libc/src/__support/FPUtil/rounding_mode.h

   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