1 // Copyright 2021 The Abseil Authors
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 // https://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 ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
16 #define ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
17
18 #include <limits>
19
20 #include "absl/base/config.h"
21
22 namespace absl {
23 ABSL_NAMESPACE_BEGIN
24 namespace numeric_internal {
25
26 // Returns true iff long double is represented as a pair of doubles added
27 // together.
IsDoubleDouble()28 inline constexpr bool IsDoubleDouble() {
29 // A double-double value always has exactly twice the precision of a double
30 // value--one double carries the high digits and one double carries the low
31 // digits. This property is not shared with any other common floating-point
32 // representation, so this test won't trigger false positives. For reference,
33 // this table gives the number of bits of precision of each common
34 // floating-point representation:
35 //
36 // type precision
37 // IEEE single 24 b
38 // IEEE double 53
39 // x86 long double 64
40 // double-double 106
41 // IEEE quadruple 113
42 //
43 // Note in particular that a quadruple-precision float has greater precision
44 // than a double-double float despite taking up the same amount of memory; the
45 // quad has more of its bits allocated to the mantissa than the double-double
46 // has.
47 return std::numeric_limits<long double>::digits ==
48 2 * std::numeric_limits<double>::digits;
49 }
50
51 } // namespace numeric_internal
52 ABSL_NAMESPACE_END
53 } // namespace absl
54
55 #endif // ABSL_NUMERIC_INTERNAL_REPRESENTATION_H_
56