Reland "[bigint] Karatsuba multiplication"

This is a reland of 59eff3bfaa

Original change's description:
> [bigint] Karatsuba multiplication
>
> The Karatsuba algorithm is used for BigInts with 34 or more internal
> digits, and thanks to better asymptotic complexity provides greater
> speedups the bigger the inputs.
>
> Bug: v8:11515
> Change-Id: I5ab0e318173ea4a02ced3f156d3c17e0259c5036
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2782283
> Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74916}

Bug: v8:11515
Change-Id: I5ece2ff29ef11ea304980c053887d9746cfc80bc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2933497
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74922}

BUILD.gn

@@ -4898,7 +4898,9 @@ v8_source_set("v8_bigint") {
     "src/bigint/bigint-internal.h",
     "src/bigint/bigint.h",
     "src/bigint/digit-arithmetic.h",
+    "src/bigint/mul-karatsuba.cc",
     "src/bigint/mul-schoolbook.cc",
+    "src/bigint/util.h",
     "src/bigint/vector-arithmetic.cc",
     "src/bigint/vector-arithmetic.h",
   ]

src/bigint/bigint-internal.cc

@@ -30,7 +30,8 @@ void ProcessorImpl::Multiply(RWDigits Z, Digits X, Digits Y) {
   if (X.len() == 0 || Y.len() == 0) return Z.Clear();
   if (X.len() < Y.len()) std::swap(X, Y);
   if (Y.len() == 1) return MultiplySingle(Z, X, Y[0]);
-  return MultiplySchoolbook(Z, X, Y);
+  if (Y.len() < kKaratsubaThreshold) return MultiplySchoolbook(Z, X, Y);
+  return MultiplyKaratsuba(Z, X, Y);
 }
 
 Status Processor::Multiply(RWDigits Z, Digits X, Digits Y) {

src/bigint/bigint-internal.h

@@ -10,6 +10,8 @@
 namespace v8 {
 namespace bigint {
 
+constexpr int kKaratsubaThreshold = 34;
+
 class ProcessorImpl : public Processor {
  public:
   explicit ProcessorImpl(Platform* platform);
@@ -21,6 +23,11 @@ class ProcessorImpl : public Processor {
   void MultiplySingle(RWDigits Z, Digits X, digit_t y);
   void MultiplySchoolbook(RWDigits Z, Digits X, Digits Y);
 
+  void MultiplyKaratsuba(RWDigits Z, Digits X, Digits Y);
+  void KaratsubaStart(RWDigits Z, Digits X, Digits Y, RWDigits scratch, int k);
+  void KaratsubaChunk(RWDigits Z, Digits X, Digits Y, RWDigits scratch);
+  void KaratsubaMain(RWDigits Z, Digits X, Digits Y, RWDigits scratch, int n);
+
  private:
   // Each unit is supposed to represent approximately one CPU {mul} instruction.
   // Doesn't need to be accurate; we just want to make sure to check for
@@ -59,6 +66,28 @@ class ProcessorImpl : public Processor {
 #define DCHECK(cond) (void(0))
 #endif
 
+// RAII memory for a Digits array.
+class Storage {
+ public:
+  explicit Storage(int count) : ptr_(new digit_t[count]) {}
+
+  digit_t* get() { return ptr_.get(); }
+
+ private:
+  std::unique_ptr<digit_t[]> ptr_;
+};
+
+// A writable Digits array with attached storage.
+class ScratchDigits : public RWDigits {
+ public:
+  explicit ScratchDigits(int len) : RWDigits(nullptr, len), storage_(len) {
+    digits_ = storage_.get();
+  }
+
+ private:
+  Storage storage_;
+};
+
 }  // namespace bigint
 }  // namespace v8
 

src/bigint/digit-arithmetic.h

@@ -45,6 +45,36 @@ inline digit_t digit_add3(digit_t a, digit_t b, digit_t c, digit_t* carry) {
 #endif
 }
 
+// {borrow} will be set to 0 or 1.
+inline digit_t digit_sub(digit_t a, digit_t b, digit_t* borrow) {
+#if HAVE_TWODIGIT_T
+  twodigit_t result = twodigit_t{a} - b;
+  *borrow = (result >> kDigitBits) & 1;
+  return static_cast<digit_t>(result);
+#else
+  digit_t result = a - b;
+  *borrow = (result > a) ? 1 : 0;
+  return result;
+#endif
+}
+
+// {borrow_out} will be set to 0 or 1.
+inline digit_t digit_sub2(digit_t a, digit_t b, digit_t borrow_in,
+                          digit_t* borrow_out) {
+#if HAVE_TWODIGIT_T
+  twodigit_t subtrahend = twodigit_t{b} + borrow_in;
+  twodigit_t result = twodigit_t{a} - subtrahend;
+  *borrow_out = (result >> kDigitBits) & 1;
+  return static_cast<digit_t>(result);
+#else
+  digit_t result = a - b;
+  *borrow_out = (result > a) ? 1 : 0;
+  if (result < borrow_in) *borrow_out += 1;
+  result -= borrow_in;
+  return result;
+#endif
+}
+
 // Returns the low half of the result. High half is in {high}.
 inline digit_t digit_mul(digit_t a, digit_t b, digit_t* high) {
 #if HAVE_TWODIGIT_T

src/bigint/mul-karatsuba.cc

@@ -0,0 +1,189 @@
+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Karatsuba multiplication. This is loosely based on Go's implementation
+// found at https://golang.org/src/math/big/nat.go, licensed 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 [1].
+//
+// [1] https://golang.org/LICENSE
+
+#include <algorithm>
+#include <utility>
+
+#include "src/bigint/bigint-internal.h"
+#include "src/bigint/digit-arithmetic.h"
+#include "src/bigint/util.h"
+#include "src/bigint/vector-arithmetic.h"
+
+namespace v8 {
+namespace bigint {
+
+namespace {
+
+// The Karatsuba algorithm sometimes finishes more quickly when the
+// input length is rounded up a bit. This method encodes some heuristics
+// to accomplish this. The details have been determined experimentally.
+int RoundUpLen(int len) {
+  if (len <= 36) return RoundUp(len, 2);
+  // Keep the 4 or 5 most significant non-zero bits.
+  int shift = BitLength(len) - 5;
+  if ((len >> shift) >= 0x18) {
+    shift++;
+  }
+  // Round up, unless we're only just above the threshold. This smoothes
+  // the steps by which time goes up as input size increases.
+  int additive = ((1 << shift) - 1);
+  if (shift >= 2 && (len & additive) < (1 << (shift - 2))) {
+    return len;
+  }
+  return ((len + additive) >> shift) << shift;
+}
+
+// This method makes the final decision how much to bump up the input size.
+int KaratsubaLength(int n) {
+  n = RoundUpLen(n);
+  int i = 0;
+  while (n > kKaratsubaThreshold) {
+    n >>= 1;
+    i++;
+  }
+  return n << i;
+}
+
+// Performs the specific subtraction required by {KaratsubaMain} below.
+void KaratsubaSubtractionHelper(RWDigits result, Digits X, Digits Y,
+                                int* sign) {
+  X.Normalize();
+  Y.Normalize();
+  digit_t borrow = 0;
+  int i = 0;
+  if (!GreaterThanOrEqual(X, Y)) {
+    *sign = -(*sign);
+    std::swap(X, Y);
+  }
+  for (; i < Y.len(); i++) {
+    result[i] = digit_sub2(X[i], Y[i], borrow, &borrow);
+  }
+  for (; i < X.len(); i++) {
+    result[i] = digit_sub(X[i], borrow, &borrow);
+  }
+  DCHECK(borrow == 0);  // NOLINT(readability/check)
+  for (; i < result.len(); i++) result[i] = 0;
+}
+
+}  // namespace
+
+void ProcessorImpl::MultiplyKaratsuba(RWDigits Z, Digits X, Digits Y) {
+  DCHECK(X.len() >= Y.len());
+  DCHECK(Y.len() >= kKaratsubaThreshold);
+  DCHECK(Z.len() >= X.len() + Y.len());
+  int k = KaratsubaLength(Y.len());
+  int scratch_len = 4 * k;
+  ScratchDigits scratch(scratch_len);
+  KaratsubaStart(Z, X, Y, scratch, k);
+}
+
+// Entry point for Karatsuba-based multiplication, takes care of inputs
+// with unequal lengths by chopping the larger into chunks.
+void ProcessorImpl::KaratsubaStart(RWDigits Z, Digits X, Digits Y,
+                                   RWDigits scratch, int k) {
+  KaratsubaMain(Z, X, Y, scratch, k);
+  if (should_terminate()) return;
+  for (int i = 2 * k; i < Z.len(); i++) Z[i] = 0;
+  if (k < Y.len() || X.len() != Y.len()) {
+    ScratchDigits T(2 * k);
+    // Add X0 * Y1 * b.
+    Digits X0(X, 0, k);
+    Digits Y1 = Y + std::min(k, Y.len());
+    if (Y1.len() > 0) {
+      KaratsubaChunk(T, X0, Y1, scratch);
+      if (should_terminate()) return;
+      AddAt(Z + k, T);
+    }
+
+    // Add Xi * Y0 << i and Xi * Y1 * b << (i + k).
+    Digits Y0(Y, 0, k);
+    for (int i = k; i < X.len(); i += k) {
+      Digits Xi(X, i, k);
+      KaratsubaChunk(T, Xi, Y0, scratch);
+      if (should_terminate()) return;
+      AddAt(Z + i, T);
+      if (Y1.len() > 0) {
+        KaratsubaChunk(T, Xi, Y1, scratch);
+        if (should_terminate()) return;
+        AddAt(Z + (i + k), T);
+      }
+    }
+  }
+}
+
+// Entry point for chunk-wise multiplications, selects an appropriate
+// algorithm for the inputs based on their sizes.
+void ProcessorImpl::KaratsubaChunk(RWDigits Z, Digits X, Digits Y,
+                                   RWDigits scratch) {
+  X.Normalize();
+  Y.Normalize();
+  if (X.len() == 0 || Y.len() == 0) return Z.Clear();
+  if (X.len() < Y.len()) std::swap(X, Y);
+  if (Y.len() == 1) return MultiplySingle(Z, X, Y[0]);
+  if (Y.len() < kKaratsubaThreshold) return MultiplySchoolbook(Z, X, Y);
+  int k = KaratsubaLength(Y.len());
+  DCHECK(scratch.len() >= 4 * k);
+  return KaratsubaStart(Z, X, Y, scratch, k);
+}
+
+// The main recursive Karatsuba method.
+void ProcessorImpl::KaratsubaMain(RWDigits Z, Digits X, Digits Y,
+                                  RWDigits scratch, int n) {
+  if (n < kKaratsubaThreshold) {
+    X.Normalize();
+    Y.Normalize();
+    if (X.len() >= Y.len()) {
+      return MultiplySchoolbook(RWDigits(Z, 0, 2 * n), X, Y);
+    } else {
+      return MultiplySchoolbook(RWDigits(Z, 0, 2 * n), Y, X);
+    }
+  }
+  DCHECK(scratch.len() >= 4 * n);
+  DCHECK((n & 1) == 0);  // NOLINT(readability/check)
+  int n2 = n >> 1;
+  Digits X0(X, 0, n2);
+  Digits X1(X, n2, n2);
+  Digits Y0(Y, 0, n2);
+  Digits Y1(Y, n2, n2);
+  RWDigits scratch_for_recursion(scratch, 2 * n, 2 * n);
+  RWDigits P0(scratch, 0, n);
+  KaratsubaMain(P0, X0, Y0, scratch_for_recursion, n2);
+  if (should_terminate()) return;
+  for (int i = 0; i < n; i++) Z[i] = P0[i];
+  RWDigits P2(scratch, n, n);
+  KaratsubaMain(P2, X1, Y1, scratch_for_recursion, n2);
+  if (should_terminate()) return;
+  RWDigits Z1 = Z + n;
+  int end = std::min(Z1.len(), P2.len());
+  for (int i = 0; i < end; i++) Z1[i] = P2[i];
+  for (int i = end; i < n; i++) {
+    DCHECK(P2[i] == 0);  // NOLINT(readability/check)
+  }
+  AddAt(Z + n2, P0);
+  AddAt(Z + n2, P2);
+  RWDigits X_diff(scratch, 0, n2);
+  RWDigits Y_diff(scratch, n2, n2);
+  int sign = 1;
+  KaratsubaSubtractionHelper(X_diff, X1, X0, &sign);
+  KaratsubaSubtractionHelper(Y_diff, Y0, Y1, &sign);
+  RWDigits P1(scratch, n, n);
+  KaratsubaMain(P1, X_diff, Y_diff, scratch_for_recursion, n2);
+  if (sign > 0) {
+    AddAt(Z + n2, P1);
+  } else {
+    SubAt(Z + n2, P1);
+  }
+}
+
+}  // namespace bigint
+}  // namespace v8

src/bigint/mul-schoolbook.cc

@@ -71,7 +71,6 @@ void ProcessorImpl::MultiplySchoolbook(RWDigits Z, Digits X, Digits Y) {
     next_carry = 0;
     BODY(0, i);
     AddWorkEstimate(i);
-    if (should_terminate()) return;
   }
   // Last part: i exceeds Y now, we have to be careful about bounds.
   int loop_end = X.len() + Y.len() - 2;
@@ -85,7 +84,6 @@ void ProcessorImpl::MultiplySchoolbook(RWDigits Z, Digits X, Digits Y) {
     next_carry = 0;
     BODY(min_x_index, max_x_index);
     AddWorkEstimate(max_x_index - min_x_index);
-    if (should_terminate()) return;
   }
   // Write the last digit, and zero out any extra space in Z.
   Z[i++] = digit_add2(next, carry, &carry);

src/bigint/util.h

@@ -0,0 +1,60 @@
+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// "Generic" helper functions (not specific to BigInts).
+
+#include <stdint.h>
+
+#include <type_traits>
+
+#ifdef _MSC_VER
+#include <intrin.h>  // For _BitScanReverse.
+#endif
+
+#ifndef V8_BIGINT_UTIL_H_
+#define V8_BIGINT_UTIL_H_
+
+// Integer division, rounding up.
+#define DIV_CEIL(x, y) (((x)-1) / (y) + 1)
+
+namespace v8 {
+namespace bigint {
+
+// Rounds up x to a multiple of y.
+inline constexpr int RoundUp(int x, int y) { return (x + y - 1) & -y; }
+
+// Different environments disagree on how 64-bit uintptr_t and uint64_t are
+// defined, so we have to use templates to be generic.
+template <typename T, typename = typename std::enable_if<
+                          std::is_unsigned<T>::value && sizeof(T) == 8>::type>
+constexpr int CountLeadingZeros(T value) {
+#if __GNUC__ || __clang__
+  return value == 0 ? 64 : __builtin_clzll(value);
+#elif _MSC_VER
+  unsigned long index = 0;  // NOLINT(runtime/int). MSVC insists.
+  return _BitScanReverse64(&index, value) ? 63 - index : 64;
+#else
+#error Unsupported compiler.
+#endif
+}
+
+constexpr int CountLeadingZeros(uint32_t value) {
+#if __GNUC__ || __clang__
+  return value == 0 ? 32 : __builtin_clz(value);
+#elif _MSC_VER
+  unsigned long index = 0;  // NOLINT(runtime/int). MSVC insists.
+  return _BitScanReverse(&index, value) ? 31 - index : 32;
+#else
+#error Unsupported compiler.
+#endif
+}
+
+inline constexpr int BitLength(int n) {
+  return 32 - CountLeadingZeros(static_cast<uint32_t>(n));
+}
+
+}  // namespace bigint
+}  // namespace v8
+
+#endif  // V8_BIGINT_UTIL_H_

src/bigint/vector-arithmetic.cc

@@ -4,9 +4,36 @@
 
 #include "src/bigint/vector-arithmetic.h"
 
+#include "src/bigint/digit-arithmetic.h"
+
 namespace v8 {
 namespace bigint {
 
+void AddAt(RWDigits Z, Digits X) {
+  X.Normalize();
+  if (X.len() == 0) return;
+  digit_t carry = 0;
+  int i = 0;
+  for (; i < X.len(); i++) {
+    Z[i] = digit_add3(Z[i], X[i], carry, &carry);
+  }
+  for (; carry != 0; i++) {
+    Z[i] = digit_add2(Z[i], carry, &carry);
+  }
+}
+
+void SubAt(RWDigits Z, Digits X) {
+  X.Normalize();
+  digit_t borrow = 0;
+  int i = 0;
+  for (; i < X.len(); i++) {
+    Z[i] = digit_sub2(Z[i], X[i], borrow, &borrow);
+  }
+  for (; borrow != 0; i++) {
+    Z[i] = digit_sub(Z[i], borrow, &borrow);
+  }
+}
+
 int Compare(Digits A, Digits B) {
   A.Normalize();
   B.Normalize();

src/bigint/vector-arithmetic.h

@@ -12,8 +12,18 @@
 namespace v8 {
 namespace bigint {
 
+// Z += X.
+void AddAt(RWDigits Z, Digits X);
+
+// Z -= X.
+void SubAt(RWDigits Z, Digits X);
+
 inline bool IsDigitNormalized(Digits X) { return X.len() == 0 || X.msd() != 0; }
 
+inline bool GreaterThanOrEqual(Digits A, Digits B) {
+  return Compare(A, B) >= 0;
+}
+
 }  // namespace bigint
 }  // namespace v8
 

test/BUILD.gn

@@ -19,6 +19,7 @@ group("gn_all") {
   ]
 
   deps = [
+    "bigint:bigint_shell",
     "inspector:inspector-test",
     "mkgrokdump:mkgrokdump",
   ]

test/bigint/BUILD.gn

@@ -0,0 +1,25 @@
+# Copyright 2021 The V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("../../gni/v8.gni")
+
+v8_executable("bigint_shell") {
+  testonly = true
+
+  deps = [
+    "../..:v8_bigint",
+    "//build/win:default_exe_manifest",
+  ]
+
+  data_deps = [ "../../tools:v8_testrunner" ]
+
+  data = [
+    "testcfg.py",
+    "bigint.status",
+  ]
+
+  configs = [ "../..:internal_config_base" ]
+
+  sources = [ "bigint-shell.cc" ]
+}

test/bigint/DEPS

@@ -0,0 +1,3 @@
+include_rules = [
+  "+src/bigint",
+]

test/bigint/bigint-shell.cc

@@ -0,0 +1,263 @@
+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/bigint/bigint-internal.h"
+#include "src/bigint/util.h"
+
+namespace v8 {
+namespace bigint {
+namespace test {
+
+int PrintHelp(char** argv) {
+  std::cerr << "Usage:\n"
+            << argv[0] << " --help\n"
+            << "    Print this help and exit.\n"
+            << argv[0] << " --list\n"
+            << "    List supported tests.\n"
+            << argv[0] << " <testname>\n"
+            << "    Run the specified test (see --list for a list).\n"
+            << "\nOptions when running tests:\n"
+            << "--random-seed R\n"
+            << "    Initialize the random number generator with this seed.\n"
+            << "--runs N\n"
+            << "    Repeat the test N times.\n";
+  return 1;
+}
+
+#define TESTS(V) V(kKaratsuba, "karatsuba")
+
+enum Operation { kNoOp, kList, kTest };
+
+enum Test {
+#define TEST(kName, name) kName,
+  TESTS(TEST)
+#undef TEST
+};
+
+class RNG {
+ public:
+  RNG() = default;
+
+  void Initialize(int64_t seed) {
+    state0_ = MurmurHash3(static_cast<uint64_t>(seed));
+    state1_ = MurmurHash3(~state0_);
+    CHECK(state0_ != 0 || state1_ != 0);
+  }
+
+  uint64_t NextUint64() {
+    XorShift128(&state0_, &state1_);
+    return static_cast<uint64_t>(state0_ + state1_);
+  }
+
+  static inline void XorShift128(uint64_t* state0, uint64_t* state1) {
+    uint64_t s1 = *state0;
+    uint64_t s0 = *state1;
+    *state0 = s0;
+    s1 ^= s1 << 23;
+    s1 ^= s1 >> 17;
+    s1 ^= s0;
+    s1 ^= s0 >> 26;
+    *state1 = s1;
+  }
+
+  static uint64_t MurmurHash3(uint64_t h) {
+    h ^= h >> 33;
+    h *= uint64_t{0xFF51AFD7ED558CCD};
+    h ^= h >> 33;
+    h *= uint64_t{0xC4CEB9FE1A85EC53};
+    h ^= h >> 33;
+    return h;
+  }
+
+ private:
+  uint64_t state0_;
+  uint64_t state1_;
+};
+
+static constexpr int kCharsPerDigit = kDigitBits / 4;
+
+static const char kConversionChars[] = "0123456789abcdef";
+
+std::string FormatHex(Digits X) {
+  X.Normalize();
+  if (X.len() == 0) return "0";
+  digit_t msd = X.msd();
+  const int msd_leading_zeros = CountLeadingZeros(msd);
+  const size_t bit_length = X.len() * kDigitBits - msd_leading_zeros;
+  const size_t chars = DIV_CEIL(bit_length, 4);
+
+  if (chars > 100000) {
+    return std::string("<BigInt with ") + std::to_string(bit_length) +
+           std::string(" bits>");
+  }
+
+  std::unique_ptr<char[]> result(new char[chars]);
+  for (size_t i = 0; i < chars; i++) result[i] = '?';
+  // Print the number into the string, starting from the last position.
+  int pos = static_cast<int>(chars - 1);
+  for (int i = 0; i < X.len() - 1; i++) {
+    digit_t d = X[i];
+    for (int j = 0; j < kCharsPerDigit; j++) {
+      result[pos--] = kConversionChars[d & 15];
+      d = static_cast<digit_t>(d >> 4u);
+    }
+  }
+  while (msd != 0) {
+    result[pos--] = kConversionChars[msd & 15];
+    msd = static_cast<digit_t>(msd >> 4u);
+  }
+  CHECK(pos == -1);
+  return std::string(result.get(), chars);
+}
+
+class Runner {
+ public:
+  Runner() = default;
+
+  void Initialize() {
+    rng_.Initialize(random_seed_);
+    processor_.reset(Processor::New(new Platform()));
+  }
+
+  ProcessorImpl* processor() {
+    return static_cast<ProcessorImpl*>(processor_.get());
+  }
+
+  int Run() {
+    if (op_ == kList) {
+      ListTests();
+    } else if (op_ == kTest) {
+      RunTest();
+    } else {
+      DCHECK(false);  // Unreachable.
+    }
+    return 0;
+  }
+
+  void ListTests() {
+#define PRINT(kName, name) std::cout << name << "\n";
+    TESTS(PRINT)
+#undef PRINT
+  }
+
+  void AssertEquals(Digits input1, Digits input2, Digits expected,
+                    Digits actual) {
+    if (Compare(expected, actual) == 0) return;
+    std::cerr << "Input 1:  " << FormatHex(input1) << "\n";
+    std::cerr << "Input 2:  " << FormatHex(input2) << "\n";
+    std::cerr << "Expected: " << FormatHex(expected) << "\n";
+    std::cerr << "Actual:   " << FormatHex(actual) << "\n";
+    error_ = true;
+  }
+
+  int RunTest() {
+    int count = 0;
+    if (test_ == kKaratsuba) {
+      for (int i = 0; i < runs_; i++) {
+        TestKaratsuba(&count);
+      }
+    } else {
+      DCHECK(false);  // Unreachable.
+    }
+    if (error_) return 1;
+    std::cout << count << " tests run, no error reported.\n";
+    return 0;
+  }
+
+  void TestKaratsuba(int* count) {
+    // Calling {MultiplyKaratsuba} directly is only valid if
+    // left_size >= right_size and right_size >= kKaratsubaThreshold.
+    for (int right_size = kKaratsubaThreshold;
+         right_size <= 3 * kKaratsubaThreshold; right_size++) {
+      for (int left_size = right_size; left_size <= 3 * kKaratsubaThreshold;
+           left_size++) {
+        ScratchDigits A(left_size);
+        ScratchDigits B(right_size);
+        int result_len = MultiplyResultLength(A, B);
+        ScratchDigits result(result_len);
+        ScratchDigits result_schoolbook(result_len);
+        GenerateRandom(A);
+        GenerateRandom(B);
+        processor()->MultiplyKaratsuba(result, A, B);
+        processor()->MultiplySchoolbook(result_schoolbook, A, B);
+        AssertEquals(A, B, result_schoolbook, result);
+        if (error_) return;
+        (*count)++;
+      }
+    }
+  }
+
+  int ParseOptions(int argc, char** argv) {
+    for (int i = 1; i < argc; i++) {
+      if (strcmp(argv[i], "--list") == 0) {
+        op_ = kList;
+      } else if (strcmp(argv[i], "--help") == 0 || strcmp(argv[i], "-h") == 0) {
+        PrintHelp(argv);
+        return 0;
+      } else if (strcmp(argv[i], "--random-seed") == 0 ||
+                 strcmp(argv[i], "--random_seed") == 0) {
+        random_seed_ = std::stoi(argv[++i]);
+      } else if (strncmp(argv[i], "--random-seed=", 14) == 0 ||
+                 strncmp(argv[i], "--random_seed=", 14) == 0) {
+        random_seed_ = std::stoi(argv[i] + 14);
+      } else if (strcmp(argv[i], "--runs") == 0) {
+        runs_ = std::stoi(argv[++i]);
+      } else if (strncmp(argv[i], "--runs=", 7) == 0) {
+        runs_ = std::stoi(argv[i] + 7);
+      }
+#define TEST(kName, name)                \
+  else if (strcmp(argv[i], name) == 0) { \
+    op_ = kTest;                         \
+    test_ = kName;                       \
+  }
+      TESTS(TEST)
+#undef TEST
+      else {
+        std::cerr << "Warning: ignored argument: " << argv[i] << "\n";
+      }
+    }
+    if (op_ == kNoOp) return PrintHelp(argv);  // op is mandatory.
+    return 0;
+  }
+
+ private:
+  void GenerateRandom(RWDigits Z) {
+    if (Z.len() == 0) return;
+    if (sizeof(digit_t) == 8) {
+      for (int i = 0; i < Z.len(); i++) {
+        Z[i] = static_cast<digit_t>(rng_.NextUint64());
+      }
+    } else {
+      for (int i = 0; i < Z.len(); i += 2) {
+        uint64_t random = rng_.NextUint64();
+        Z[i] = static_cast<digit_t>(random);
+        if (i + 1 < Z.len()) Z[i + 1] = static_cast<digit_t>(random >> 32);
+      }
+    }
+    // Special case: we don't want the MSD to be zero.
+    while (Z.msd() == 0) {
+      Z[Z.len() - 1] = static_cast<digit_t>(rng_.NextUint64());
+    }
+  }
+
+  Operation op_{kNoOp};
+  Test test_;
+  bool error_{false};
+  int runs_ = 1;
+  int64_t random_seed_{314159265359};
+  RNG rng_;
+  std::unique_ptr<Processor, Processor::Destroyer> processor_;
+};
+
+}  // namespace test
+}  // namespace bigint
+}  // namespace v8
+
+int main(int argc, char** argv) {
+  v8::bigint::test::Runner runner;
+  int ret = runner.ParseOptions(argc, argv);
+  if (ret != 0) return ret;
+  runner.Initialize();
+  return runner.Run();
+}

test/bigint/bigint.status

@@ -0,0 +1,5 @@
+# Copyright 2021 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+[]

test/bigint/testcfg.py

@@ -0,0 +1,63 @@
+# Copyright 2021 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# for py2/py3 compatibility
+from __future__ import print_function
+
+import os
+
+from testrunner.local import command
+from testrunner.local import utils
+from testrunner.local import testsuite
+from testrunner.objects import testcase
+
+SHELL = 'bigint_shell'
+
+class VariantsGenerator(testsuite.VariantsGenerator):
+  def _get_variants(self, test):
+    return self._standard_variant
+
+
+class TestLoader(testsuite.TestLoader):
+  def _list_test_filenames(self):
+    shell = os.path.abspath(
+      os.path.join(self.test_config.shell_dir, SHELL))
+    if utils.IsWindows():
+      shell += ".exe"
+    cmd = command.Command(
+      cmd_prefix=self.test_config.command_prefix,
+      shell=shell,
+      args=['--list'] + self.test_config.extra_flags)
+    output = cmd.execute()
+
+    if output.exit_code != 0:
+      print("Test executable failed to list the tests.\n\nCmd:")
+      print(cmd)
+      print("\nStdout:")
+      print(output.stdout)
+      print("\nStderr:")
+      print(output.stderr)
+      print("\nExit code: %d" % output.exit_code)
+
+    return sorted(output.stdout.strip().split())
+
+
+class TestSuite(testsuite.TestSuite):
+  def _test_loader_class(self):
+    return TestLoader
+
+  def _test_class(self):
+    return TestCase
+
+
+class TestCase(testcase.TestCase):
+  def _get_files_params(self):
+    return [self.path]
+
+  def get_shell(self):
+    return SHELL
+
+
+def GetSuite(*args, **kwargs):
+  return TestSuite(*args, **kwargs)

tools/dev/gm.py

@@ -36,8 +36,8 @@ USE_PTY = "linux" in sys.platform
 if USE_PTY:
   import pty
 
-BUILD_TARGETS_TEST = ["d8", "cctest", "inspector-test", "unittests",
-                      "wasm_api_tests"]
+BUILD_TARGETS_TEST = ["d8", "bigint_shell", "cctest", "inspector-test",
+                      "unittests", "wasm_api_tests"]
 BUILD_TARGETS_ALL = ["all"]
 
 # All arches that this script understands.
@@ -51,7 +51,8 @@ MODES = ["release", "debug", "optdebug"]
 DEFAULT_MODES = ["release", "debug"]
 # Build targets that can be manually specified.
 TARGETS = ["d8", "cctest", "unittests", "v8_fuzzers", "wasm_api_tests", "wee8",
-           "mkgrokdump", "generate-bytecode-expectations", "inspector-test"]
+           "mkgrokdump", "generate-bytecode-expectations", "inspector-test",
+           "bigint_shell"]
 # Build targets that get built when you don't specify any (and specified tests
 # don't imply any other targets).
 DEFAULT_TARGETS = ["d8"]
@@ -81,6 +82,7 @@ HELP = """<arch> can be any of: %(arches)s
        "targets": ", ".join(TARGETS)}
 
 TESTSUITES_TARGETS = {"benchmarks": "d8",
+              "bigint": "bigint_shell",
               "cctest": "cctest",
               "debugger": "d8",
               "fuzzer": "v8_fuzzers",