1//===---- Free-standing function to detect rounding mode --------*- 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 LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
10#define LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
11
12#include "hdr/fenv_macros.h"
13#include "src/__support/CPP/type_traits.h" // is_constant_evaluated
14#include "src/__support/macros/attributes.h" // LIBC_INLINE
15#include "src/__support/macros/config.h"
16
17namespace LIBC_NAMESPACE_DECL {
18namespace fputil {
19
20namespace generic {
21
22// Quick free-standing test whether fegetround() == FE_UPWARD.
23// Using the following observation:
24// 1.0f + 2^-25 = 1.0f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
25// = 0x1.000002f for FE_UPWARD.
26LIBC_INLINE bool fenv_is_round_up() {
27#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
28 return false;
29#else
30 static volatile float x = 0x1.0p-25f;
31 return (1.0f + x != 1.0f);
32#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
33}
34
35// Quick free-standing test whether fegetround() == FE_DOWNWARD.
36// Using the following observation:
37// -1.0f - 2^-25 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
38// = -0x1.000002f for FE_DOWNWARD.
39LIBC_INLINE bool fenv_is_round_down() {
40#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
41 return false;
42#else
43 static volatile float x = 0x1.0p-25f;
44 return (-1.0f - x != -1.0f);
45#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
46}
47
48// Quick free-standing test whether fegetround() == FE_TONEAREST.
49// Using the following observation:
50// 1.5f + 2^-24 = 1.5f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
51// = 0x1.100002p0f for FE_UPWARD,
52// 1.5f - 2^-24 = 1.5f for FE_TONEAREST, FE_UPWARD
53// = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
54LIBC_INLINE bool fenv_is_round_to_nearest() {
55#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
56 return true;
57#else
58 static volatile float x = 0x1.0p-24f;
59 float y = 1.5f + x;
60 return (y == 1.5f - x);
61#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
62}
63
64// Quick free-standing test whether fegetround() == FE_TOWARDZERO.
65// Using the following observation:
66// 1.0f + 2^-23 + 2^-24 = 0x1.000002p0f for FE_DOWNWARD, FE_TOWARDZERO
67// = 0x1.000004p0f for FE_TONEAREST, FE_UPWARD,
68// -1.0f - 2^-24 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
69// = -0x1.000002p0f for FE_DOWNWARD
70// So:
71// (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
72// = 2^-22 for FE_TONEAREST, FE_UPWARD
73// = 0 for FE_DOWNWARD
74LIBC_INLINE bool fenv_is_round_to_zero() {
75#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
76 return false;
77#else
78 static volatile float x = 0x1.0p-24f;
79 float y = x;
80 return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
81#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
82}
83
84// Quick free standing get rounding mode based on the above observations.
85LIBC_INLINE int quick_get_round() {
86#ifdef LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
87 return FE_TONEAREST;
88#else
89 static volatile float x = 0x1.0p-24f;
90 float y = x;
91 float z = (0x1.000002p0f + y) + (-1.0f - y);
92
93 if (z == 0.0f)
94 return FE_DOWNWARD;
95 if (z == 0x1.0p-23f)
96 return FE_TOWARDZERO;
97 return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
98#endif // LIBC_MATH_HAS_ASSUME_ROUND_NEAREST_ONLY
99}
100
101} // namespace generic
102
103LIBC_INLINE LIBC_CONSTEXPR_DEFAULT bool fenv_is_round_up() {
104 if (cpp::is_constant_evaluated()) {
105 return false;
106 } else {
107 return generic::fenv_is_round_up();
108 }
109}
110
111LIBC_INLINE LIBC_CONSTEXPR_DEFAULT bool fenv_is_round_down() {
112 if (cpp::is_constant_evaluated()) {
113 return false;
114 } else {
115 return generic::fenv_is_round_down();
116 }
117}
118
119LIBC_INLINE LIBC_CONSTEXPR_DEFAULT bool fenv_is_round_to_nearest() {
120 if (cpp::is_constant_evaluated()) {
121 return true;
122 } else {
123 return generic::fenv_is_round_to_nearest();
124 }
125}
126
127LIBC_INLINE LIBC_CONSTEXPR_DEFAULT bool fenv_is_round_to_zero() {
128 if (cpp::is_constant_evaluated()) {
129 return false;
130 } else {
131 return generic::fenv_is_round_to_zero();
132 }
133}
134
135// Quick free standing get rounding mode based on the above observations.
136LIBC_INLINE LIBC_CONSTEXPR_DEFAULT int quick_get_round() {
137 if (cpp::is_constant_evaluated()) {
138 return FE_TONEAREST;
139 } else {
140 return generic::quick_get_round();
141 }
142}
143
144} // namespace fputil
145} // namespace LIBC_NAMESPACE_DECL
146
147#endif // LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
148