diff options
Diffstat (limited to 'absl/random/internal/distribution_impl.h')
| -rw-r--r-- | absl/random/internal/distribution_impl.h | 260 |
1 files changed, 260 insertions, 0 deletions
diff --git a/absl/random/internal/distribution_impl.h b/absl/random/internal/distribution_impl.h new file mode 100644 index 000000000000..9b6ffb0fb504 --- /dev/null +++ b/absl/random/internal/distribution_impl.h @@ -0,0 +1,260 @@ +// 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_DISTRIBUTION_IMPL_H_ +#define ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_ + +// This file contains some implementation details which are used by one or more +// of the absl random number distributions. + +#include <cfloat> +#include <cstddef> +#include <cstdint> +#include <cstring> +#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/fastmath.h" +#include "absl/random/internal/traits.h" + +namespace absl { +namespace random_internal { + +// Creates a double from `bits`, with the template fields controlling the +// output. +// +// RandU64To is both more efficient and generates more unique values in the +// result interval than known implementations of std::generate_canonical(). +// +// The `Signed` parameter controls whether positive, negative, or both are +// returned (thus affecting the output interval). +// When Signed == SignedValueT, range is U(-1, 1) +// When Signed == NegativeValueT, range is U(-1, 0) +// When Signed == PositiveValueT, range is U(0, 1) +// +// When the `IncludeZero` parameter is true, the function may return 0 for some +// inputs, otherwise it never returns 0. +// +// The `ExponentBias` parameter determines the scale of the output range by +// adjusting the exponent. +// +// When a value in U(0,1) is required, use: +// RandU64ToDouble<PositiveValueT, true, 0>(); +// +// When a value in U(-1,1) is required, use: +// RandU64ToDouble<SignedValueT, false, 0>() => U(-1, 1) +// This generates more distinct values than the mathematically equivalent +// expression `U(0, 1) * 2.0 - 1.0`, and is preferable. +// +// Scaling the result by powers of 2 (and avoiding a multiply) is also possible: +// RandU64ToDouble<PositiveValueT, false, 1>(); => U(0, 2) +// RandU64ToDouble<PositiveValueT, false, -1>(); => U(0, 0.5) +// + +// Tristate types controlling the output. +struct PositiveValueT {}; +struct NegativeValueT {}; +struct SignedValueT {}; + +// RandU64ToDouble is the double-result variant of RandU64To, described above. +template <typename Signed, bool IncludeZero, int ExponentBias = 0> +inline double RandU64ToDouble(uint64_t bits) { + static_assert(std::is_same<Signed, PositiveValueT>::value || + std::is_same<Signed, NegativeValueT>::value || + std::is_same<Signed, SignedValueT>::value, + ""); + + // Maybe use the left-most bit for a sign bit. + uint64_t sign = std::is_same<Signed, NegativeValueT>::value + ? 0x8000000000000000ull + : 0; // Sign bits. + + if (std::is_same<Signed, SignedValueT>::value) { + sign = bits & 0x8000000000000000ull; + bits = bits & 0x7FFFFFFFFFFFFFFFull; + } + if (IncludeZero) { + if (bits == 0u) return 0; + } + + // Number of leading zeros is mapped to the exponent: 2^-clz + int clz = base_internal::CountLeadingZeros64(bits); + // Shift number left to erase leading zeros. + bits <<= IncludeZero ? clz : (clz & 63); + + // Shift number right to remove bits that overflow double mantissa. The + // direction of the shift depends on `clz`. + bits >>= (64 - DBL_MANT_DIG); + + // Compute IEEE 754 double exponent. + // In the Signed case, bits is a 63-bit number with a 0 msb. Adjust the + // exponent to account for that. + const uint64_t exp = + (std::is_same<Signed, SignedValueT>::value ? 1023U : 1022U) + + static_cast<uint64_t>(ExponentBias - clz); + constexpr int kExp = DBL_MANT_DIG - 1; + // Construct IEEE 754 double from exponent and mantissa. + const uint64_t val = sign | (exp << kExp) | (bits & ((1ULL << kExp) - 1U)); + + double res; + static_assert(sizeof(res) == sizeof(val), "double is not 64 bit"); + // Memcpy value from "val" to "res" to avoid aliasing problems. Assumes that + // endian-ness is same for double and uint64_t. + std::memcpy(&res, &val, sizeof(res)); + + return res; +} + +// RandU64ToFloat is the float-result variant of RandU64To, described above. +template <typename Signed, bool IncludeZero, int ExponentBias = 0> +inline float RandU64ToFloat(uint64_t bits) { + static_assert(std::is_same<Signed, PositiveValueT>::value || + std::is_same<Signed, NegativeValueT>::value || + std::is_same<Signed, SignedValueT>::value, + ""); + + // Maybe use the left-most bit for a sign bit. + uint64_t sign = std::is_same<Signed, NegativeValueT>::value + ? 0x80000000ul + : 0; // Sign bits. + + if (std::is_same<Signed, SignedValueT>::value) { + uint64_t a = bits & 0x8000000000000000ull; + sign = static_cast<uint32_t>(a >> 32); + bits = bits & 0x7FFFFFFFFFFFFFFFull; + } + if (IncludeZero) { + if (bits == 0u) return 0; + } + + // Number of leading zeros is mapped to the exponent: 2^-clz + int clz = base_internal::CountLeadingZeros64(bits); + // Shift number left to erase leading zeros. + bits <<= IncludeZero ? clz : (clz & 63); + // Shift number right to remove bits that overflow double mantissa. The + // direction of the shift depends on `clz`. + bits >>= (64 - FLT_MANT_DIG); + + // Construct IEEE 754 float exponent. + // In the Signed case, bits is a 63-bit number with a 0 msb. Adjust the + // exponent to account for that. + const uint32_t exp = + (std::is_same<Signed, SignedValueT>::value ? 127U : 126U) + + static_cast<uint32_t>(ExponentBias - clz); + constexpr int kExp = FLT_MANT_DIG - 1; + const uint32_t val = sign | (exp << kExp) | (bits & ((1U << kExp) - 1U)); + + float res; + static_assert(sizeof(res) == sizeof(val), "float is not 32 bit"); + // Assumes that endian-ness is same for float and uint32_t. + std::memcpy(&res, &val, sizeof(res)); + + return res; +} + +template <typename Result> +struct RandU64ToReal { + template <typename Signed, bool IncludeZero, int ExponentBias = 0> + static inline Result Value(uint64_t bits) { + return RandU64ToDouble<Signed, IncludeZero, ExponentBias>(bits); + } +}; + +template <> +struct RandU64ToReal<float> { + template <typename Signed, bool IncludeZero, int ExponentBias = 0> + static inline float Value(uint64_t bits) { + return RandU64ToFloat<Signed, IncludeZero, ExponentBias>(bits); + } +}; + +inline uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) { +#if defined(ABSL_HAVE_INTRINSIC_INT128) + return 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 = uint128; + + static result_type multiply(uint64_t a, uint64_t b) { + return MultiplyU64ToU128(a, b); + } + + static uint64_t hi(result_type r) { return Uint128High64(r); } + static uint64_t lo(result_type r) { return Uint128Low64(r); } +}; +#endif + +} // namespace random_internal +} // namespace absl + +#endif // ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_ |