1 // Copyright (c) 2018, NVIDIA CORPORATION. All rights reserved. 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // http://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 15 #ifndef FORTRAN_EVALUATE_ROUNDING_BITS_H_ 16 #define FORTRAN_EVALUATE_ROUNDING_BITS_H_ 17 18 // A helper class used by Real<> to determine rounding of rational results 19 // to floating-point values. Bits lost from intermediate computations by 20 // being shifted rightward are accumulated in instances of this class. 21 22 namespace Fortran::evaluate::value { 23 24 class RoundingBits { 25 public: 26 constexpr RoundingBits( 27 bool guard = false, bool round = false, bool sticky = false) 28 : guard_{guard}, round_{round}, sticky_{sticky} {} 29 30 template<typename FRACTION> RoundingBits(const FRACTION & fraction,int rshift)31 constexpr RoundingBits(const FRACTION &fraction, int rshift) { 32 if (rshift > 0 && rshift < fraction.bits + 1) { 33 guard_ = fraction.BTEST(rshift - 1); 34 } 35 if (rshift > 1 && rshift < fraction.bits + 2) { 36 round_ = fraction.BTEST(rshift - 2); 37 } 38 if (rshift > 2) { 39 if (rshift >= fraction.bits + 2) { 40 sticky_ = !fraction.IsZero(); 41 } else { 42 auto mask{fraction.MASKR(rshift - 2)}; 43 sticky_ = !fraction.IAND(mask).IsZero(); 44 } 45 } 46 } 47 guard()48 constexpr bool guard() const { return guard_; } round()49 constexpr bool round() const { return round_; } sticky()50 constexpr bool sticky() const { return sticky_; } empty()51 constexpr bool empty() const { return !(guard_ | round_ | sticky_); } 52 Negate()53 constexpr bool Negate() { 54 bool carry{!sticky_}; 55 if (carry) { 56 carry = !round_; 57 } else { 58 round_ = !round_; 59 } 60 if (carry) { 61 carry = !guard_; 62 } else { 63 guard_ = !guard_; 64 } 65 return carry; 66 } 67 ShiftLeft()68 constexpr bool ShiftLeft() { 69 bool oldGuard{guard_}; 70 guard_ = round_; 71 round_ = sticky_; 72 return oldGuard; 73 } 74 ShiftRight(bool newGuard)75 constexpr void ShiftRight(bool newGuard) { 76 sticky_ |= round_; 77 round_ = guard_; 78 guard_ = newGuard; 79 } 80 81 // Determines whether a value should be rounded by increasing its 82 // fraction, given a rounding mode and a summary of the lost bits. MustRound(Rounding rounding,bool isNegative,bool isOdd)83 constexpr bool MustRound( 84 Rounding rounding, bool isNegative, bool isOdd) const { 85 bool round{false}; // to dodge bogus g++ warning about missing return 86 switch (rounding.mode) { 87 case RoundingMode::TiesToEven: 88 round = guard_ && (round_ | sticky_ | isOdd); 89 break; 90 case RoundingMode::ToZero: break; 91 case RoundingMode::Down: round = isNegative && !empty(); break; 92 case RoundingMode::Up: round = !isNegative && !empty(); break; 93 case RoundingMode::TiesAwayFromZero: round = guard_; break; 94 } 95 return round; 96 } 97 98 private: 99 bool guard_{false}; // 0.5 * ulp (unit in lowest place) 100 bool round_{false}; // 0.25 * ulp 101 bool sticky_{false}; // true if any lesser-valued bit would be set 102 }; 103 } 104 #endif // FORTRAN_EVALUATE_ROUNDING_BITS_H_ 105