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 16 namespace LIBC_NAMESPACE_DECL { 17 namespace 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. 23 LIBC_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. 32 LIBC_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 43 LIBC_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 59 LIBC_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. 66 LIBC_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