sysdeps/ieee754/dbl-64/wordsize-64/s_roundeven.c

   1 /* Round to nearest integer value, rounding halfway cases to even.
   2    dbl-64/wordsize-64 version.
   3    Copyright (C) 2016 Free Software Foundation, Inc.
   4    This file is part of the GNU C Library.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 #include <math.h>
  21 #include <math_private.h>
  22 #include <stdint.h>
  23
  24 #define BIAS 0x3ff
  25 #define MANT_DIG 53
  26 #define MAX_EXP (2 * BIAS + 1)
  27
  28 double
  29 roundeven (double x)
  30 {
  31   uint64_t ix, ux;
  32   EXTRACT_WORDS64 (ix, x);
  33   ux = ix & 0x7fffffffffffffffULL;
  34   int exponent = ux >> (MANT_DIG - 1);
  35   if (exponent >= BIAS + MANT_DIG - 1)
  36     {
  37       /* Integer, infinity or NaN.  */
  38       if (exponent == MAX_EXP)
  39         /* Infinity or NaN; quiet signaling NaNs.  */
  40         return x + x;
  41       else
  42         return x;
  43     }
  44   else if (exponent >= BIAS)
  45     {
  46       /* At least 1; not necessarily an integer.  Locate the bits with
  47          exponents 0 and -1 (when the unbiased exponent is 0, the bit
  48          with exponent 0 is implicit, but as the bias is odd it is OK
  49          to take it from the low bit of the exponent).  */
  50       int int_pos = (BIAS + MANT_DIG - 1) - exponent;
  51       int half_pos = int_pos - 1;
  52       uint64_t half_bit = 1ULL << half_pos;
  53       uint64_t int_bit = 1ULL << int_pos;
  54       if ((ix & (int_bit | (half_bit - 1))) != 0)
  55         /* Carry into the exponent works correctly.  No need to test
  56            whether HALF_BIT is set.  */
  57         ix += half_bit;
  58       ix &= ~(int_bit - 1);
  59     }
  60   else if (exponent == BIAS - 1 && ux > 0x3fe0000000000000ULL)
  61     /* Interval (0.5, 1).  */
  62     ix = (ix & 0x8000000000000000ULL) | 0x3ff0000000000000ULL;
  63   else
  64     /* Rounds to 0.  */
  65     ix &= 0x8000000000000000ULL;
  66   INSERT_WORDS64 (x, ix);
  67   return x;
  68 }
  69 hidden_def (roundeven)
  70 #ifdef NO_LONG_DOUBLE
  71 weak_alias (roundeven, roundevenl)
  72 #endif