1df60f3ceb
Original-commit: flang-compiler/f18@a34afdc73b Reviewed-on: https://github.com/flang-compiler/f18/pull/231 Tree-same-pre-rewrite: false
104 lines
3.2 KiB
C++
104 lines
3.2 KiB
C++
// Copyright (c) 2018, NVIDIA CORPORATION. All rights reserved.
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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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// http://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 FORTRAN_EVALUATE_ROUNDING_BITS_H_
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#define FORTRAN_EVALUATE_ROUNDING_BITS_H_
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// A helper class used by Real<> to determine rounding of rational results
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// to floating-point values. Bits lost from intermediate computations by
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// being shifted rightward are accumulated in instances of this class.
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namespace Fortran::evaluate::value {
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class RoundingBits {
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public:
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constexpr RoundingBits(
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bool guard = false, bool round = false, bool sticky = false)
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: guard_{guard}, round_{round}, sticky_{sticky} {}
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template<typename FRACTION>
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constexpr RoundingBits(const FRACTION &fraction, int rshift) {
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if (rshift > 0 && rshift < fraction.bits + 1) {
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guard_ = fraction.BTEST(rshift - 1);
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}
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if (rshift > 1 && rshift < fraction.bits + 2) {
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round_ = fraction.BTEST(rshift - 2);
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}
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if (rshift > 2) {
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if (rshift >= fraction.bits + 2) {
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sticky_ = !fraction.IsZero();
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} else {
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auto mask{fraction.MASKR(rshift - 2)};
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sticky_ = !fraction.IAND(mask).IsZero();
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}
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}
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}
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constexpr bool guard() const { return guard_; }
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constexpr bool round() const { return round_; }
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constexpr bool sticky() const { return sticky_; }
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constexpr bool empty() const { return !(guard_ | round_ | sticky_); }
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constexpr bool Negate() {
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bool carry{!sticky_};
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if (carry) {
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carry = !round_;
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} else {
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round_ = !round_;
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}
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if (carry) {
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carry = !guard_;
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} else {
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guard_ = !guard_;
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}
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return carry;
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}
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constexpr bool ShiftLeft() {
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bool oldGuard{guard_};
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guard_ = round_;
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round_ = sticky_;
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return oldGuard;
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}
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constexpr void ShiftRight(bool newGuard) {
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sticky_ |= round_;
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round_ = guard_;
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guard_ = newGuard;
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}
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// Determines whether a value should be rounded by increasing its
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// fraction, given a rounding mode and a summary of the lost bits.
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constexpr bool MustRound(
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Rounding rounding, bool isNegative, bool isOdd) const {
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bool round{false}; // to dodge bogus g++ warning about missing return
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switch (rounding) {
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case Rounding::TiesToEven:
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round = guard_ && (round_ | sticky_ | isOdd);
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break;
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case Rounding::ToZero: break;
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case Rounding::Down: round = isNegative && !empty(); break;
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case Rounding::Up: round = !isNegative && !empty(); break;
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case Rounding::TiesAwayFromZero: round = guard_; break;
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}
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return round;
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}
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private:
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bool guard_{false}; // 0.5 * ulp (unit in lowest place)
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bool round_{false}; // 0.25 * ulp
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bool sticky_{false}; // true if any lesser-valued bit would be set
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};
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}
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#endif // FORTRAN_EVALUATE_ROUNDING_BITS_H_
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