libm/s_rint.c

   1 /*
   2  * ====================================================
   3  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
   4  *
   5  * Developed at SunPro, a Sun Microsystems, Inc. business.
   6  * Permission to use, copy, modify, and distribute this
   7  * software is freely granted, provided that this notice
   8  * is preserved.
   9  * ====================================================
  10  */
  11
  12 /*
  13  * rint(x)
  14  * Return x rounded to integral value according to the prevailing
  15  * rounding mode.
  16  * Method:
  17  *      Using floating addition.
  18  * Exception:
  19  *      Inexact flag raised if x not equal to rint(x).
  20  */
  21
  22 #include "math.h"
  23 #include "math_private.h"
  24
  25 static const double
  26 TWO52[2]={
  27   4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
  28  -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
  29 };
  30
  31 double rint(double x)
  32 {
  33         int32_t i0, _j0, sx;
  34         u_int32_t i,i1;
  35         double t;
  36         /* We use w = x + 2^52; t = w - 2^52; trick to round x to integer.
  37          * This trick requires that compiler does not optimize it
  38          * by keeping intermediate result w in a register wider than double.
  39          * Declaring w volatile assures that value gets truncated to double
  40          * (unfortunately, it also forces store+load):
  41          */
  42         volatile double w;
  43
  44         EXTRACT_WORDS(i0,i1,x);
  45         /* Unbiased exponent */
  46         _j0 = ((((u_int32_t)i0) >> 20)&0x7ff)-0x3ff;
  47
  48         if (_j0 > 51) {
  49                 //Why bother? Just returning x works too
  50                 //if (_j0 == 0x400)  /* inf or NaN */
  51                 //      return x+x;
  52                 return x;  /* x is integral */
  53         }
  54
  55         /* Sign */
  56         sx = ((u_int32_t)i0) >> 31;
  57
  58         if (_j0<20) {
  59             if (_j0<0) { /* |x| < 1 */
  60                 if (((i0&0x7fffffff)|i1)==0) return x;
  61                 i1 |= (i0&0x0fffff);
  62                 i0 &= 0xfffe0000;
  63                 i0 |= ((i1|-i1)>>12)&0x80000;
  64                 SET_HIGH_WORD(x,i0);
  65                 w = TWO52[sx]+x;
  66                 t = w-TWO52[sx];
  67                 GET_HIGH_WORD(i0,t);
  68                 SET_HIGH_WORD(t,(i0&0x7fffffff)|(sx<<31));
  69                 return t;
  70             } else {
  71                 i = (0x000fffff)>>_j0;
  72                 if (((i0&i)|i1)==0) return x; /* x is integral */
  73                 i>>=1;
  74                 if (((i0&i)|i1)!=0) {
  75                     if (_j0==19) i1 = 0x40000000;
  76                     else i0 = (i0&(~i))|((0x20000)>>_j0);
  77                 }
  78             }
  79         } else {
  80             i = ((u_int32_t)(0xffffffff))>>(_j0-20);
  81             if ((i1&i)==0) return x;    /* x is integral */
  82             i>>=1;
  83             if ((i1&i)!=0) i1 = (i1&(~i))|((0x40000000)>>(_j0-20));
  84         }
  85         INSERT_WORDS(x,i0,i1);
  86         w = TWO52[sx]+x;
  87         return w-TWO52[sx];
  88 }
  89 libm_hidden_def(rint)
  90
  91 strong_alias(rint, nearbyint)
  92 libm_hidden_def(nearbyint)