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/macros/attributes.h" // LIBC_INLINE
14
15namespace LIBC_NAMESPACE::fputil {
16
17// Quick free-standing test whether fegetround() == FE_UPWARD.
18// Using the following observation:
19// 1.0f + 2^-25 = 1.0f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
20// = 0x1.000002f for FE_UPWARD.
21LIBC_INLINE bool fenv_is_round_up() {
22 volatile float x = 0x1.0p-25f;
23 return (1.0f + x != 1.0f);
24}
25
26// Quick free-standing test whether fegetround() == FE_DOWNWARD.
27// Using the following observation:
28// -1.0f - 2^-25 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
29// = -0x1.000002f for FE_DOWNWARD.
30LIBC_INLINE bool fenv_is_round_down() {
31 volatile float x = 0x1.0p-25f;
32 return (-1.0f - x != -1.0f);
33}
34
35// Quick free-standing test whether fegetround() == FE_TONEAREST.
36// Using the following observation:
37// 1.5f + 2^-24 = 1.5f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
38// = 0x1.100002p0f for FE_UPWARD,
39// 1.5f - 2^-24 = 1.5f for FE_TONEAREST, FE_UPWARD
40// = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
41LIBC_INLINE bool fenv_is_round_to_nearest() {
42 static volatile float x = 0x1.0p-24f;
43 float y = x;
44 return (1.5f + y == 1.5f - y);
45}
46
47// Quick free-standing test whether fegetround() == FE_TOWARDZERO.
48// Using the following observation:
49// 1.0f + 2^-23 + 2^-24 = 0x1.000002p0f for FE_DOWNWARD, FE_TOWARDZERO
50// = 0x1.000004p0f for FE_TONEAREST, FE_UPWARD,
51// -1.0f - 2^-24 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
52// = -0x1.000002p0f for FE_DOWNWARD
53// So:
54// (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
55// = 2^-22 for FE_TONEAREST, FE_UPWARD
56// = 0 for FE_DOWNWARD
57LIBC_INLINE bool fenv_is_round_to_zero() {
58 static volatile float x = 0x1.0p-24f;
59 float y = x;
60 return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
61}
62
63// Quick free standing get rounding mode based on the above observations.
64LIBC_INLINE int quick_get_round() {
65 static volatile float x = 0x1.0p-24f;
66 float y = x;
67 float z = (0x1.000002p0f + y) + (-1.0f - y);
68
69 if (z == 0.0f)
70 return FE_DOWNWARD;
71 if (z == 0x1.0p-23f)
72 return FE_TOWARDZERO;
73 return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
74}
75
76} // namespace LIBC_NAMESPACE::fputil
77
78#endif // LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
79

source code of libc/src/__support/FPUtil/rounding_mode.h