += DigitBits;
+
+      // Add in bits from another digit, if any, if we still have unfilled
+      // significand bits.
+      if (bitsFilled < SignificandWidth + 1 && length >= 3) {
+        Digit third = x->digit(length - 3);
+        shiftedMantissa |= uint64_t(third) >> msdIncludedBits;
+        // The second and third 32-bit digits contributed 64 bits total, filling
+        // well beyond the mantissa.
+        bitsFilled = 64;
+      }
+    } else {
+      shiftedMantissa |= second >> msdIncludedBits;
+      // A full 64-bit digit's worth of bits (some from the most significant
+      // digit, the rest from the next) fills well beyond the mantissa.
+      bitsFilled = 64;
+    }
+  }
+
+  // Round the overall result, if necessary.  (It's possible we don't need to
+  // round -- the number might not have enough bits to round.)
+  if (bitsFilled >= SignificandWidth + 1) {
+    constexpr uint64_t LeastSignificantBit = uint64_t(1)
+                                             << (64 - SignificandWidth);
+    constexpr uint64_t ExtraBit = LeastSignificantBit >> 1;
+
+    // When the first bit outside the significand is set, the overall value
+    // is rounded: downward (i.e. no change to the bits) if the least
+    // significant bit in the significand is zero, upward if it instead is
+    // one.
+    if ((shiftedMantissa & ExtraBit) &&
+        (shiftedMantissa & LeastSignificantBit)) {
+      // We're rounding upward: add to the significand bits.  If they
+      // overflow, the exponent must also be increased.  If *that*
+      // overflows, return the appropriate infinity.
+      uint64_t before = shiftedMantissa;
+      shiftedMantissa += ExtraBit;
+      if (shiftedMantissa < before) {
+        exponent++;
+        if (exponent > ExponentBias) {
+          return x->isNegative() ? NegativeInfinity<double>()
+                                 : PositiveInfinity<double>();
+        }
+      }
+    }
+  }
+
+  uint64_t significandBits = shiftedMantissa >> (64 - SignificandWidth);
+  uint64_t signBit = uint64_t(x->isNegative() ? 1 : 0) << SignShift;
+  uint64_t exponentBits = (exponent + ExponentBias) << ExponentShift;
+  return mozilla::BitwiseCast<double>(signBit | exponentBits | significandBits);
 }
 
-bool
-BigInt::toBoolean()
-{
+int8_t BigInt::compare(BigInt* x, BigInt* y) {
+  // Sanity checks to catch negative zeroes escaping to the wild.
+  MOZ_ASSERT(!x->isNegative() || !x->isZero());
+  MOZ_ASSERT(!y->isNegative() || !y->isZero());
+
+  bool xSign = x->isNegative();
+
+  if (xSign != y->isNegative()) {
+    return xSign ? -1 : 1;
+  }
+
+  if (xSign) {
+    mozilla::Swap(x, y);
+  }
+
+  return absoluteCompare(x, y);
+}
+
+bool BigInt::equal(BigInt* lhs, BigInt* rhs) {
+  if (lhs == rhs) {
+    return true;
+  }
+  if (lhs->digitLength() != rhs->digitLength()) {
+    return false;
+  }
+  if (lhs->isNegative() != rhs->isNegative()) {
     return false;
+  }
+  for (size_t i = 0; i < lhs->digitLength(); i++) {
+    if (lhs->digit(i) != rhs->digit(i)) {
+      return false;
+    }
+  }
+  return true;
 }
 
-js::HashNumber
-BigInt::hash()
-{
-    return 0;
+int8_t BigInt::compare(BigInt* x, double y) {
+  MOZ_ASSERT(!mozilla::IsNaN(y));
+
+  constexpr int LessThan = -1, Equal = 0, GreaterThan = 1;
+
+  // ±Infinity exceeds a finite bigint value.
+  if (!mozilla::IsFinite(y)) {
+    return y > 0 ? LessThan : GreaterThan;
+  }
+
+  // Handle `x === 0n` and `y == 0` special cases.
+  if (x->isZero()) {
+    if (y == 0) {
+      // -0 and +0 are treated identically.
+      return Equal;
+    }
+
+    return y > 0 ? LessThan : GreaterThan;
+  }
+
+  const bool xNegative = x->isNegative();
+  if (y == 0) {
+    return xNegative ? LessThan : GreaterThan;
+  }
+
+  // Nonzero `x` and `y` with different signs are trivially compared.
+  const bool yNegative = y < 0;
+  if (xNegative != yNegative) {
+    return xNegative ? LessThan : GreaterThan;
+  }
+
+  // `x` and `y` are same-signed.  Determine which has greater magnitude,
+  // then combine that with the signedness just computed to reach a result.
+  const int exponent = mozilla::ExponentComponent(y);
+  if (exponent < 0) {
+    // `y` is a nonzero fraction of magnitude less than 1.
+    return xNegative ? LessThan : GreaterThan;
+  }
+
+  size_t xLength = x->digitLength();
+  MOZ_ASSERT(xLength > 0);
+
+  Digit xMSD = x->digit(xLength - 1);
+  const int shift = DigitLeadingZeroes(xMSD);
+  int xBitLength = xLength * DigitBits - shift;
+
+  // Differing bit-length makes for a simple comparison.
+  int yBitLength = exponent + 1;
+  if (xBitLength < yBitLength) {
+    return xNegative ? GreaterThan : LessThan;
+  }
+  if (xBitLength > yBitLength) {
+    return xNegative ? LessThan : GreaterThan;
+  }
+
+  // Compare the high 64 bits of both numbers.  (Lower-order bits not present
+  // in either number are zeroed.)  Either that distinguishes `x` and `y`, or
+  // `x` and `y` differ only if a subsequent nonzero bit in `x` means `x` has
+  // larger magnitude.
+
+  using Double = mozilla::FloatingPoint<double>;
+  constexpr uint8_t SignificandWidth = Double::kSignificandWidth;
+  constexpr uint64_t SignificandBits = Double::kSignificandBits;
+
+  const uint64_t doubleBits = mozilla::BitwiseCast<uint64_t>(y);
+  const uint64_t significandBits = doubleBits & SignificandBits;
+
+  // Readd the implicit-one bit when constructing `y`'s high 64 bits.
+  const uint64_t yHigh64Bits =
+      ((uint64_t(1) << SignificandWidth) | significandBits)
+      << (64 - SignificandWidth - 1);
+
+  // Cons up `x`'s high 64 bits, backfilling zeroes for binary fractions of 1
+  // if `x` doesn't have 64 bits.
+  uint8_t xBitsFilled = DigitBits - shift;
+  uint64_t xHigh64Bits = uint64_t(xMSD) << (64 - xBitsFilled);
+
+  // At this point we no longer need to look at the most significant digit.
+  xLength--;
+
+  // The high 64 bits from `x` will probably not align to a digit boundary.
+  // `xHasNonZeroLeftoverBits` will be set to true if any remaining
+  // least-significant bit from the digit holding xHigh64Bits's
+  // least-significant bit is nonzero.
+  bool xHasNonZeroLeftoverBits = false;
+
+  if (xBitsFilled < std::min(xBitLength, 64)) {
+    MOZ_ASSERT(xLength >= 1,
+               "If there are more bits to fill, there should be "
+               "more digits to fill them from");
+
+    Digit second = x->digit(--xLength);
+    if (DigitBits == 32) {
+      xBitsFilled += 32;
+      xHigh64Bits |= uint64_t(second) << (64 - xBitsFilled);
+      if (xBitsFilled < 64 && xLength >= 1) {
+        Digit third = x->digit(--xLength);
+        const uint8_t neededBits = 64 - xBitsFilled;
+        xHigh64Bits |= uint64_t(third) >> (DigitBits - neededBits);
+        xHasNonZeroLeftoverBits = (third << neededBits) != 0;
+      }
+    } else {
+      const uint8_t neededBits = 64 - xBitsFilled;
+      xHigh64Bits |= uint64_t(second) >> (DigitBits - neededBits);
+      xHasNonZeroLeftoverBits = (second << neededBits) != 0;
+    }
+  }
+
+  // If high bits are unequal, the larger one has greater magnitude.
+  if (yHigh64Bits > xHigh64Bits) {
+    return xNegative ? GreaterThan : LessThan;
+  }
+  if (xHigh64Bits > yHigh64Bits) {
+    return xNegative ? LessThan : GreaterThan;
+  }
+
+  // Otherwise the top 64 bits of both are equal.  If the values differ, a
+  // lower-order bit in `x` is nonzero and `x` has greater magnitude than
+  // `y`; otherwise `x == y`.
+  if (xHasNonZeroLeftoverBits) {
+    return xNegative ? LessThan : GreaterThan;
+  }
+  while (xLength != 0) {
+    if (x->digit(--xLength) != 0) {
+      return xNegative ? LessThan : GreaterThan;
+    }
+  }
+
+  return Equal;
 }
 
-size_t
-BigInt::sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const
-{
-    return 0;
+bool BigInt::equal(BigInt* lhs, double rhs) {
+  if (mozilla::IsNaN(rhs)) {
+    return false;
+  }
+  return compare(lhs, rhs) == 0;
+}
+
+// BigInt proposal section 3.2.5
+JS::Result<bool> BigInt::looselyEqual(ExclusiveContext* cx, HandleBigInt lhs,
+                                      HandleValue rhs) {
+  // Step 1.
+  if (rhs.isBigInt()) {
+    return equal(lhs, rhs.toBigInt());
+  }
+
+  // Steps 2-5 (not applicable).
+
+  // Steps 6-7.
+  if (rhs.isString()) {
+    RootedBigInt rhsBigInt(cx);
+    RootedString rhsString(cx, rhs.toString());
+    MOZ_TRY_VAR(rhsBigInt, StringToBigInt(cx, rhsString));
+    if (!rhsBigInt) {
+      return false;
+    }
+    return equal(lhs, rhsBigInt);
+  }
+
+  // Steps 8-9 (not applicable).
+
+  // Steps 10-11.
+  if (rhs.isObject()) {
+    RootedValue rhsPrimitive(cx, rhs);
+    if (!cx->isJSContext() || !ToPrimitive(cx->asJSContext(), &rhsPrimitive)) {
+      return cx->alreadyReportedError();
+    }
+    return looselyEqual(cx, lhs, rhsPrimitive);
+  }
+
+  // Step 12.
+  if (rhs.isNumber()) {
+    return equal(lhs, rhs.toNumber());
+  }
+
+  // Step 13.
+  return false;
+}
+
+// BigInt proposal section 1.1.12. BigInt::lessThan ( x, y )
+bool BigInt::lessThan(BigInt* x, BigInt* y) { return compare(x, y) < 0; }
+
+Maybe<bool> BigInt::lessThan(BigInt* lhs, double rhs) {
+  if (mozilla::IsNaN(rhs)) {
+    return Maybe<bool>(Nothing());
+  }
+  return Some(compare(lhs, rhs) < 0);
+}
+
+Maybe<bool> BigInt::lessThan(double lhs, BigInt* rhs) {
+  if (mozilla::IsNaN(lhs)) {
+    return Maybe<bool>(Nothing());
+  }
+  return Some(-compare(rhs, lhs) < 0);
+}
+
+bool BigInt::lessThan(ExclusiveContext* cx, HandleBigInt lhs, HandleString rhs,
+                      Maybe<bool>& res) {
+  RootedBigInt rhsBigInt(cx);
+  JS_TRY_VAR_OR_RETURN_FALSE(cx, rhsBigInt, StringToBigInt(cx, rhs));
+  if (!rhsBigInt) {
+    res = Nothing();
+    return true;
+  }
+  res = Some(lessThan(lhs, rhsBigInt));
+  return true;
+}
+
+bool BigInt::lessThan(ExclusiveContext* cx, HandleString lhs, HandleBigInt rhs,
+                      Maybe<bool>& res) {
+  RootedBigInt lhsBigInt(cx);
+  JS_TRY_VAR_OR_RETURN_FALSE(cx, lhsBigInt, StringToBigInt(cx, lhs));
+  if (!lhsBigInt) {
+    res = Nothing();
+    return true;
+  }
+  res = Some(lessThan(lhsBigInt, rhs));
+  return true;
+}
+
+bool BigInt::lessThan(ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                      Maybe<bool>& res) {
+  if (lhs.isBigInt()) {
+    if (rhs.isString()) {
+      RootedBigInt lhsBigInt(cx, lhs.toBigInt());
+      RootedString rhsString(cx, rhs.toString());
+      return lessThan(cx, lhsBigInt, rhsString, res);
+    }
+
+    if (rhs.isNumber()) {
+      res = lessThan(lhs.toBigInt(), rhs.toNumber());
+      return true;
+    }
+
+    MOZ_ASSERT(rhs.isBigInt());
+    res = Some(lessThan(lhs.toBigInt(), rhs.toBigInt()));
+    return true;
+  }
+
+  MOZ_ASSERT(rhs.isBigInt());
+  if (lhs.isString()) {
+    RootedString lhsString(cx, lhs.toString());
+    RootedBigInt rhsBigInt(cx, rhs.toBigInt());
+    return lessThan(cx, lhsString, rhsBigInt, res);
+  }
+
+  MOZ_ASSERT(lhs.isNumber());
+  res = lessThan(lhs.toNumber(), rhs.toBigInt());
+  return true;
+}
+
+JSLinearString* BigInt::toString(ExclusiveContext* cx, HandleBigInt x, uint8_t radix) {
+  MOZ_ASSERT(2 <= radix && radix <= 36);
+
+  if (x->isZero()) {
+    return cx->staticStrings().getInt(0);
+  }
+
+  if (mozilla::IsPowerOfTwo(radix)) {
+    return toStringBasePowerOfTwo(cx, x, radix);
+  }
+
+  return toStringGeneric(cx, x, radix);
+}
+
+template <typename CharT>
+static inline BigInt* ParseStringBigIntLiteral(ExclusiveContext* cx,
+                                               Range<const CharT> range,
+                                               bool* haveParseError) {
+  auto start = range.begin();
+  auto end = range.end();
+
+  while (start < end && unicode::IsSpace(start[0])) {
+    start++;
+  }
+
+  while (start < end && unicode::IsSpace(end[-1])) {
+    end--;
+  }
+
+  if (start == end) {
+    return BigInt::zero(cx);
+  }
+
+  // StringNumericLiteral ::: StrDecimalLiteral, but without Infinity, decimal
+  // points, or exponents.  Note that the raw '+' or '-' cases fall through
+  // because the string is too short, and eventually signal a parse error.
+  if (end - start > 1) {
+    if (start[0] == '+') {
+      bool isNegative = false;
+      start++;
+      return BigInt::parseLiteralDigits(cx, Range<const CharT>(start, end), 10,
+                                        isNegative, haveParseError);
+    } else if (start[0] == '-') {
+      bool isNegative = true;
+      start++;
+      return BigInt::parseLiteralDigits(cx, Range<const CharT>(start, end), 10,
+                                        isNegative, haveParseError);
+    }
+  }
+
+  return BigInt::parseLiteral(cx, Range<const CharT>(start, end),
+                              haveParseError);
+}
+
+// Called from BigInt constructor.
+JS::Result<BigInt*, JS::OOM&> js::StringToBigInt(ExclusiveContext* cx,
+                                                 HandleString str) {
+  JSLinearString* linear = str->ensureLinear(cx);
+  if (!linear) {
+    return cx->alreadyReportedOOM();
+  }
+
+  BigInt* res = nullptr;
+  bool parseError = false;
+  
+  if(cx->isJSContext()) {
+    AutoStableStringChars chars(cx->asJSContext());
+    if (!chars.init(cx->asJSContext(), str)) {
+      return cx->alreadyReportedOOM();
+    }
+
+    if (chars.isLatin1()) {
+      res = ParseStringBigIntLiteral(cx->asJSContext(), chars.latin1Range(), &parseError);
+    } else {
+      res = ParseStringBigIntLiteral(cx->asJSContext(), chars.twoByteRange(), &parseError);
+    }
+  }
+
+  // A nullptr result can indicate either a parse error or out-of-memory.
+  if (!res && !parseError) {
+    return cx->alreadyReportedOOM();
+  }
+
+  return res;
+}
+
+// Called from parser with already trimmed and validated token.
+BigInt* js::ParseBigIntLiteral(ExclusiveContext* cx,
+                               const Range<const char16_t>& chars) {
+  bool parseError = false;
+  BigInt* res = BigInt::parseLiteral(cx, chars, &parseError);
+  if (!res) {
+    return nullptr;
+  }
+  MOZ_RELEASE_ASSERT(!parseError);
+  return res;
+}
+
+JSAtom* js::BigIntToAtom(ExclusiveContext* cx, HandleBigInt bi) {
+  JSString* str = BigInt::toString(cx, bi, 10);
+  if (!str) {
+    return nullptr;
+  }
+  return AtomizeString(cx, str);
+}
+
+JS::ubi::Node::Size JS::ubi::Concrete<BigInt>::size(
+    mozilla::MallocSizeOf mallocSizeOf) const {
+  BigInt& bi = get();
+  MOZ_ASSERT(bi.isTenured());
+  size_t size = js::gc::Arena::thingSize(bi.asTenured().getAllocKind());
+  size += bi.sizeOfExcludingThis(mallocSizeOf);
+  return size;
 }
 
-JS::ubi::Node::Size
-JS::ubi::Concrete<BigInt>::size(mozilla::MallocSizeOf mallocSizeOf) const
-{
-    MOZ_ASSERT(get().isTenured());
-    return js::gc::Arena::thingSize(get().asTenured().getAllocKind());
+#if 0 // Future XDR support
+template <XDRMode mode>
+XDRResult js::XDRBigInt(XDRState<mode>* xdr, MutableHandleBigInt bi) {
+  JSContext* cx = xdr->cx();
+
+  uint8_t sign;
+  uint32_t length;
+
+  if (mode == XDR_ENCODE) {
+    cx->check(bi);
+    sign = static_cast<uint8_t>(bi->isNegative());
+    uint64_t sz = bi->digitLength() * sizeof(BigInt::Digit);
+    // As the maximum source code size is currently UINT32_MAX code units
+    // (see BytecodeCompiler::checkLength), any bigint literal's length in
+    // word-sized digits will be less than UINT32_MAX as well.  That could
+    // change or FoldConstants could start creating these though, so leave
+    // this as a release-enabled assert.
+    MOZ_RELEASE_ASSERT(sz <= UINT32_MAX);
+    length = static_cast<uint32_t>(sz);
+  }
+
+  MOZ_TRY(xdr->codeUint8(&sign));
+  MOZ_TRY(xdr->codeUint32(&length));
+
+  MOZ_RELEASE_ASSERT(length % sizeof(BigInt::Digit) == 0);
+  uint32_t digitLength = length / sizeof(BigInt::Digit);
+  auto buf = cx->make_pod_array<BigInt::Digit>(digitLength);
+  if (!buf) {
+    return xdr->fail(JS::TranscodeResult_Throw);
+  }
+
+  if (mode == XDR_ENCODE) {
+    std::uninitialized_copy_n(bi->digits().Elements(), digitLength, buf.get());
+  }
+
+  MOZ_TRY(xdr->codeBytes(buf.get(), length));
+
+  if (mode == XDR_DECODE) {
+    BigInt* res = BigInt::createUninitialized(cx, digitLength, sign);
+    if (!res) {
+      return xdr->fail(JS::TranscodeResult_Throw);
+    }
+    std::uninitialized_copy_n(buf.get(), digitLength, bi->digits().Elements());
+    bi.set(res);
+  }
+
+  return Ok();
 }
+
+template XDRResult js::XDRBigInt(XDRState<XDR_ENCODE>* xdr,
+                                 MutableHandleBigInt bi);
+
+template XDRResult js::XDRBigInt(XDRState<XDR_DECODE>* xdr,
+                                 MutableHandleBigInt bi);
+#endif
diff --git a/js/src/vm/BigIntType.h b/js/src/vm/BigIntType.h
index 8d934271a3..f2a02e9b82 100644
--- a/js/src/vm/BigIntType.h
+++ b/js/src/vm/BigIntType.h
@@ -1,4 +1,4 @@
-/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
@@ -6,62 +6,374 @@
 #ifndef vm_BigIntType_h
 #define vm_BigIntType_h
 
+#include "mozilla/Range.h"
+#include "mozilla/Span.h"
+
 #include "gc/Barrier.h"
 #include "gc/Marking.h"
 #include "gc/Heap.h"
 #include "js/GCHashTable.h"
 #include "js/RootingAPI.h"
+#include "js/TraceKind.h"
 #include "js/TypeDecls.h"
 #include "vm/String.h"
+#include "vm/Xdr.h"
+
+// Handle future js::gc::Cell::ReservedBits, we have no reserved bits...
+#define js_gc_Cell_ReservedBits 0
+// Handle future js::gc::MinCellSize, 16 bytes, twice our js:gc:CellSize
+#define js_gc_MinCellSize (js::gc::CellSize*2)
+
+namespace JS {
+
+#if 0 // Future XDR support
+class BigInt;
+
+}  // namespace JS
+
+namespace js {
+
+template <XDRMode mode>
+XDRResult XDRBigInt(XDRState<mode>* xdr, MutableHandle<JS::BigInt*> bi);
+
+}  // namespace js
 
 namespace JS {
+#endif
+
+class BigInt final : public js::gc::TenuredCell {
+ public:
+  using Digit = uintptr_t;
+
+ private:
+  // The low js::gc::Cell::ReservedBits are reserved.
+  static constexpr uintptr_t SignBit = JS_BIT(js_gc_Cell_ReservedBits);
+  static constexpr uintptr_t LengthShift = js_gc_Cell_ReservedBits + 1;
+  static constexpr size_t InlineDigitsLength =
+      (js_gc_MinCellSize - sizeof(uintptr_t)) / sizeof(Digit);
+
+  uintptr_t lengthSignAndReservedBits_;
+
+  // The digit storage starts with the least significant digit (little-endian
+  // digit order).  Byte order within a digit is of course native endian.
+  union {
+    Digit* heapDigits_;
+    Digit inlineDigits_[InlineDigitsLength];
+  };
+
+ public:
+  static const JS::TraceKind TraceKind = JS::TraceKind::BigInt;
+
+  size_t digitLength() const {
+    return lengthSignAndReservedBits_ >> LengthShift;
+  }
+
+  bool hasInlineDigits() const { return digitLength() <= InlineDigitsLength; }
+  bool hasHeapDigits() const { return !hasInlineDigits(); }
+
+  using Digits = mozilla::Span<Digit>;
+  Digits digits() {
+    return Digits(hasInlineDigits() ? inlineDigits_ : heapDigits_,
+                  digitLength());
+  }
+  Digit digit(size_t idx) { return digits()[idx]; }
+  void setDigit(size_t idx, Digit digit) { digits()[idx] = digit; }
+
+  bool isZero() const { return digitLength() == 0; }
+  bool isNegative() const { return lengthSignAndReservedBits_ & SignBit; }
+
+  void initializeDigitsToZero();
+
+  void traceChildren(JSTracer* trc);
+  void finalize(js::FreeOp* fop);
+  js::HashNumber hash();
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+
+  static BigInt* createUninitialized(js::ExclusiveContext* cx, size_t length,
+                                     bool isNegative);
+  static BigInt* createFromDouble(js::ExclusiveContext* cx, double d);
+  static BigInt* createFromUint64(js::ExclusiveContext* cx, uint64_t n);
+  static BigInt* createFromInt64(js::ExclusiveContext* cx, int64_t n);
+  // FIXME: Cache these values.
+  static BigInt* zero(js::ExclusiveContext* cx);
+  static BigInt* one(js::ExclusiveContext* cx);
+
+  static BigInt* copy(js::ExclusiveContext* cx, Handle<BigInt*> x);
+  static BigInt* add(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* sub(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* mul(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* div(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* mod(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* pow(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* neg(js::ExclusiveContext* cx, Handle<BigInt*> x);
+  static BigInt* lsh(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* rsh(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* bitAnd(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* bitXor(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* bitOr(js::ExclusiveContext* cx, Handle<BigInt*> x, Handle<BigInt*> y);
+  static BigInt* bitNot(js::ExclusiveContext* cx, Handle<BigInt*> x);
+
+  static int64_t toInt64(BigInt* x);
+  static uint64_t toUint64(BigInt* x);
+
+  static BigInt* asIntN(js::ExclusiveContext* cx, Handle<BigInt*> x, uint64_t bits);
+  static BigInt* asUintN(js::ExclusiveContext* cx, Handle<BigInt*> x, uint64_t bits);
+
+  // Type-checking versions of arithmetic operations. These methods
+  // must be called with at least one BigInt operand. Binary
+  // operations will throw a TypeError if one of the operands is not a
+  // BigInt value.
+  static bool add(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool sub(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool mul(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool div(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool mod(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool pow(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool neg(js::ExclusiveContext* cx, Handle<Value> operand,
+                  MutableHandle<Value> res);
+  static bool lsh(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool rsh(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                  MutableHandle<Value> res);
+  static bool bitAnd(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                     MutableHandle<Value> res);
+  static bool bitXor(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                     MutableHandle<Value> res);
+  static bool bitOr(js::ExclusiveContext* cx, Handle<Value> lhs, Handle<Value> rhs,
+                    MutableHandle<Value> res);
+  static bool bitNot(js::ExclusiveContext* cx, Handle<Value> operand,
+                     MutableHandle<Value> res);
+
+  static double numberValue(BigInt* x);
 
-class BigInt final : public js::gc::TenuredCell
-{
-  private:
-    // The minimum allocation size is currently 16 bytes (see
-    // SortedArenaList in gc/ArenaList.h).
-    uint8_t unused_[16 + (16 % js::gc::CellSize)];
+  static JSLinearString* toString(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                  uint8_t radix);
+  template <typename CharT>
+  static BigInt* parseLiteral(js::ExclusiveContext* cx,
+                              const mozilla::Range<const CharT> chars,
+                              bool* haveParseError);
+  template <typename CharT>
+  static BigInt* parseLiteralDigits(js::ExclusiveContext* cx,
+                                    const mozilla::Range<const CharT> chars,
+                                    unsigned radix, bool isNegative,
+                                    bool* haveParseError);
 
-  public:
-    // Allocate and initialize a BigInt value
-    static BigInt* create(js::ExclusiveContext* cx);
+  static int8_t compare(BigInt* lhs, BigInt* rhs);
+  static bool equal(BigInt* lhs, BigInt* rhs);
+  static JS::Result<bool> looselyEqual(js::ExclusiveContext* cx, Handle<BigInt*> lhs,
+                                       HandleValue rhs);
 
-    static BigInt* create(js::ExclusiveContext* cx, double d);
+  static bool lessThan(BigInt* x, BigInt* y);
+  // These methods return Nothing when the non-BigInt operand is NaN
+  // or a string that can't be interpreted as a BigInt.
+  static mozilla::Maybe<bool> lessThan(BigInt* lhs, double rhs);
+  static mozilla::Maybe<bool> lessThan(double lhs, BigInt* rhs);
+  static bool lessThan(js::ExclusiveContext* cx, Handle<BigInt*> lhs, HandleString rhs,
+                       mozilla::Maybe<bool>& res);
+  static bool lessThan(js::ExclusiveContext* cx, HandleString lhs, Handle<BigInt*> rhs,
+                       mozilla::Maybe<bool>& res);
+  static bool lessThan(js::ExclusiveContext* cx, HandleValue lhs, HandleValue rhs,
+                       mozilla::Maybe<bool>& res);
 
-    static const JS::TraceKind TraceKind = JS::TraceKind::BigInt;
+ private:
+  static constexpr size_t DigitBits = sizeof(Digit) * CHAR_BIT;
+  static constexpr size_t HalfDigitBits = DigitBits / 2;
+  static constexpr Digit HalfDigitMask = (1ull << HalfDigitBits) - 1;
 
-    void traceChildren(JSTracer* trc);
+  static_assert(DigitBits == 32 || DigitBits == 64,
+                "Unexpected BigInt Digit size");
 
-    void finalize(js::FreeOp* fop);
+  // The maximum number of digits that the current implementation supports
+  // would be 0x7fffffff / DigitBits. However, we use a lower limit for now,
+  // because raising it later is easier than lowering it.  Support up to 1
+  // million bits.
+  static constexpr size_t MaxBitLength = 1024 * 1024;
+  static constexpr size_t MaxDigitLength = MaxBitLength / DigitBits;
 
-    js::HashNumber hash();
+  // BigInts can be serialized to strings of radix between 2 and 36.  For a
+  // given bigint, radix 2 will take the most characters (one per bit).
+  // Ensure that the max bigint size is small enough so that we can fit the
+  // corresponding character count into a size_t, with space for a possible
+  // sign prefix.
+  static_assert(MaxBitLength <= std::numeric_limits<size_t>::max() - 1,
+                "BigInt max length must be small enough to be serialized as a "
+                "binary string");
 
-    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+  static size_t calculateMaximumCharactersRequired(HandleBigInt x,
+                                                   unsigned radix);
+  static MOZ_MUST_USE bool calculateMaximumDigitsRequired(js::ExclusiveContext* cx,
+                                                          uint8_t radix,
+                                                          size_t charCount,
+                                                          size_t* result);
 
-    bool toBoolean();
+  static bool absoluteDivWithDigitDivisor(
+      js::ExclusiveContext* cx, Handle<BigInt*> x, Digit divisor,
+      const mozilla::Maybe<MutableHandle<BigInt*>>& quotient, Digit* remainder,
+      bool quotientNegative);
+  static void internalMultiplyAdd(BigInt* source, Digit factor, Digit summand,
+                                  unsigned, BigInt* result);
+  static void multiplyAccumulate(BigInt* multiplicand, Digit multiplier,
+                                 BigInt* accumulator,
+                                 unsigned accumulatorIndex);
+  static bool absoluteDivWithBigIntDivisor(
+      js::ExclusiveContext* cx, Handle<BigInt*> dividend, Handle<BigInt*> divisor,
+      const mozilla::Maybe<MutableHandle<BigInt*>>& quotient,
+      const mozilla::Maybe<MutableHandle<BigInt*>>& remainder,
+      bool quotientNegative);
 
-    static BigInt* copy(js::ExclusiveContext* cx, Handle<BigInt*> x);
+  enum class LeftShiftMode { SameSizeResult, AlwaysAddOneDigit };
 
-    static JSLinearString* toString(js::ExclusiveContext* cx, BigInt* x, uint8_t radix);
+  static BigInt* absoluteLeftShiftAlwaysCopy(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                             unsigned shift, LeftShiftMode);
+  static bool productGreaterThan(Digit factor1, Digit factor2, Digit high,
+                                 Digit low);
+  static BigInt* lshByAbsolute(js::ExclusiveContext* cx, HandleBigInt x, HandleBigInt y);
+  static BigInt* rshByAbsolute(js::ExclusiveContext* cx, HandleBigInt x, HandleBigInt y);
+  static BigInt* rshByMaximum(js::ExclusiveContext* cx, bool isNegative);
+  static BigInt* truncateAndSubFromPowerOfTwo(js::ExclusiveContext* cx, HandleBigInt x,
+                                              uint64_t bits,
+                                              bool resultNegative);
 
+  Digit absoluteInplaceAdd(BigInt* summand, unsigned startIndex);
+  Digit absoluteInplaceSub(BigInt* subtrahend, unsigned startIndex);
+  void inplaceRightShiftLowZeroBits(unsigned shift);
+  void inplaceMultiplyAdd(Digit multiplier, Digit part);
+
+  // The result of an SymmetricTrim bitwise op has as many digits as the
+  // smaller operand.  A SymmetricFill bitwise op result has as many digits as
+  // the larger operand, with high digits (if any) copied from the larger
+  // operand.  AsymmetricFill is like SymmetricFill, except the result has as
+  // many digits as the first operand; this kind is used for the and-not
+  // operation.
+  enum class BitwiseOpKind { SymmetricTrim, SymmetricFill, AsymmetricFill };
+
+  template <BitwiseOpKind kind, typename BitwiseOp>
+  static BigInt* absoluteBitwiseOp(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                   Handle<BigInt*> y, BitwiseOp&& op);
+
+  // Return `|x| & |y|`.
+  static BigInt* absoluteAnd(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                             Handle<BigInt*> y);
+
+  // Return `|x| | |y|`.
+  static BigInt* absoluteOr(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                            Handle<BigInt*> y);
+
+  // Return `|x| & ~|y|`.
+  static BigInt* absoluteAndNot(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                Handle<BigInt*> y);
+
+  // Return `|x| ^ |y|`.
+  static BigInt* absoluteXor(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                             Handle<BigInt*> y);
+
+  // Return `(|x| + 1) * (resultNegative ? -1 : +1)`.
+  static BigInt* absoluteAddOne(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                bool resultNegative);
+
+  // Return `(|x| - 1) * (resultNegative ? -1 : +1)`, with the precondition that
+  // |x| != 0.
+  static BigInt* absoluteSubOne(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                                unsigned resultLength);
+
+  // Return `a + b`, incrementing `*carry` if the addition overflows.
+  static inline Digit digitAdd(Digit a, Digit b, Digit* carry) {
+    Digit result = a + b;
+    *carry += static_cast<Digit>(result < a);
+    return result;
+  }
+
+  // Return `left - right`, incrementing `*borrow` if the addition overflows.
+  static inline Digit digitSub(Digit left, Digit right, Digit* borrow) {
+    Digit result = left - right;
+    *borrow += static_cast<Digit>(result > left);
+    return result;
+  }
+
+  // Compute `a * b`, returning the low half of the result and putting the
+  // high half in `*high`.
+  static Digit digitMul(Digit a, Digit b, Digit* high);
+
+  // Divide `(high << DigitBits) + low` by `divisor`, returning the quotient
+  // and storing the remainder in `*remainder`, with the precondition that
+  // `high < divisor` so that the result fits in a Digit.
+  static Digit digitDiv(Digit high, Digit low, Digit divisor, Digit* remainder);
+
+  // Return `(|x| + |y|) * (resultNegative ? -1 : +1)`.
+  static BigInt* absoluteAdd(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                             Handle<BigInt*> y, bool resultNegative);
+
+  // Return `(|x| - |y|) * (resultNegative ? -1 : +1)`, with the precondition
+  // that |x| >= |y|.
+  static BigInt* absoluteSub(js::ExclusiveContext* cx, Handle<BigInt*> x,
+                             Handle<BigInt*> y, bool resultNegative);
+
+  // If `|x| < |y|` return -1; if `|x| == |y|` return 0; otherwise return 1.
+  static int8_t absoluteCompare(BigInt* lhs, BigInt* rhs);
+
+  static int8_t compare(BigInt* lhs, double rhs);
+
+  static bool equal(BigInt* lhs, double rhs);
+
+  static JSLinearString* toStringBasePowerOfTwo(js::ExclusiveContext* cx, Handle<BigInt*>,
+                                                unsigned radix);
+  static JSLinearString* toStringGeneric(js::ExclusiveContext* cx, Handle<BigInt*>,
+                                         unsigned radix);
+
+  static BigInt* trimHighZeroDigits(js::ExclusiveContext* cx, Handle<BigInt*> x);
+  static BigInt* destructivelyTrimHighZeroDigits(js::ExclusiveContext* cx,
+                                                 Handle<BigInt*> x);
+
+  friend struct JSStructuredCloneReader;
+  friend struct JSStructuredCloneWriter;
+#if 0 // Future XDR support
+  template <js::XDRMode mode>
+  friend js::XDRResult js::XDRBigInt(js::XDRState<mode>* xdr,
+                                     MutableHandle<BigInt*> bi);
+#endif
+
+  BigInt() = delete;
+  BigInt(const BigInt& other) = delete;
+  void operator=(const BigInt& other) = delete;
 };
 
-static_assert(sizeof(BigInt) >= js::gc::CellSize,
-              "sizeof(BigInt) must be greater than the minimum allocation size");
+static_assert(
+    sizeof(BigInt) >= js_gc_MinCellSize,
+    "sizeof(BigInt) must be greater than the minimum allocation size");
+
+static_assert(
+    sizeof(BigInt) == js_gc_MinCellSize,
+    "sizeof(BigInt) intended to be the same as the minimum allocation size");
 
-} // namespace JS
+}  // namespace JS
 
 namespace js {
 
-extern JSAtom*
-BigIntToAtom(ExclusiveContext* cx, JS::BigInt* bi);
+extern JSAtom* BigIntToAtom(js::ExclusiveContext* cx, JS::HandleBigInt bi);
+
+extern JS::BigInt* NumberToBigInt(js::ExclusiveContext* cx, double d);
+
+// Parse a BigInt from a string, using the method specified for StringToBigInt.
+// Used by the BigInt constructor among other places.
+extern JS::Result<JS::BigInt*, JS::OOM&> StringToBigInt(
+    js::ExclusiveContext* cx, JS::Handle<JSString*> str);
+
+// Parse a BigInt from an already-validated numeric literal.  Used by the
+// parser.  Can only fail in out-of-memory situations.
+extern JS::BigInt* ParseBigIntLiteral(
+    js::ExclusiveContext* cx, const mozilla::Range<const char16_t>& chars);
+
+extern JS::BigInt* ToBigInt(js::ExclusiveContext* cx, JS::Handle<JS::Value> v);
 
-extern JS::BigInt*
-NumberToBigInt(ExclusiveContext* cx, double d);
+}  // namespace js
 
-extern JS::BigInt*
-ToBigInt(ExclusiveContext* cx, JS::Handle<JS::Value> v);
-} // namespace js
+#undef js_gc_MinCellSize
+#undef js_gc_Cell_ReservedBits
 
 #endif
diff --git a/js/src/vm/StringBuffer.cpp b/js/src/vm/StringBuffer.cpp
index 58a3c3e164..ce0fc4e719 100644
--- a/js/src/vm/StringBuffer.cpp
+++ b/js/src/vm/StringBuffer.cpp
@@ -171,7 +171,8 @@ js::ValueToStringBufferSlow(JSContext* cx, const Value& arg, StringBuffer& sb)
         return false;
     }
     if (v.isBigInt()) {
-        JSLinearString* str = BigInt::toString(cx, v.toBigInt(), 10);
+        RootedBigInt i(cx, v.toBigInt());
+        JSLinearString* str = BigInt::toString(cx, i, 10);
         if (!str)
             return false;
         return sb.append(str);
diff --git a/js/src/vm/StructuredClone.cpp b/js/src/vm/StructuredClone.cpp
index 26e57976fc..ac73df30e6 100644
--- a/js/src/vm/StructuredClone.cpp
+++ b/js/src/vm/StructuredClone.cpp
@@ -31,6 +31,7 @@
 #include "mozilla/CheckedInt.h"
 #include "mozilla/EndianUtils.h"
 #include "mozilla/FloatingPoint.h"
+#include "mozilla/RangedPtr.h"
 
 #include <algorithm>
 
@@ -39,6 +40,8 @@
 #include "jsdate.h"
 #include "jswrapper.h"
 
+#include "vm/BigIntType.h"
+
 #include "builtin/MapObject.h"
 #include "js/Date.h"
 #include "js/GCHashTable.h"
@@ -57,6 +60,7 @@ using mozilla::IsNaN;
 using mozilla::LittleEndian;
 using mozilla::NativeEndian;
 using mozilla::NumbersAreIdentical;
+using mozilla::RangedPtr;
 using JS::CanonicalizeNaN;
 
 // When you make updates here, make sure you consider whether you need to bump the
@@ -104,6 +108,9 @@ enum StructuredDataType : uint32_t {
 
     SCTAG_SHARED_ARRAY_BUFFER_OBJECT,
 
+    SCTAG_BIGINT,
+    SCTAG_BIGINT_OBJECT,
+
     SCTAG_TYPED_ARRAY_V1_MIN = 0xFFFF0100,
     SCTAG_TYPED_ARRAY_V1_INT8 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Int8,
     SCTAG_TYPED_ARRAY_V1_UINT8 = SCTAG_TYPED_ARRAY_V1_MIN + Scalar::Uint8,
@@ -349,6 +356,8 @@ struct JSStructuredCloneReader {
     JSString* readStringImpl(uint32_t nchars);
     JSString* readString(uint32_t data);
 
+    BigInt* readBigInt(uint32_t data);
+
     bool checkDouble(double d);
     bool readTypedArray(uint32_t arrayType, uint32_t nelems, MutableHandleValue vp,
                         bool v1Read = false);
@@ -439,6 +448,8 @@ struct JSStructuredCloneWriter {
     bool traverseSet(HandleObject obj);
     bool traverseSavedFrame(HandleObject obj);
 
+    bool writeBigInt(uint32_t tag, BigInt* bi);
+
     bool reportDataCloneError(uint32_t errorId);
 
     bool parseTransferable();
@@ -1055,6 +1066,23 @@ JSStructuredCloneWriter::writeString(uint32_t tag, JSString* str)
            : out.writeChars(linear->twoByteChars(nogc), length);
 }
 
+bool
+JSStructuredCloneWriter::writeBigInt(uint32_t tag, BigInt* bi)
+{
+    bool signBit = bi->isNegative();
+    size_t length = bi->digitLength();
+    // The length must fit in 31 bits to leave room for a sign bit.
+    if (length > size_t(INT32_MAX)) {
+        return false;
+    }
+    uint32_t lengthAndSign = length | (static_cast<uint32_t>(signBit) << 31);
+
+    if (!out.writePair(tag, lengthAndSign)) {
+         return false;
+    }
+    return out.writeArray(bi->digits().data(), length);
+}
+
 inline void
 JSStructuredCloneWriter::checkStack()
 {
@@ -1388,6 +1416,8 @@ JSStructuredCloneWriter::startWrite(HandleValue v)
         return out.writePair(SCTAG_NULL, 0);
     } else if (v.isUndefined()) {
         return out.writePair(SCTAG_UNDEFINED, 0);
+    } else if (v.isBigInt()) {
+        return writeBigInt(SCTAG_BIGINT, v.toBigInt());
     } else if (v.isObject()) {
         RootedObject obj(context(), &v.toObject());
 
@@ -1441,6 +1471,12 @@ JSStructuredCloneWriter::startWrite(HandleValue v)
             return writeString(SCTAG_STRING_OBJECT, unboxed.toString());
         } else if (cls == ESClass::Map) {
             return traverseMap(obj);
+        } else if (cls == ESClass::BigInt) {
+            RootedValue unboxed(context());
+            if (!Unbox(context(), obj, &unboxed)) {
+                return false;
+            }
+            return writeBigInt(SCTAG_BIGINT_OBJECT, unboxed.toBigInt());
         } else if (cls == ESClass::Set) {
             return traverseSet(obj);
         } else if (SavedFrame::isSavedFrameOrWrapperAndNotProto(*obj)) {
@@ -1745,6 +1781,22 @@ JSStructuredCloneReader::readString(uint32_t data)
     return latin1 ? readStringImpl<Latin1Char>(nchars) : readStringImpl<char16_t>(nchars);
 }
 
+BigInt* JSStructuredCloneReader::readBigInt(uint32_t data) {
+    size_t length = data & JS_BITMASK(31);
+    bool isNegative = data & (1 << 31);
+    if (length == 0) {
+        return BigInt::zero(context());
+    }
+    BigInt* result = BigInt::createUninitialized(context(), length, isNegative);
+    if (!result) {
+        return nullptr;
+    }
+    if (!in.readArray(result->digits().data(), length)) {
+        return nullptr;
+    }
+    return result;
+}
+
 static uint32_t
 TagToV1ArrayType(uint32_t tag)
 {
@@ -2021,6 +2073,19 @@ JSStructuredCloneReader::startRead(MutableHandleValue vp)
         break;
       }
 
+      case SCTAG_BIGINT:
+      case SCTAG_BIGINT_OBJECT: {
+        RootedBigInt bi(context(), readBigInt(data));
+        if (!bi) {
+            return false;
+        }
+        vp.setBigInt(bi);
+        if (tag == SCTAG_BIGINT_OBJECT && !PrimitiveToObject(context(), vp)) {
+            return false;
+        }
+        break;
+      }
+
       case SCTAG_DATE_OBJECT: {
         double d;
         if (!in.readDouble(&d) || !checkDouble(d))