fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
111 lines
3.8 KiB
C++
111 lines
3.8 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/base/internal/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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// Helper object to multiply two 64-bit values to a 128-bit value.
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// MultiplyU64ToU128 multiplies two 64-bit values to a 128-bit value.
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// If an intrinsic is available, it is used, otherwise use native 32-bit
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// multiplies to construct the result.
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inline absl::uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
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#if defined(ABSL_HAVE_INTRINSIC_INT128)
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return absl::uint128(static_cast<__uint128_t>(a) * b);
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#elif defined(ABSL_INTERNAL_USE_UMUL128)
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// uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC.
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uint64_t high = 0;
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const uint64_t low = _umul128(a, b, &high);
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return absl::MakeUint128(high, low);
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#else
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// uint128(a) * uint128(b) in emulated mode computes a full 128-bit x 128-bit
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// multiply. However there are many cases where that is not necessary, and it
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// is only necessary to support a 64-bit x 64-bit = 128-bit multiply. This is
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// for those cases.
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const uint64_t a00 = static_cast<uint32_t>(a);
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const uint64_t a32 = a >> 32;
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const uint64_t b00 = static_cast<uint32_t>(b);
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const uint64_t b32 = b >> 32;
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const uint64_t c00 = a00 * b00;
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const uint64_t c32a = a00 * b32;
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const uint64_t c32b = a32 * b00;
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const uint64_t c64 = a32 * b32;
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const uint32_t carry =
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static_cast<uint32_t>(((c00 >> 32) + static_cast<uint32_t>(c32a) +
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static_cast<uint32_t>(c32b)) >>
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32);
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return absl::MakeUint128(c64 + (c32a >> 32) + (c32b >> 32) + carry,
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c00 + (c32a << 32) + (c32b << 32));
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#endif
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}
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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) { return r >> kN; }
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static input_type lo(result_type r) { return 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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#ifndef ABSL_HAVE_INTRINSIC_INT128
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template <>
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struct wide_multiply<uint64_t> {
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using input_type = uint64_t;
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using result_type = absl::uint128;
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static result_type multiply(uint64_t a, uint64_t b) {
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return MultiplyU64ToU128(a, b);
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}
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static uint64_t hi(result_type r) { return absl::Uint128High64(r); }
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static uint64_t lo(result_type r) { return absl::Uint128Low64(r); }
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};
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#endif
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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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