llvm/flang/lib/evaluate/rounding-bits.h

104 lines
3.2 KiB
C++

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