libc/src/__support/FPUtil/nearest_integer.h

   1 //===-- Fast rounding to nearest integer for floating point -----*- 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_NEAREST_INTEGER_H
  10 #define LLVM_LIBC_SRC_SUPPORT_FPUTIL_NEAREST_INTEGER_H
  11
  12 #include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
  13 #include "src/__support/macros/properties/architectures.h"
  14 #include "src/__support/macros/properties/cpu_features.h"
  15
  16 #if (defined(LIBC_TARGET_ARCH_IS_X86_64) && defined(LIBC_TARGET_CPU_HAS_SSE4_2))
  17 #include "x86_64/nearest_integer.h"
  18 #elif defined(LIBC_TARGET_ARCH_IS_AARCH64)
  19 #include "aarch64/nearest_integer.h"
  20 #else
  21
  22 namespace __llvm_libc {
  23 namespace fputil {
  24
  25 // This is a fast implementation for rounding to a nearest integer that.
  26 //
  27 // Notice that for AARCH64 and x86-64 with SSE4.2 support, we will use their
  28 // corresponding rounding instruction instead.  And in those cases, the results
  29 // are rounded to the nearest integer, tie-to-even.
  30 LIBC_INLINE float nearest_integer(float x) {
  31   if (x < 0x1p24f && x > -0x1p24f) {
  32     float r = x < 0 ? (x - 0x1.0p23f) + 0x1.0p23f : (x + 0x1.0p23f) - 0x1.0p23f;
  33     float diff = x - r;
  34     // The expression above is correct for the default rounding mode, round-to-
  35     // nearest, tie-to-even.  For other rounding modes, it might be off by 1,
  36     // which is corrected below.
  37     if (LIBC_UNLIKELY(diff > 0.5f))
  38       return r + 1.0f;
  39     if (LIBC_UNLIKELY(diff < -0.5f))
  40       return r - 1.0f;
  41     return r;
  42   }
  43   return x;
  44 }
  45
  46 LIBC_INLINE double nearest_integer(double x) {
  47   if (x < 0x1p53 && x > -0x1p53) {
  48     double r = x < 0 ? (x - 0x1.0p52) + 0x1.0p52 : (x + 0x1.0p52) - 0x1.0p52;
  49     double diff = x - r;
  50     // The expression above is correct for the default rounding mode, round-to-
  51     // nearest, tie-to-even.  For other rounding modes, it might be off by 1,
  52     // which is corrected below.
  53     if (LIBC_UNLIKELY(diff > 0.5))
  54       return r + 1.0;
  55     if (LIBC_UNLIKELY(diff < -0.5))
  56       return r - 1.0;
  57     return r;
  58   }
  59   return x;
  60 }
  61
  62 } // namespace fputil
  63 } // namespace __llvm_libc
  64
  65 #endif
  66 #endif // LLVM_LIBC_SRC_SUPPORT_FPUTIL_NEAREST_INTEGER_H