97 lines
2.9 KiB
C++
97 lines
2.9 KiB
C++
// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
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#define ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
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#include <cstdint>
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#include <limits>
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#include <type_traits>
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#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64)
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#include <intrin.h> // NOLINT(build/include_order)
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#pragma intrinsic(_umul128)
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#define ABSL_INTERNAL_USE_UMUL128 1
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#endif
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#include "absl/base/config.h"
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#include "absl/numeric/bits.h"
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#include "absl/numeric/int128.h"
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#include "absl/random/internal/traits.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace random_internal {
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// wide_multiply<T> multiplies two N-bit values to a 2N-bit result.
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template <typename UIntType>
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struct wide_multiply {
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static constexpr size_t kN = std::numeric_limits<UIntType>::digits;
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using input_type = UIntType;
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using result_type = typename random_internal::unsigned_bits<kN * 2>::type;
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static result_type multiply(input_type a, input_type b) {
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return static_cast<result_type>(a) * b;
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}
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static input_type hi(result_type r) {
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return static_cast<input_type>(r >> kN);
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}
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static input_type lo(result_type r) { return static_cast<input_type>(r); }
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static_assert(std::is_unsigned<UIntType>::value,
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"Class-template wide_multiply<> argument must be unsigned.");
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};
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// MultiplyU128ToU256 multiplies two 128-bit values to a 256-bit value.
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inline U256 MultiplyU128ToU256(uint128 a, uint128 b) {
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const uint128 a00 = static_cast<uint64_t>(a);
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const uint128 a64 = a >> 64;
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const uint128 b00 = static_cast<uint64_t>(b);
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const uint128 b64 = b >> 64;
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const uint128 c00 = a00 * b00;
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const uint128 c64a = a00 * b64;
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const uint128 c64b = a64 * b00;
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const uint128 c128 = a64 * b64;
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const uint64_t carry =
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static_cast<uint64_t>(((c00 >> 64) + static_cast<uint64_t>(c64a) +
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static_cast<uint64_t>(c64b)) >>
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64);
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return {c128 + (c64a >> 64) + (c64b >> 64) + carry,
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c00 + (c64a << 64) + (c64b << 64)};
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}
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template <>
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struct wide_multiply<uint128> {
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using input_type = uint128;
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using result_type = U256;
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static result_type multiply(input_type a, input_type b) {
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return MultiplyU128ToU256(a, b);
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}
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static input_type hi(result_type r) { return r.hi; }
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static input_type lo(result_type r) { return r.lo; }
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};
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} // namespace random_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
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