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#include "src/__support/macros/config.h"
15
16namespace LIBC_NAMESPACE_DECL {
17namespace fputil {
18
19// Quick free-standing test whether fegetround() == FE_UPWARD.
20// Using the following observation:
21// 1.0f + 2^-25 = 1.0f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
22// = 0x1.000002f for FE_UPWARD.
23LIBC_INLINE bool fenv_is_round_up() {
24 volatile float x = 0x1.0p-25f;
25 return (1.0f + x != 1.0f);
26}
27
28// Quick free-standing test whether fegetround() == FE_DOWNWARD.
29// Using the following observation:
30// -1.0f - 2^-25 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
31// = -0x1.000002f for FE_DOWNWARD.
32LIBC_INLINE bool fenv_is_round_down() {
33 volatile float x = 0x1.0p-25f;
34 return (-1.0f - x != -1.0f);
35}
36
37// Quick free-standing test whether fegetround() == FE_TONEAREST.
38// Using the following observation:
39// 1.5f + 2^-24 = 1.5f for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
40// = 0x1.100002p0f for FE_UPWARD,
41// 1.5f - 2^-24 = 1.5f for FE_TONEAREST, FE_UPWARD
42// = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
43LIBC_INLINE bool fenv_is_round_to_nearest() {
44 static volatile float x = 0x1.0p-24f;
45 float y = x;
46 return (1.5f + y == 1.5f - y);
47}
48
49// Quick free-standing test whether fegetround() == FE_TOWARDZERO.
50// Using the following observation:
51// 1.0f + 2^-23 + 2^-24 = 0x1.000002p0f for FE_DOWNWARD, FE_TOWARDZERO
52// = 0x1.000004p0f for FE_TONEAREST, FE_UPWARD,
53// -1.0f - 2^-24 = -1.0f for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
54// = -0x1.000002p0f for FE_DOWNWARD
55// So:
56// (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
57// = 2^-22 for FE_TONEAREST, FE_UPWARD
58// = 0 for FE_DOWNWARD
59LIBC_INLINE bool fenv_is_round_to_zero() {
60 static volatile float x = 0x1.0p-24f;
61 float y = x;
62 return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
63}
64
65// Quick free standing get rounding mode based on the above observations.
66LIBC_INLINE int quick_get_round() {
67 static volatile float x = 0x1.0p-24f;
68 float y = x;
69 float z = (0x1.000002p0f + y) + (-1.0f - y);
70
71 if (z == 0.0f)
72 return FE_DOWNWARD;
73 if (z == 0x1.0p-23f)
74 return FE_TOWARDZERO;
75 return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
76}
77
78} // namespace fputil
79} // namespace LIBC_NAMESPACE_DECL
80
81#endif // LLVM_LIBC_SRC___SUPPORT_FPUTIL_ROUNDING_MODE_H
82