musl/src/math/rint.c

/* origin: FreeBSD /usr/src/lib/msun/src/s_rint.c */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */
/*
 * rint(x)
 * Return x rounded to integral value according to the prevailing
 * rounding mode.
 * Method:
 *      Using floating addition.
 * Exception:
 *      Inexact flag raised if x not equal to rint(x).
 */

#include "libm.h"

static const double
TWO52[2] = {
  4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
 -4.50359962737049600000e+15, /* 0xC3300000, 0x00000000 */
};

double rint(double x)
{
	int32_t i0,j0,sx;
	uint32_t i,i1;
	double w,t;

	EXTRACT_WORDS(i0, i1, x);
	// FIXME: signed shift
	sx = (i0>>31) & 1;
	j0 = ((i0>>20)&0x7ff) - 0x3ff;
	if (j0 < 20) {
		if (j0 < 0) {
			if (((i0&0x7fffffff)|i1) == 0)
				return x;
			i1 |= i0 & 0x0fffff;
			i0 &= 0xfffe0000;
			i0 |= ((i1|-i1)>>12) & 0x80000;
			SET_HIGH_WORD(x, i0);
			STRICT_ASSIGN(double, w, TWO52[sx] + x);
			t = w - TWO52[sx];
			GET_HIGH_WORD(i0, t);
			SET_HIGH_WORD(t, (i0&0x7fffffff)|(sx<<31));
			return t;
		} else {
			i = 0x000fffff>>j0;
			if (((i0&i)|i1) == 0)
				return x; /* x is integral */
			i >>= 1;
			if (((i0&i)|i1) != 0) {
				/*
				 * Some bit is set after the 0.5 bit.  To avoid the
				 * possibility of errors from double rounding in
				 * w = TWO52[sx]+x, adjust the 0.25 bit to a lower
				 * guard bit.  We do this for all j0<=51.  The
				 * adjustment is trickiest for j0==18 and j0==19
				 * since then it spans the word boundary.
				 */
				if (j0 == 19)
					i1 = 0x40000000;
				else if (j0 == 18)
					i1 = 0x80000000;
				else
					i0 = (i0 & ~i)|(0x20000>>j0);
			}
		}
	} else if (j0 > 51) {
		if (j0 == 0x400)
			return x+x;  /* inf or NaN */
		return x;            /* x is integral */
	} else {
		i = (uint32_t)0xffffffff>>(j0-20);
		if ((i1&i) == 0)
			return x;    /* x is integral */
		i >>= 1;
		if ((i1&i) != 0)
			i1 = (i1 & ~i)|(0x40000000>>(j0-20));
	}
	INSERT_WORDS(x, i0, i1);
	STRICT_ASSIGN(double, w, TWO52[sx] + x);
	return w - TWO52[sx];
}