s_scalbn.c

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/* @(#)s_scalbn.c 5.1 93/09/24 */
/*
 * ====================================================
 * 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.
 * ====================================================
 */

#if defined(LIBM_SCCS) && !defined(lint)
static const char rcsid[] =
    "$NetBSD: s_scalbn.c,v 1.8 1995/05/10 20:48:08 jtc Exp $";
#endif

/*
 * scalbn (double x, int n)
 * scalbn(x,n) returns x* 2**n  computed by  exponent
 * manipulation rather than by actually performing an
 * exponentiation or a multiplication.
 */

#include "math_libm.h"
#include "math_private.h"

libm_hidden_proto(copysign)
#ifdef __STDC__
     static const double
#else
     static double
#endif
       two54 = 1.80143985094819840000e+16,      /* 0x43500000, 0x00000000 */
         twom54 = 5.55111512312578270212e-17,   /* 0x3C900000, 0x00000000 */
         huge_val = 1.0e+300, tiny = 1.0e-300;

libm_hidden_proto(scalbn)
#ifdef __STDC__
     double scalbn(double x, int n)
#else
     double scalbn(x, n)
     double x;
     int n;
#endif
{
    int32_t k, hx, lx;
    EXTRACT_WORDS(hx, lx, x);
    k = (hx & 0x7ff00000) >> 20;        /* extract exponent */
    if (k == 0) {               /* 0 or subnormal x */
        if ((lx | (hx & 0x7fffffff)) == 0)
            return x;           /* +-0 */
        x *= two54;
        GET_HIGH_WORD(hx, x);
        k = ((hx & 0x7ff00000) >> 20) - 54;
        if (n < -50000)
            return tiny * x;    /* underflow */
    }
    if (k == 0x7ff)
        return x + x;           /* NaN or Inf */
    k = k + n;
    if (k > 0x7fe)
        return huge_val * copysign(huge_val, x);        /* overflow  */
    if (k > 0) {                /* normal result */
        SET_HIGH_WORD(x, (hx & 0x800fffff) | (k << 20));
        return x;
    }
    if (k <= -54) {
        if (n > 50000)          /* in case integer overflow in n+k */
            return huge_val * copysign(huge_val, x);    /* overflow */
        else
            return tiny * copysign(tiny, x);    /* underflow */
    }
    k += 54;                    /* subnormal result */
    SET_HIGH_WORD(x, (hx & 0x800fffff) | (k << 20));
    return x * twom54;
}

libm_hidden_def(scalbn)