LibJS+LibCrypto: Use a bitwise approach for BigInt's as*IntN methods

This speeds up expressions such as `BigInt.asIntN(0x4000000000000, 1n)`
(#3615). And those involving very large bigints.

Libraries/LibCrypto/BigInt/Algorithms/BitwiseOperations.cpp

@@ -131,7 +131,7 @@ FLATTEN void UnsignedBigIntegerAlgorithms::bitwise_xor_without_allocation(
 /**
  * Complexity: O(N) where N is the number of words
  */
-FLATTEN void UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_without_allocation(
+FLATTEN ErrorOr<void> UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_without_allocation(
     UnsignedBigInteger const& right,
     size_t index,
     UnsignedBigInteger& output)
@@ -139,16 +139,16 @@ FLATTEN void UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_w
     // If the value is invalid, the output value is invalid as well.
     if (right.is_invalid()) {
         output.invalidate();
-        return;
+        return {};
     }
 
     if (index == 0) {
         output.set_to_0();
-        return;
+        return {};
     }
     size_t size = (index + UnsignedBigInteger::BITS_IN_WORD - 1) / UnsignedBigInteger::BITS_IN_WORD;
 
-    output.m_words.resize_and_keep_capacity(size);
+    TRY(output.m_words.try_resize_and_keep_capacity(size));
     VERIFY(size > 0);
     for (size_t i = 0; i < size - 1; ++i)
         output.m_words[i] = ~(i < right.length() ? right.words()[i] : 0);
@@ -158,6 +158,8 @@ FLATTEN void UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_w
     auto last_word = last_word_index < right.length() ? right.words()[last_word_index] : 0;
 
     output.m_words[last_word_index] = (NumericLimits<UnsignedBigInteger::Word>::max() >> (UnsignedBigInteger::BITS_IN_WORD - index)) & ~last_word;
+
+    return {};
 }
 
 FLATTEN void UnsignedBigIntegerAlgorithms::shift_left_without_allocation(

Libraries/LibCrypto/BigInt/Algorithms/UnsignedBigIntegerAlgorithms.h

@@ -20,13 +20,13 @@ public:
     static void bitwise_or_without_allocation(UnsignedBigInteger const& left, UnsignedBigInteger const& right, UnsignedBigInteger& output);
     static void bitwise_and_without_allocation(UnsignedBigInteger const& left, UnsignedBigInteger const& right, UnsignedBigInteger& output);
     static void bitwise_xor_without_allocation(UnsignedBigInteger const& left, UnsignedBigInteger const& right, UnsignedBigInteger& output);
-    static void bitwise_not_fill_to_one_based_index_without_allocation(UnsignedBigInteger const& left, size_t, UnsignedBigInteger& output);
     static void shift_left_without_allocation(UnsignedBigInteger const& number, size_t bits_to_shift_by, UnsignedBigInteger& temp_result, UnsignedBigInteger& temp_plus, UnsignedBigInteger& output);
     static void shift_right_without_allocation(UnsignedBigInteger const& number, size_t num_bits, UnsignedBigInteger& output);
     static void multiply_without_allocation(UnsignedBigInteger const& left, UnsignedBigInteger const& right, UnsignedBigInteger& temp_shift_result, UnsignedBigInteger& temp_shift_plus, UnsignedBigInteger& temp_shift, UnsignedBigInteger& output);
     static void divide_without_allocation(UnsignedBigInteger const& numerator, UnsignedBigInteger const& denominator, UnsignedBigInteger& quotient, UnsignedBigInteger& remainder);
     static void divide_u16_without_allocation(UnsignedBigInteger const& numerator, UnsignedBigInteger::Word denominator, UnsignedBigInteger& quotient, UnsignedBigInteger& remainder);
 
+    static ErrorOr<void> bitwise_not_fill_to_one_based_index_without_allocation(UnsignedBigInteger const& left, size_t, UnsignedBigInteger& output);
     static ErrorOr<void> try_shift_left_without_allocation(UnsignedBigInteger const& number, size_t bits_to_shift_by, UnsignedBigInteger& temp_result, UnsignedBigInteger& temp_plus, UnsignedBigInteger& output);
 
     static void extended_GCD_without_allocation(UnsignedBigInteger const& a, UnsignedBigInteger const& b, UnsignedBigInteger& x, UnsignedBigInteger& y, UnsignedBigInteger& gcd, UnsignedBigInteger& temp_quotient, UnsignedBigInteger& temp_1, UnsignedBigInteger& temp_2, UnsignedBigInteger& temp_shift_result, UnsignedBigInteger& temp_shift_plus, UnsignedBigInteger& temp_shift, UnsignedBigInteger& temp_r, UnsignedBigInteger& temp_s, UnsignedBigInteger& temp_t);

Libraries/LibCrypto/BigInt/SignedBigInteger.cpp

@@ -291,6 +291,17 @@ FLATTEN SignedBigInteger SignedBigInteger::shift_right(size_t num_bits) const
     return SignedBigInteger { m_unsigned_data.shift_right(num_bits), m_sign };
 }
 
+FLATTEN ErrorOr<SignedBigInteger> SignedBigInteger::mod_power_of_two(size_t power_of_two) const
+{
+    auto const lower_bits = m_unsigned_data.as_n_bits(power_of_two);
+
+    if (is_positive())
+        return SignedBigInteger(lower_bits);
+
+    // twos encode lower bits
+    return SignedBigInteger(TRY(lower_bits.try_bitwise_not_fill_to_one_based_index(power_of_two)).plus(1).as_n_bits(power_of_two));
+}
+
 FLATTEN SignedBigInteger SignedBigInteger::multiplied_by(SignedBigInteger const& other) const
 {
     bool result_sign = m_sign ^ other.m_sign;

Libraries/LibCrypto/BigInt/SignedBigInteger.h

@@ -120,6 +120,7 @@ public:
     [[nodiscard]] SignedBigInteger multiplied_by(SignedBigInteger const& other) const;
     [[nodiscard]] SignedDivisionResult divided_by(SignedBigInteger const& divisor) const;
 
+    [[nodiscard]] ErrorOr<SignedBigInteger> mod_power_of_two(size_t power_of_two) const;
     [[nodiscard]] ErrorOr<SignedBigInteger> try_shift_left(size_t num_bits) const;
 
     [[nodiscard]] SignedBigInteger plus(UnsignedBigInteger const& other) const;

Libraries/LibCrypto/BigInt/UnsignedBigInteger.cpp

@@ -473,10 +473,15 @@ FLATTEN UnsignedBigInteger UnsignedBigInteger::bitwise_xor(UnsignedBigInteger co
 }
 
 FLATTEN UnsignedBigInteger UnsignedBigInteger::bitwise_not_fill_to_one_based_index(size_t size) const
+{
+    return MUST(try_bitwise_not_fill_to_one_based_index(size));
+}
+
+FLATTEN ErrorOr<UnsignedBigInteger> UnsignedBigInteger::try_bitwise_not_fill_to_one_based_index(size_t size) const
 {
     UnsignedBigInteger result;
 
-    UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_without_allocation(*this, size, result);
+    TRY(UnsignedBigIntegerAlgorithms::bitwise_not_fill_to_one_based_index_without_allocation(*this, size, result));
 
     return result;
 }
@@ -506,6 +511,33 @@ FLATTEN UnsignedBigInteger UnsignedBigInteger::shift_right(size_t num_bits) cons
     return output;
 }
 
+FLATTEN UnsignedBigInteger UnsignedBigInteger::as_n_bits(size_t n) const
+{
+    if (auto const num_bits = one_based_index_of_highest_set_bit(); n >= num_bits)
+        return *this;
+
+    UnsignedBigInteger output;
+    output.set_to(*this);
+
+    auto const word_index = n / BITS_IN_WORD;
+
+    auto const bits_to_keep = n % BITS_IN_WORD;
+    auto const bits_to_discard = BITS_IN_WORD - bits_to_keep;
+
+    output.m_words.resize(word_index + 1);
+
+    auto const last_word = output.m_words[word_index];
+    Word new_last_word = 0;
+
+    // avoid UB from a 32 bit shift on a u32
+    if (bits_to_keep != 0)
+        new_last_word = last_word << bits_to_discard >> bits_to_discard;
+
+    output.m_words[word_index] = new_last_word;
+
+    return output;
+}
+
 FLATTEN UnsignedBigInteger UnsignedBigInteger::multiplied_by(UnsignedBigInteger const& other) const
 {
     UnsignedBigInteger result;

Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -117,9 +117,11 @@ public:
     [[nodiscard]] UnsignedBigInteger bitwise_not_fill_to_one_based_index(size_t) const;
     [[nodiscard]] UnsignedBigInteger shift_left(size_t num_bits) const;
     [[nodiscard]] UnsignedBigInteger shift_right(size_t num_bits) const;
+    [[nodiscard]] UnsignedBigInteger as_n_bits(size_t n) const;
     [[nodiscard]] UnsignedBigInteger multiplied_by(UnsignedBigInteger const& other) const;
     [[nodiscard]] UnsignedDivisionResult divided_by(UnsignedBigInteger const& divisor) const;
 
+    [[nodiscard]] ErrorOr<UnsignedBigInteger> try_bitwise_not_fill_to_one_based_index(size_t) const;
     [[nodiscard]] ErrorOr<UnsignedBigInteger> try_shift_left(size_t num_bits) const;
 
     [[nodiscard]] u32 hash() const;

Libraries/LibJS/Runtime/BigIntConstructor.cpp

@@ -18,6 +18,7 @@ namespace JS {
 GC_DEFINE_ALLOCATOR(BigIntConstructor);
 
 static Crypto::SignedBigInteger const BIGINT_ONE { 1 };
+static Crypto::SignedBigInteger const BIGINT_ZERO { 0 };
 
 BigIntConstructor::BigIntConstructor(Realm& realm)
     : NativeFunction(realm.vm().names.BigInt.as_string(), realm.intrinsics().function_prototype())
@@ -72,20 +73,25 @@ JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_int_n)
     // 2. Set bigint to ? ToBigInt(bigint).
     auto bigint = TRY(vm.argument(1).to_bigint(vm));
 
+    // OPTIMIZATION: mod = bigint (mod 2^0) = 0 < 2^(0-1) = 0.5
+    if (bits == 0)
+        return BigInt::create(vm, BIGINT_ZERO);
+
     // 3. Let mod be ℝ(bigint) modulo 2^bits.
-    // FIXME: For large values of `bits`, this can likely be improved with a SignedBigInteger API to
-    //        drop the most significant bits.
-    auto bits_shift_left = TRY_OR_THROW_OOM(vm, BIGINT_ONE.try_shift_left(bits));
-    auto mod = modulo(bigint->big_integer(), bits_shift_left);
+    auto const mod = TRY_OR_THROW_OOM(vm, bigint->big_integer().mod_power_of_two(bits));
 
-    // 4. If mod ≥ 2^(bits-1), return ℤ(mod - 2^bits); otherwise, return ℤ(mod).
-    // NOTE: Some of the below conditionals are non-standard, but are to protect SignedBigInteger from
-    //       allocating an absurd amount of memory if `bits - 1` overflows to NumericLimits<size_t>::max.
-    if ((bits == 0) && (mod >= BIGINT_ONE))
-        return BigInt::create(vm, mod.minus(bits_shift_left));
-    if ((bits > 0) && (mod >= BIGINT_ONE.shift_left(bits - 1)))
-        return BigInt::create(vm, mod.minus(bits_shift_left));
+    // OPTIMIZATION: mod < 2^(bits-1)
+    if (mod.is_zero())
+        return BigInt::create(vm, BIGINT_ZERO);
 
+    // 4. If mod ≥ 2^(bits-1), return ℤ(mod - 2^bits); ...
+    if (auto top_bit_index = mod.unsigned_value().one_based_index_of_highest_set_bit(); top_bit_index >= bits) {
+        // twos complement decode
+        auto decoded = TRY_OR_THROW_OOM(vm, mod.unsigned_value().try_bitwise_not_fill_to_one_based_index(bits)).plus(1);
+        return BigInt::create(vm, Crypto::SignedBigInteger { std::move(decoded), true });
+    }
+
+    // ... otherwise, return ℤ(mod).
     return BigInt::create(vm, mod);
 }
 
@@ -98,10 +104,10 @@ JS_DEFINE_NATIVE_FUNCTION(BigIntConstructor::as_uint_n)
     // 2. Set bigint to ? ToBigInt(bigint).
     auto bigint = TRY(vm.argument(1).to_bigint(vm));
 
-    // 3. Return the BigInt value that represents ℝ(bigint) modulo 2bits.
-    // FIXME: For large values of `bits`, this can likely be improved with a SignedBigInteger API to
-    //        drop the most significant bits.
-    return BigInt::create(vm, modulo(bigint->big_integer(), TRY_OR_THROW_OOM(vm, BIGINT_ONE.try_shift_left(bits))));
+    // 3. Return the BigInt value that represents ℝ(bigint) modulo 2^bits.
+    auto const mod = TRY_OR_THROW_OOM(vm, bigint->big_integer().mod_power_of_two(bits));
+
+    return BigInt::create(vm, mod);
 }
 
 }

Libraries/LibJS/Tests/builtins/BigInt/BigInt.asIntN.js

@@ -22,12 +22,6 @@ describe("errors", () => {
             BigInt.asIntN(1, "foo");
         }).toThrowWithMessage(SyntaxError, "Invalid value for BigInt: foo");
     });
-
-    test("large allocation", () => {
-        expect(() => {
-            BigInt.asIntN(0x4000000000000, 1n);
-        }).toThrowWithMessage(InternalError, "Out of memory");
-    });
 });
 
 describe("correct behavior", () => {
@@ -82,4 +76,23 @@ describe("correct behavior", () => {
         expect(BigInt.asIntN(128, -extremelyBigInt)).toBe(99061374399389259395070030194384019691n);
         expect(BigInt.asIntN(256, -extremelyBigInt)).toBe(-extremelyBigInt);
     });
+
+    test("large bit values", () => {
+        expect(BigInt.asIntN(0x4000000000000, 1n)).toBe(1n);
+        expect(BigInt.asIntN(0x8ffffffffffff, 1n)).toBe(1n);
+        expect(BigInt.asIntN(2 ** 53 - 1, 2n)).toBe(2n);
+
+        // These incur large intermediate values that 00M. For now, ensure they don't crash
+        expect(() => {
+            BigInt.asIntN(0x4000000000000, -1n);
+        }).toThrowWithMessage(InternalError, "Out of memory");
+
+        expect(() => {
+            BigInt.asIntN(0x8ffffffffffff, -1n);
+        }).toThrowWithMessage(InternalError, "Out of memory");
+
+        expect(() => {
+            BigInt.asIntN(2 ** 53 - 1, -2n);
+        }).toThrowWithMessage(InternalError, "Out of memory");
+    });
 });

Libraries/LibJS/Tests/builtins/BigInt/BigInt.asUintN.js

@@ -22,12 +22,6 @@ describe("errors", () => {
             BigInt.asUintN(1, "foo");
         }).toThrowWithMessage(SyntaxError, "Invalid value for BigInt: foo");
     });
-
-    test("large allocation", () => {
-        expect(() => {
-            BigInt.asUintN(0x4000000000000, 1n);
-        }).toThrowWithMessage(InternalError, "Out of memory");
-    });
 });
 
 describe("correct behavior", () => {
@@ -80,4 +74,20 @@ describe("correct behavior", () => {
             115792089237316195423570861551898784396480861208851440582668460551124006183147n
         );
     });
+
+    test("large bit values", () => {
+        const INDEX_MAX = 2 ** 53 - 1;
+        const LAST_8_DIGITS = 10n ** 8n;
+
+        expect(BigInt.asUintN(0x400000000, 1n)).toBe(1n);
+        expect(BigInt.asUintN(0x4000, -1n) % LAST_8_DIGITS).toBe(64066815n);
+
+        expect(BigInt.asUintN(0x400000000, 2n)).toBe(2n);
+        expect(BigInt.asUintN(0x4000, -2n) % LAST_8_DIGITS).toBe(64066814n);
+
+        expect(BigInt.asUintN(INDEX_MAX, 2n)).toBe(2n);
+        expect(() => {
+            BigInt.asUintN(INDEX_MAX, -2n);
+        }).toThrowWithMessage(InternalError, "Out of memory");
+    });
 });