ldexp.c

/*	$OpenBSD: ldexp.c,v 1.10 2015/10/27 05:54:49 guenther Exp $	*/
/* @(#)s_scalbn.c 5.1 93/09/24 */
/* @(#)fdlibm.h 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.
 * ====================================================
 */

#include <sys/types.h>

#include "puffy.h"

/* Bit fiddling routines copied from msun/src/math_private.h,v 1.15 */

/* XXX: need big endian version */
typedef union
{
  double value;
  struct
  {
    uint32_t lsw;
    uint32_t msw;
  } parts;
} ieee_double_shape_type;

/* Get two 32 bit ints from a double.  */

#define EXTRACT_WORDS(ix0,ix1,d)				\
do {								\
  ieee_double_shape_type ew_u;					\
  ew_u.value = (d);						\
  (ix0) = ew_u.parts.msw;					\
  (ix1) = ew_u.parts.lsw;					\
} while (0)

/* Get the more significant 32 bit int from a double.  */

#define GET_HIGH_WORD(i,d)					\
do {								\
  ieee_double_shape_type gh_u;					\
  gh_u.value = (d);						\
  (i) = gh_u.parts.msw;						\
} while (0)

/* Set the more significant 32 bits of a double from an int.  */

#define SET_HIGH_WORD(d,v)					\
do {								\
  ieee_double_shape_type sh_u;					\
  sh_u.value = (d);						\
  sh_u.parts.msw = (v);						\
  (d) = sh_u.value;						\
} while (0)


static const double
two54   =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
twom54  =  5.55111512312578270212e-17, /* 0x3C900000, 0x00000000 */
huge   = 1.0e+300,
tiny   = 1.0e-300;

static double
_copysign(double x, double y)
{
	uint32_t hx,hy;
	GET_HIGH_WORD(hx,x);
	GET_HIGH_WORD(hy,y);
	SET_HIGH_WORD(x,(hx&0x7fffffff)|(hy&0x80000000));
	return x;
}

double
ldexp(double x, int n)
{
	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*_copysign(huge,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*_copysign(huge,x);	/*overflow*/
	    else return tiny*_copysign(tiny,x); 	/*underflow*/
	}
        k += 54;				/* subnormal result */
	SET_HIGH_WORD(x,(hx&0x800fffff)|(k<<20));
        return x*twom54;
}