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Diffstat (limited to 'third_party/abseil_cpp/absl/random/internal/wide_multiply.h')
-rw-r--r-- | third_party/abseil_cpp/absl/random/internal/wide_multiply.h | 111 |
1 files changed, 111 insertions, 0 deletions
diff --git a/third_party/abseil_cpp/absl/random/internal/wide_multiply.h b/third_party/abseil_cpp/absl/random/internal/wide_multiply.h new file mode 100644 index 000000000000..0afcbe08e229 --- /dev/null +++ b/third_party/abseil_cpp/absl/random/internal/wide_multiply.h @@ -0,0 +1,111 @@ +// Copyright 2017 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#ifndef ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_ +#define ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_ + +#include <cstdint> +#include <limits> +#include <type_traits> + +#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64) +#include <intrin.h> // NOLINT(build/include_order) +#pragma intrinsic(_umul128) +#define ABSL_INTERNAL_USE_UMUL128 1 +#endif + +#include "absl/base/config.h" +#include "absl/base/internal/bits.h" +#include "absl/numeric/int128.h" +#include "absl/random/internal/traits.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace random_internal { + +// Helper object to multiply two 64-bit values to a 128-bit value. +// MultiplyU64ToU128 multiplies two 64-bit values to a 128-bit value. +// If an intrinsic is available, it is used, otherwise use native 32-bit +// multiplies to construct the result. +inline absl::uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) { +#if defined(ABSL_HAVE_INTRINSIC_INT128) + return absl::uint128(static_cast<__uint128_t>(a) * b); +#elif defined(ABSL_INTERNAL_USE_UMUL128) + // uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC. + uint64_t high = 0; + const uint64_t low = _umul128(a, b, &high); + return absl::MakeUint128(high, low); +#else + // uint128(a) * uint128(b) in emulated mode computes a full 128-bit x 128-bit + // multiply. However there are many cases where that is not necessary, and it + // is only necessary to support a 64-bit x 64-bit = 128-bit multiply. This is + // for those cases. + const uint64_t a00 = static_cast<uint32_t>(a); + const uint64_t a32 = a >> 32; + const uint64_t b00 = static_cast<uint32_t>(b); + const uint64_t b32 = b >> 32; + + const uint64_t c00 = a00 * b00; + const uint64_t c32a = a00 * b32; + const uint64_t c32b = a32 * b00; + const uint64_t c64 = a32 * b32; + + const uint32_t carry = + static_cast<uint32_t>(((c00 >> 32) + static_cast<uint32_t>(c32a) + + static_cast<uint32_t>(c32b)) >> + 32); + + return absl::MakeUint128(c64 + (c32a >> 32) + (c32b >> 32) + carry, + c00 + (c32a << 32) + (c32b << 32)); +#endif +} + +// wide_multiply<T> multiplies two N-bit values to a 2N-bit result. +template <typename UIntType> +struct wide_multiply { + static constexpr size_t kN = std::numeric_limits<UIntType>::digits; + using input_type = UIntType; + using result_type = typename random_internal::unsigned_bits<kN * 2>::type; + + static result_type multiply(input_type a, input_type b) { + return static_cast<result_type>(a) * b; + } + + static input_type hi(result_type r) { return r >> kN; } + static input_type lo(result_type r) { return r; } + + static_assert(std::is_unsigned<UIntType>::value, + "Class-template wide_multiply<> argument must be unsigned."); +}; + +#ifndef ABSL_HAVE_INTRINSIC_INT128 +template <> +struct wide_multiply<uint64_t> { + using input_type = uint64_t; + using result_type = absl::uint128; + + static result_type multiply(uint64_t a, uint64_t b) { + return MultiplyU64ToU128(a, b); + } + + static uint64_t hi(result_type r) { return absl::Uint128High64(r); } + static uint64_t lo(result_type r) { return absl::Uint128Low64(r); } +}; +#endif + +} // namespace random_internal +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_ |