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 "src/__support/macros/attributes.h" // LIBC_INLINE
  13
  14 #include <fenv.h>
  15
  16 namespace __llvm_libc::fputil {
  17
  18 // Quick free-standing test whether fegetround() == FE_UPWARD.
  19 // Using the following observation:
  20 //   1.0f + 2^-25 = 1.0f        for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
  21 //                = 0x1.000002f for FE_UPWARD.
  22 LIBC_INLINE bool fenv_is_round_up() {
  23   volatile float x = 0x1.0p-25f;
  24   return (1.0f + x != 1.0f);
  25 }
  26
  27 // Quick free-standing test whether fegetround() == FE_DOWNWARD.
  28 // Using the following observation:
  29 //   -1.0f - 2^-25 = -1.0f        for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
  30 //                 = -0x1.000002f for FE_DOWNWARD.
  31 LIBC_INLINE bool fenv_is_round_down() {
  32   volatile float x = 0x1.0p-25f;
  33   return (-1.0f - x != -1.0f);
  34 }
  35
  36 // Quick free-standing test whether fegetround() == FE_TONEAREST.
  37 // Using the following observation:
  38 //   1.5f + 2^-24 = 1.5f           for FE_TONEAREST, FE_DOWNWARD, FE_TOWARDZERO
  39 //                = 0x1.100002p0f  for FE_UPWARD,
  40 //   1.5f - 2^-24 = 1.5f           for FE_TONEAREST, FE_UPWARD
  41 //                = 0x1.0ffffep-1f for FE_DOWNWARD, FE_TOWARDZERO
  42 LIBC_INLINE bool fenv_is_round_to_nearest() {
  43   static volatile float x = 0x1.0p-24f;
  44   float y = x;
  45   return (1.5f + y == 1.5f - y);
  46 }
  47
  48 // Quick free-standing test whether fegetround() == FE_TOWARDZERO.
  49 // Using the following observation:
  50 //   1.0f + 2^-23 + 2^-24 = 0x1.000002p0f for FE_DOWNWARD, FE_TOWARDZERO
  51 //                        = 0x1.000004p0f for FE_TONEAREST, FE_UPWARD,
  52 //  -1.0f - 2^-24 = -1.0f          for FE_TONEAREST, FE_UPWARD, FE_TOWARDZERO
  53 //                = -0x1.000002p0f for FE_DOWNWARD
  54 // So:
  55 // (0x1.000002p0f + 2^-24) + (-1.0f - 2^-24) = 2^-23 for FE_TOWARDZERO
  56 //                                           = 2^-22 for FE_TONEAREST, FE_UPWARD
  57 //                                           = 0 for FE_DOWNWARD
  58 LIBC_INLINE bool fenv_is_round_to_zero() {
  59   static volatile float x = 0x1.0p-24f;
  60   float y = x;
  61   return ((0x1.000002p0f + y) + (-1.0f - y) == 0x1.0p-23f);
  62 }
  63
  64 // Quick free standing get rounding mode based on the above observations.
  65 LIBC_INLINE int quick_get_round() {
  66   static volatile float x = 0x1.0p-24f;
  67   float y = x;
  68   float z = (0x1.000002p0f + y) + (-1.0f - y);
  69
  70   if (z == 0.0f)
  71     return FE_DOWNWARD;
  72   if (z == 0x1.0p-23f)
  73     return FE_TOWARDZERO;
  74   return (2.0f + y == 2.0f) ? FE_TONEAREST : FE_UPWARD;
  75 }
  76
  77 } // namespace __llvm_libc::fputil
  78
  79 #endif // LLVM_LIBC_SRC_SUPPORT_FPUTIL_ROUNDING_MODE_H