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| 1 | //===-- include/flang/Evaluate/rounding-bits.h ------------------*- C++ -*-===// |
|---|---|
| 2 | // |
| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| 4 | // See https://llvm.org/LICENSE.txt for license information. |
| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| 6 | // |
| 7 | //===----------------------------------------------------------------------===// |
| 8 | |
| 9 | #ifndef FORTRAN_EVALUATE_ROUNDING_BITS_H_ |
| 10 | #define FORTRAN_EVALUATE_ROUNDING_BITS_H_ |
| 11 | |
| 12 | #include "flang/Evaluate/target.h" |
| 13 | |
| 14 | // A helper class used by Real<> to determine rounding of rational results |
| 15 | // to floating-point values. Bits lost from intermediate computations by |
| 16 | // being shifted rightward are accumulated in instances of this class. |
| 17 | |
| 18 | namespace Fortran::evaluate::value { |
| 19 | |
| 20 | class RoundingBits { |
| 21 | public: |
| 22 | constexpr RoundingBits( |
| 23 | bool guard = false, bool round = false, bool sticky = false) |
| 24 | : guard_{guard}, round_{round}, sticky_{sticky} {} |
| 25 | |
| 26 | template <typename FRACTION> |
| 27 | constexpr RoundingBits(const FRACTION &fraction, int rshift) { |
| 28 | if (rshift > 0 && rshift < fraction.bits + 1) { |
| 29 | guard_ = fraction.BTEST(rshift - 1); |
| 30 | } |
| 31 | if (rshift > 1 && rshift < fraction.bits + 2) { |
| 32 | round_ = fraction.BTEST(rshift - 2); |
| 33 | } |
| 34 | if (rshift > 2) { |
| 35 | if (rshift >= fraction.bits + 2) { |
| 36 | sticky_ = !fraction.IsZero(); |
| 37 | } else { |
| 38 | auto mask{fraction.MASKR(rshift - 2)}; |
| 39 | sticky_ = !fraction.IAND(mask).IsZero(); |
| 40 | } |
| 41 | } |
| 42 | } |
| 43 | |
| 44 | constexpr bool guard() const { return guard_; } |
| 45 | constexpr bool round() const { return round_; } |
| 46 | constexpr bool sticky() const { return sticky_; } |
| 47 | constexpr bool empty() const { return !(guard_ | round_ | sticky_); } |
| 48 | |
| 49 | constexpr bool Negate() { |
| 50 | bool carry{!sticky_}; |
| 51 | if (carry) { |
| 52 | carry = !round_; |
| 53 | } else { |
| 54 | round_ = !round_; |
| 55 | } |
| 56 | if (carry) { |
| 57 | carry = !guard_; |
| 58 | } else { |
| 59 | guard_ = !guard_; |
| 60 | } |
| 61 | return carry; |
| 62 | } |
| 63 | |
| 64 | constexpr bool ShiftLeft() { |
| 65 | bool oldGuard{guard_}; |
| 66 | guard_ = round_; |
| 67 | round_ = sticky_; |
| 68 | return oldGuard; |
| 69 | } |
| 70 | |
| 71 | constexpr void ShiftRight(bool newGuard) { |
| 72 | sticky_ |= round_; |
| 73 | round_ = guard_; |
| 74 | guard_ = newGuard; |
| 75 | } |
| 76 | |
| 77 | // Determines whether a value should be rounded by increasing its |
| 78 | // fraction, given a rounding mode and a summary of the lost bits. |
| 79 | constexpr bool MustRound( |
| 80 | Rounding rounding, bool isNegative, bool isOdd) const { |
| 81 | bool round{false}; // to dodge bogus g++ warning about missing return |
| 82 | switch (rounding.mode) { |
| 83 | case common::RoundingMode::TiesToEven: |
| 84 | round = guard_ && (round_ | sticky_ | isOdd); |
| 85 | break; |
| 86 | case common::RoundingMode::ToZero: |
| 87 | break; |
| 88 | case common::RoundingMode::Down: |
| 89 | round = isNegative && !empty(); |
| 90 | break; |
| 91 | case common::RoundingMode::Up: |
| 92 | round = !isNegative && !empty(); |
| 93 | break; |
| 94 | case common::RoundingMode::TiesAwayFromZero: |
| 95 | round = guard_; |
| 96 | break; |
| 97 | } |
| 98 | return round; |
| 99 | } |
| 100 | |
| 101 | private: |
| 102 | bool guard_{false}; // 0.5 * ulp (unit in lowest place) |
| 103 | bool round_{false}; // 0.25 * ulp |
| 104 | bool sticky_{false}; // true if any lesser-valued bit would be set |
| 105 | }; |
| 106 | } // namespace Fortran::evaluate::value |
| 107 | #endif // FORTRAN_EVALUATE_ROUNDING_BITS_H_ |
| 108 |
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