Subversion Repositories openrisc_2011-10-31

/* 2009 for Newlib:  Sun's s_ilogb.c converted to be s_logb.c.  */

/* @(#)s_ilogb.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.

 * ====================================================

 */

/*

FUNCTION

       <<logb>>, <<logbf>>--get exponent of floating-point number

INDEX

        logb

INDEX

        logbf

ANSI_SYNOPSIS

        #include <math.h>

        double logb(double <[x]>);

        float logbf(float <[x]>);

DESCRIPTION

The <<logb>> functions extract the exponent of <[x]>, as a signed integer value

in floating-point format.  If <[x]> is subnormal it is treated as though it were

normalized; thus, for positive finite <[x]>,

@ifnottex

1 <= (<[x]> * FLT_RADIX to the power (-logb(<[x]>))) < FLT_RADIX.

@end ifnottex

@tex

$1 \leq ( x \cdot FLT\_RADIX ^ {-logb(x)} ) < FLT\_RADIX$.

@end tex

A domain error may occur if the argument is zero.

In this floating-point implementation, FLT_RADIX is 2.  Which also means

that for finite <[x]>, <<logb>>(<[x]>) = <<floor>>(<<log2>>(<<fabs>>(<[x]>))).

All nonzero, normal numbers can be described as

@ifnottex

<[m]> * 2**<[p]>, where 1.0 <= <[m]> < 2.0.

@end ifnottex

@tex

$m \cdot 2^p$, where $1.0 \leq m < 2.0$.

@end tex

The <<logb>> functions examine the argument <[x]>, and return <[p]>.

The <<frexp>> functions are similar to the <<logb>> functions, but

returning <[m]> adjusted to the interval [.5, 1) or 0, and <[p]>+1.

RETURNS

@comment Formatting note:  "$@" forces a new line

When <[x]> is:@*

+inf or -inf, +inf is returned;@*

NaN, NaN is returned;@*

0, -inf is returned, and the divide-by-zero exception is raised;@*

otherwise, the <<logb>> functions return the signed exponent of <[x]>.

PORTABILITY

ANSI C, POSIX

SEEALSO

frexp, ilogb

*/

/* double logb(double x)

 * return the binary exponent of non-zero x

 * logb(0) = -inf, raise divide-by-zero floating point exception

 * logb(+inf|-inf) = +inf (no signal is raised)

 * logb(NaN) = NaN (no signal is raised)

 * Per C99 recommendation, a NaN argument is returned unchanged.

 */

#include "fdlibm.h"

#ifndef _DOUBLE_IS_32BITS

double

#ifdef __STDC__

logb(double x)

#else

logb(x)

double x;

#endif

{

        __int32_t hx,lx,ix;

        EXTRACT_WORDS(hx,lx,x);

        hx &= 0x7fffffff;               /* high |x| */

        if(hx<0x00100000) {             /* 0 or subnormal */

            if((hx|lx)==0)  {

                double  xx;

                /* arg==0:  return -inf and raise divide-by-zero exception */

                INSERT_WORDS(xx,hx,lx); /* +0.0 */

                return -1./xx;  /* logb(0) = -inf */

                }

            else                        /* subnormal x */

                if(hx==0) {

                    for (ix = -1043; lx>0; lx<<=1) ix -=1;

                } else {

                    for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1;

                }

            return (double) ix;

        }

        else if (hx<0x7ff00000) return (hx>>20)-1023;   /* normal # */

        else if (hx>0x7ff00000 || lx)  return x;        /* x==NaN */

        else  return HUGE_VAL;  /* x==inf (+ or -) */

}

#endif /* _DOUBLE_IS_32BITS */