utils/FPUtil/FPBits.h

//===-- Abstract class for bit manipulation of float numbers. ---*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_UTILS_FPUTIL_FP_BITS_H
#define LLVM_LIBC_UTILS_FPUTIL_FP_BITS_H

#include "utils/CPP/TypeTraits.h"

#include <stdint.h>

namespace __llvm_libc {
namespace fputil {

template <typename T> struct MantissaWidth {};
template <> struct MantissaWidth<float> {
  static constexpr unsigned value = 23;
};
template <> struct MantissaWidth<double> {
  static constexpr unsigned value = 52;
};

template <typename T> struct ExponentWidth {};
template <> struct ExponentWidth<float> {
  static constexpr unsigned value = 8;
};
template <> struct ExponentWidth<double> {
  static constexpr unsigned value = 11;
};
template <> struct ExponentWidth<long double> {
  static constexpr unsigned value = 15;
};

template <typename T> struct FPUIntType {};
template <> struct FPUIntType<float> { using Type = uint32_t; };
template <> struct FPUIntType<double> { using Type = uint64_t; };

#if !(defined(__x86_64__) || defined(__i386__))
// TODO: This has to be extended for visual studio where long double and
// double are equivalent.
template <> struct MantissaWidth<long double> {
  static constexpr unsigned value = 112;
};

template <> struct FPUIntType<long double> { using Type = __uint128_t; };
#endif

// A generic class to represent single precision, double precision, and quad
// precision IEEE 754 floating point formats.
// On most platforms, the 'float' type corresponds to single precision floating
// point numbers, the 'double' type corresponds to double precision floating
// point numers, and the 'long double' type corresponds to the quad precision
// floating numbers. On x86 platforms however, the 'long double' type maps to
// an x87 floating point format. This format is an IEEE 754 extension format.
// It is handled as an explicit specialization of this class.
template <typename T> struct __attribute__((packed)) FPBits {
  static_assert(cpp::IsFloatingPointType<T>::Value,
                "FPBits instantiated with invalid type.");

  // Reinterpreting bits as an integer value and interpreting the bits of an
  // integer value as a floating point value is used in tests. So, a convenient
  // type is provided for such reinterpretations.
  using UIntType = typename FPUIntType<T>::Type;

  UIntType mantissa : MantissaWidth<T>::value;
  uint16_t exponent : ExponentWidth<T>::value;
  uint8_t sign : 1;

  static constexpr int exponentBias = (1 << (ExponentWidth<T>::value - 1)) - 1;
  static constexpr int maxExponent = (1 << ExponentWidth<T>::value) - 1;

  static constexpr UIntType minSubnormal = UIntType(1);
  static constexpr UIntType maxSubnormal =
      (UIntType(1) << MantissaWidth<T>::value) - 1;
  static constexpr UIntType minNormal =
      (UIntType(1) << MantissaWidth<T>::value);
  static constexpr UIntType maxNormal =
      ((UIntType(maxExponent) - 1) << MantissaWidth<T>::value) | maxSubnormal;

  // We don't want accidental type promotions/conversions so we require exact
  // type match.
  template <typename XType,
            cpp::EnableIfType<cpp::IsSame<T, XType>::Value ||
                                  (cpp::IsIntegral<XType>::Value &&
                                   (sizeof(XType) == sizeof(UIntType))),
                              int> = 0>
  explicit FPBits(XType x) {
    *this = *reinterpret_cast<FPBits<T> *>(&x);
  }

  operator T() { return *reinterpret_cast<T *>(this); }

  int getExponent() const { return int(exponent) - exponentBias; }

  bool isZero() const { return mantissa == 0 && exponent == 0; }

  bool isInf() const { return mantissa == 0 && exponent == maxExponent; }

  bool isNaN() const { return exponent == maxExponent && mantissa != 0; }

  bool isInfOrNaN() const { return exponent == maxExponent; }

  // Methods below this are used by tests.
  // The to and from integer bits converters are only used in tests. Hence,
  // the potential software implementations of UIntType will not slow real
  // code.

  UIntType bitsAsUInt() const {
    return *reinterpret_cast<const UIntType *>(this);
  }

  static FPBits<T> zero() { return FPBits(T(0.0)); }

  static FPBits<T> negZero() {
    FPBits<T> bits(T(0.0));
    bits.sign = 1;
    return bits;
  }

  static FPBits<T> inf() {
    FPBits<T> bits(T(0.0));
    bits.exponent = maxExponent;
    return bits;
  }

  static FPBits<T> negInf() {
    FPBits<T> bits(T(0.0));
    bits.exponent = maxExponent;
    bits.sign = 1;
    return bits;
  }

  static T buildNaN(UIntType v) {
    FPBits<T> bits(T(0.0));
    bits.exponent = maxExponent;
    bits.mantissa = v;
    return bits;
  }
};

} // namespace fputil
} // namespace __llvm_libc

#if defined(__x86_64__) || defined(__i386__)
#include "utils/FPUtil/LongDoubleBitsX86.h"
#endif

#endif // LLVM_LIBC_UTILS_FPUTIL_FP_BITS_H