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 static volatile float x = 0x1.0p-25f;
28 return (1.0f + x != 1.0f);
29}
30
31// Quick free-standing test whether fegetround() == FE_DOWNWARD.
32// Using the following observation:
33// -1.0f - 2^-25 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
34// = -0x1.000002f for FE_DOWNWARD.
35LIBC_INLINE bool fenv_is_round_down() {
36 static volatile float x = 0x1.0p-25f;
37 return (-1.0f - x != -1.0f);
38}
39
40// Quick free-standing test whether fegetround() == FE_TONEAREST.
41// Using the following observation:
42// 1.5f + 2^-24 = 1.5f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
43// = 0x1.100002p0f for FE_UPWARD,
44// 1.5f - 2^-24 = 1.5f for FE_TONEAREST, FE_UPWARD
45// = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
46LIBC_INLINE bool fenv_is_round_to_nearest() {
47 static volatile float x = 0x1.0p-24f;
48 float y = 1.5f + x;
49 return (y == 1.5f - x);
50}
51
52// Quick free-standing test whether fegetround() == FE_TOWARDZERO.
53// Using the following observation:
54// 1.0f + 2^-23 + 2^-24 = 0x1.000002p0f for FE_DOWNWARD, FE_TOWARDZERO
55// = 0x1.000004p0f for FE_TONEAREST, FE_UPWARD,
56// -1.0f - 2^-24 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
57// = -0x1.000002p0f for FE_DOWNWARD
58// So:
59// (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
60// = 2^-22 for FE_TONEAREST, FE_UPWARD
61// = 0 for FE_DOWNWARD
62LIBC_INLINE bool fenv_is_round_to_zero() {
63 static volatile float x = 0x1.0p-24f;
64 float y = x;
65 return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
66}
67
68// Quick free standing get rounding mode based on the above observations.
69LIBC_INLINE int quick_get_round() {
70 static volatile float x = 0x1.0p-24f;
71 float y = x;
72 float z = (0x1.000002p0f + y) + (-1.0f - y);
73
74 if (z == 0.0f)
75 return FE_DOWNWARD;
76 if (z == 0x1.0p-23f)
77 return FE_TOWARDZERO;
78 return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
79}
80
81} // namespace generic
82
83LIBC_INLINE static constexpr bool fenv_is_round_up() {
84 if (cpp::is_constant_evaluated()) {
85 return false;
86 } else {
87 return generic::fenv_is_round_up();
88 }
89}
90
91LIBC_INLINE static constexpr bool fenv_is_round_down() {
92 if (cpp::is_constant_evaluated()) {
93 return false;
94 } else {
95 return generic::fenv_is_round_down();
96 }
97}
98
99LIBC_INLINE static constexpr bool fenv_is_round_to_nearest() {
100 if (cpp::is_constant_evaluated()) {
101 return true;
102 } else {
103 return generic::fenv_is_round_to_nearest();
104 }
105}
106
107LIBC_INLINE static constexpr bool fenv_is_round_to_zero() {
108 if (cpp::is_constant_evaluated()) {
109 return false;
110 } else {
111 return generic::fenv_is_round_to_zero();
112 }
113}
114
115// Quick free standing get rounding mode based on the above observations.
116LIBC_INLINE static constexpr int quick_get_round() {
117 if (cpp::is_constant_evaluated()) {
118 return FE_TONEAREST;
119 } else {
120 return generic::quick_get_round();
121 }
122}
123
124} // namespace fputil
125} // namespace LIBC_NAMESPACE_DECL
126
127#endif // LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
128