double complex	clog(double complex);

float complex	clogf(float complex);

long double complex

		clogl(long double complex);

MLINKS+=clog.3 clogf.3 clog.3 clogl.3

	clog;

	clogf;

	clogl;

.\" Copyright (c) 2017 Steven G. Kargl <kargl@FreeBSD.org>

.\" All rights reserved.

.\"

.\" Redistribution and use in source and binary forms, with or without

.\" modification, are permitted provided that the following conditions

.\" are met:

.\" 1. Redistributions of source code must retain the above copyright

.\"    notice, this list of conditions and the following disclaimer.

.\" 2. Redistributions in binary form must reproduce the above copyright

.\"    notice, this list of conditions and the following disclaimer in the

.\"    documentation and/or other materials provided with the distribution.

.\"

.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND

.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE

.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

.\" SUCH DAMAGE.

.\"

.\" $FreeBSD$

.\"

.Dd February 13, 2017

.Dt CLOG 3

.Os

.Sh NAME

.Nm clog ,

.Nm clogf ,

and

.Nm clogl

.Nd complex natural logrithm functions

.Sh LIBRARY

.Lb libm

.Sh SYNOPSIS

.In complex.h

.Ft double complex

.Fn clog "double complex z"

.Ft float complex

.Fn clogf "float complex z"

.Ft long double complex

.Fn clogl "long double complex z"

.Sh DESCRIPTION

The

.Fn clog ,

.Fn clogf ,

and 

.Fn clogl

functions compute the complex natural logrithm of

.Fa z .

with a branch cut along the negative real axis .

.Sh RETURN VALUES

The

.Fn clog

function returns the complex natural logarithm value, in the

range of a strip mathematically unbounded along the real axis and in

the interval [-I* \*(Pi , +I* \*(Pi ] along the imaginary axis.

The function satisfies the relationship: 

.Fo clog

.Fn conj "z" Fc

=

.Fo conj

.Fn clog "z" Fc .

.Pp

.\" Table is formatted for an 80-column xterm.

.Bl -column ".Sy +\*(If + I*\*(Na" ".Sy Return value" ".Sy Divide-by-zero exception"

.It Sy Argument          Ta Sy Return value Ta Sy Comment

.It -0 + I*0             Ta -\*(If + I*\*(Pi    Ta Divide-by-zero exception

.It                      Ta                     Ta raised

.It +0 + I*0             Ta -\*(If + I*0        Ta Divide by zero exception

.It                      Ta                     Ta raised

.It x + I*\*(If          Ta +\*(If + I*\*(Pi/2  Ta For finite x

.It x + I*\*(Na          Ta  \*(Na + I*\*(Na    Ta Optionally raises invalid

.It                      Ta                     Ta floating-point exception

.It                      Ta                     Ta for finite x

.It -\*(If + I*y         Ta +\*(If + I*\*(Pi    Ta For finite positive-signed y

.It +\*(If + I*y         Ta +\*(If + I*0        Ta For finite positive-signed y

.It -\*(If + I*\*(If     Ta +\*(If + I*3\*(Pi/4

.It +\*(If + I*\*(If     Ta +\*(If + I*\*(Pi/4

.It \*(Pm\*(If + I*\*(Na Ta +\*(If + I*\*(Na

.It \*(Na + I*y          Ta \*(Na + I*\*(Na    Ta Optionally raises invalid

.It                      Ta                    Ta floating-point exception

.It                      Ta                    Ta for finite y

.It \*(Na + I*\*(If      Ta +\*(If + I*\*(Na

.It \*(Na + I*\*(Na      Ta \*(Na + I*\*(Na 

.El

.Sh SEE ALSO

.Xr complex 3 ,

.Xr log 3 ,

.Xr math 3

.Sh STANDARDS

The

.Fn clog ,

.Fn cexpf ,

and

.Fn clogl

functions conform to

.St -isoC-99 .

.Ss Natural logrithm Function

.Cl

clog	natural logrithm

.El

/*-

 * Copyright (c) 2013 Bruce D. Evans

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice unmodified, this list of conditions, and the following

 *    disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

#include <sys/cdefs.h>

__FBSDID("$FreeBSD$");

#include <complex.h>

#include <float.h>

#include "fpmath.h"

#include "math.h"

#include "math_private.h"

#define	MANT_DIG	DBL_MANT_DIG

#define	MAX_EXP		DBL_MAX_EXP

#define	MIN_EXP		DBL_MIN_EXP

static const double

ln2_hi = 6.9314718055829871e-1,		/*  0x162e42fefa0000.0p-53 */

ln2_lo = 1.6465949582897082e-12;	/*  0x1cf79abc9e3b3a.0p-92 */

double complex

clog(double complex z)

{

	double_t ax, ax2h, ax2l, axh, axl, ay, ay2h, ay2l, ayh, ayl, sh, sl, t;

	double x, y, v;

	uint32_t hax, hay;

	int kx, ky;

	x = creal(z);

	y = cimag(z);

	v = atan2(y, x);

	ax = fabs(x);

	ay = fabs(y);

	if (ax < ay) {

		t = ax;

		ax = ay;

		ay = t;

	}

	GET_HIGH_WORD(hax, ax);

	kx = (hax >> 20) - 1023;

	GET_HIGH_WORD(hay, ay);

	ky = (hay >> 20) - 1023;

	/* Handle NaNs and Infs using the general formula. */

	if (kx == MAX_EXP || ky == MAX_EXP)

		return (CMPLX(log(hypot(x, y)), v));

	/* Avoid spurious underflow, and reduce inaccuracies when ax is 1. */

	if (ax == 1) {

		if (ky < (MIN_EXP - 1) / 2)

			return (CMPLX((ay / 2) * ay, v));

		return (CMPLX(log1p(ay * ay) / 2, v));

	}

	/* Avoid underflow when ax is not small.  Also handle zero args. */

	if (kx - ky > MANT_DIG || ay == 0)

		return (CMPLX(log(ax), v));

	/* Avoid overflow. */

	if (kx >= MAX_EXP - 1)

		return (CMPLX(log(hypot(x * 0x1p-1022, y * 0x1p-1022)) +

		    (MAX_EXP - 2) * ln2_lo + (MAX_EXP - 2) * ln2_hi, v));

	if (kx >= (MAX_EXP - 1) / 2)

		return (CMPLX(log(hypot(x, y)), v));

	/* Reduce inaccuracies and avoid underflow when ax is denormal. */

	if (kx <= MIN_EXP - 2)

		return (CMPLX(log(hypot(x * 0x1p1023, y * 0x1p1023)) +

		    (MIN_EXP - 2) * ln2_lo + (MIN_EXP - 2) * ln2_hi, v));

	/* Avoid remaining underflows (when ax is small but not denormal). */

	if (ky < (MIN_EXP - 1) / 2 + MANT_DIG)

		return (CMPLX(log(hypot(x, y)), v));

	/* Calculate ax*ax and ay*ay exactly using Dekker's algorithm. */

	t = (double)(ax * (0x1p27 + 1));

	axh = (double)(ax - t) + t;

	axl = ax - axh;

	ax2h = ax * ax;

	ax2l = axh * axh - ax2h + 2 * axh * axl + axl * axl;

	t = (double)(ay * (0x1p27 + 1));

	ayh = (double)(ay - t) + t;

	ayl = ay - ayh;

	ay2h = ay * ay;

	ay2l = ayh * ayh - ay2h + 2 * ayh * ayl + ayl * ayl;

	/*

	 * When log(|z|) is far from 1, accuracy in calculating the sum

	 * of the squares is not very important since log() reduces

	 * inaccuracies.  We depended on this to use the general

	 * formula when log(|z|) is very far from 1.  When log(|z|) is

	 * moderately far from 1, we go through the extra-precision

	 * calculations to reduce branches and gain a little accuracy.

	 *

	 * When |z| is near 1, we subtract 1 and use log1p() and don't

	 * leave it to log() to subtract 1, since we gain at least 1 bit

	 * of accuracy in this way.

	 *

	 * When |z| is very near 1, subtracting 1 can cancel almost

	 * 3*MANT_DIG bits.  We arrange that subtracting 1 is exact in

	 * doubled precision, and then do the rest of the calculation

	 * in sloppy doubled precision.  Although large cancellations

	 * often lose lots of accuracy, here the final result is exact

	 * in doubled precision if the large calculation occurs (because

	 * then it is exact in tripled precision and the cancellation

	 * removes enough bits to fit in doubled precision).  Thus the

	 * result is accurate in sloppy doubled precision, and the only

	 * significant loss of accuracy is when it is summed and passed

	 * to log1p().

	 */

	sh = ax2h;

	sl = ay2h;

	_2sumF(sh, sl);

	if (sh < 0.5 || sh >= 3)

		return (CMPLX(log(ay2l + ax2l + sl + sh) / 2, v));

	sh -= 1;

	_2sum(sh, sl);

	_2sum(ax2l, ay2l);

	/* Briggs-Kahan algorithm (except we discard the final low term): */

	_2sum(sh, ax2l);

	_2sum(sl, ay2l);

	t = ax2l + sl;

	_2sumF(sh, t);

	return (CMPLX(log1p(ay2l + t + sh) / 2, v));

}

#if (LDBL_MANT_DIG == 53)

__weak_reference(clog, clogl);

#endif

/*-

 * Copyright (c) 2013 Bruce D. Evans

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice unmodified, this list of conditions, and the following

 *    disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

#include <sys/cdefs.h>

__FBSDID("$FreeBSD$");

#include <complex.h>

#include <float.h>

#include "fpmath.h"

#include "math.h"

#include "math_private.h"

#define	MANT_DIG	FLT_MANT_DIG

#define	MAX_EXP		FLT_MAX_EXP

#define	MIN_EXP		FLT_MIN_EXP

static const float

ln2f_hi =  6.9314575195e-1,		/*  0xb17200.0p-24 */

ln2f_lo =  1.4286067653e-6;		/*  0xbfbe8e.0p-43 */

float complex

clogf(float complex z)

{

	float_t ax, ax2h, ax2l, axh, axl, ay, ay2h, ay2l, ayh, ayl, sh, sl, t;

	float x, y, v;

	uint32_t hax, hay;

	int kx, ky;

	x = crealf(z);

	y = cimagf(z);

	v = atan2f(y, x);

	ax = fabsf(x);

	ay = fabsf(y);

	if (ax < ay) {

		t = ax;

		ax = ay;

		ay = t;

	}

	GET_FLOAT_WORD(hax, ax);

	kx = (hax >> 23) - 127;

	GET_FLOAT_WORD(hay, ay);

	ky = (hay >> 23) - 127;

	/* Handle NaNs and Infs using the general formula. */

	if (kx == MAX_EXP || ky == MAX_EXP)

		return (CMPLXF(logf(hypotf(x, y)), v));

	/* Avoid spurious underflow, and reduce inaccuracies when ax is 1. */

	if (hax == 0x3f800000) {

		if (ky < (MIN_EXP - 1) / 2)

			return (CMPLXF((ay / 2) * ay, v));

		return (CMPLXF(log1pf(ay * ay) / 2, v));

	}

	/* Avoid underflow when ax is not small.  Also handle zero args. */

	if (kx - ky > MANT_DIG || hay == 0)

		return (CMPLXF(logf(ax), v));

	/* Avoid overflow. */

	if (kx >= MAX_EXP - 1)

		return (CMPLXF(logf(hypotf(x * 0x1p-126F, y * 0x1p-126F)) +

		    (MAX_EXP - 2) * ln2f_lo + (MAX_EXP - 2) * ln2f_hi, v));

	if (kx >= (MAX_EXP - 1) / 2)

		return (CMPLXF(logf(hypotf(x, y)), v));

	/* Reduce inaccuracies and avoid underflow when ax is denormal. */

	if (kx <= MIN_EXP - 2)

		return (CMPLXF(logf(hypotf(x * 0x1p127F, y * 0x1p127F)) +

		    (MIN_EXP - 2) * ln2f_lo + (MIN_EXP - 2) * ln2f_hi, v));

	/* Avoid remaining underflows (when ax is small but not denormal). */

	if (ky < (MIN_EXP - 1) / 2 + MANT_DIG)

		return (CMPLXF(logf(hypotf(x, y)), v));

	/* Calculate ax*ax and ay*ay exactly using Dekker's algorithm. */

	t = (float)(ax * (0x1p12F + 1));

	axh = (float)(ax - t) + t;

	axl = ax - axh;

	ax2h = ax * ax;

	ax2l = axh * axh - ax2h + 2 * axh * axl + axl * axl;

	t = (float)(ay * (0x1p12F + 1));

	ayh = (float)(ay - t) + t;

	ayl = ay - ayh;

	ay2h = ay * ay;

	ay2l = ayh * ayh - ay2h + 2 * ayh * ayl + ayl * ayl;

	/*

	 * When log(|z|) is far from 1, accuracy in calculating the sum

	 * of the squares is not very important since log() reduces

	 * inaccuracies.  We depended on this to use the general

	 * formula when log(|z|) is very far from 1.  When log(|z|) is

	 * moderately far from 1, we go through the extra-precision

	 * calculations to reduce branches and gain a little accuracy.

	 *

	 * When |z| is near 1, we subtract 1 and use log1p() and don't

	 * leave it to log() to subtract 1, since we gain at least 1 bit

	 * of accuracy in this way.

	 *

	 * When |z| is very near 1, subtracting 1 can cancel almost

	 * 3*MANT_DIG bits.  We arrange that subtracting 1 is exact in

	 * doubled precision, and then do the rest of the calculation

	 * in sloppy doubled precision.  Although large cancellations

	 * often lose lots of accuracy, here the final result is exact

	 * in doubled precision if the large calculation occurs (because

	 * then it is exact in tripled precision and the cancellation

	 * removes enough bits to fit in doubled precision).  Thus the

	 * result is accurate in sloppy doubled precision, and the only

	 * significant loss of accuracy is when it is summed and passed

	 * to log1p().

	 */

	sh = ax2h;

	sl = ay2h;

	_2sumF(sh, sl);

	if (sh < 0.5F || sh >= 3)

		return (CMPLXF(logf(ay2l + ax2l + sl + sh) / 2, v));

	sh -= 1;

	_2sum(sh, sl);

	_2sum(ax2l, ay2l);

	/* Briggs-Kahan algorithm (except we discard the final low term): */

	_2sum(sh, ax2l);

	_2sum(sl, ay2l);

	t = ax2l + sl;

	_2sumF(sh, t);

	return (CMPLXF(log1pf(ay2l + t + sh) / 2, v));

}

/*-

 * Copyright (c) 2013 Bruce D. Evans

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice unmodified, this list of conditions, and the following

 *    disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR

 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES

 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.

 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,

 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT

 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF

 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

#include <sys/cdefs.h>

__FBSDID("$FreeBSD$");

#include <complex.h>

#include <float.h>

#ifdef __i386__

#include <ieeefp.h>

#endif

#include "fpmath.h"

#include "math.h"

#include "math_private.h"

#define	MANT_DIG	LDBL_MANT_DIG

#define	MAX_EXP		LDBL_MAX_EXP

#define	MIN_EXP		LDBL_MIN_EXP

static const double

ln2_hi = 6.9314718055829871e-1;		/*  0x162e42fefa0000.0p-53 */

#if LDBL_MANT_DIG == 64

#define	MULT_REDUX	0x1p32		/* exponent MANT_DIG / 2 rounded up */

static const double

ln2l_lo = 1.6465949582897082e-12;	/*  0x1cf79abc9e3b3a.0p-92 */

#elif LDBL_MANT_DIG == 113

#define	MULT_REDUX	0x1p57

static const long double

ln2l_lo = 1.64659495828970812809844307550013433e-12L;	/*  0x1cf79abc9e3b39803f2f6af40f343.0p-152L */

#else

#error "Unsupported long double format"

#endif

long double complex

clogl(long double complex z)

{

	long double ax, ax2h, ax2l, axh, axl, ay, ay2h, ay2l, ayh, ayl;

	long double sh, sl, t;

	long double x, y, v;

	uint16_t hax, hay;

	int kx, ky;

	ENTERIT(long double complex);

	x = creall(z);

	y = cimagl(z);

	v = atan2l(y, x);

	ax = fabsl(x);

	ay = fabsl(y);

	if (ax < ay) {

		t = ax;

		ax = ay;

		ay = t;

	}

	GET_LDBL_EXPSIGN(hax, ax);

	kx = hax - 16383;

	GET_LDBL_EXPSIGN(hay, ay);

	ky = hay - 16383;

	/* Handle NaNs and Infs using the general formula. */

	if (kx == MAX_EXP || ky == MAX_EXP)

		RETURNI(CMPLXL(logl(hypotl(x, y)), v));

	/* Avoid spurious underflow, and reduce inaccuracies when ax is 1. */

	if (ax == 1) {

		if (ky < (MIN_EXP - 1) / 2)

			RETURNI(CMPLXL((ay / 2) * ay, v));

		RETURNI(CMPLXL(log1pl(ay * ay) / 2, v));

	}

	/* Avoid underflow when ax is not small.  Also handle zero args. */

	if (kx - ky > MANT_DIG || ay == 0)

		RETURNI(CMPLXL(logl(ax), v));

	/* Avoid overflow. */

	if (kx >= MAX_EXP - 1)

		RETURNI(CMPLXL(logl(hypotl(x * 0x1p-16382L, y * 0x1p-16382L)) +

		    (MAX_EXP - 2) * ln2l_lo + (MAX_EXP - 2) * ln2_hi, v));

	if (kx >= (MAX_EXP - 1) / 2)

		RETURNI(CMPLXL(logl(hypotl(x, y)), v));

	/* Reduce inaccuracies and avoid underflow when ax is denormal. */

	if (kx <= MIN_EXP - 2)

		RETURNI(CMPLXL(logl(hypotl(x * 0x1p16383L, y * 0x1p16383L)) +

		    (MIN_EXP - 2) * ln2l_lo + (MIN_EXP - 2) * ln2_hi, v));

	/* Avoid remaining underflows (when ax is small but not denormal). */

	if (ky < (MIN_EXP - 1) / 2 + MANT_DIG)

		RETURNI(CMPLXL(logl(hypotl(x, y)), v));

	/* Calculate ax*ax and ay*ay exactly using Dekker's algorithm. */

	t = (long double)(ax * (MULT_REDUX + 1));

	axh = (long double)(ax - t) + t;

	axl = ax - axh;

	ax2h = ax * ax;

	ax2l = axh * axh - ax2h + 2 * axh * axl + axl * axl;

	t = (long double)(ay * (MULT_REDUX + 1));

	ayh = (long double)(ay - t) + t;

	ayl = ay - ayh;

	ay2h = ay * ay;

	ay2l = ayh * ayh - ay2h + 2 * ayh * ayl + ayl * ayl;

	/*

	 * When log(|z|) is far from 1, accuracy in calculating the sum

	 * of the squares is not very important since log() reduces

	 * inaccuracies.  We depended on this to use the general

	 * formula when log(|z|) is very far from 1.  When log(|z|) is

	 * moderately far from 1, we go through the extra-precision

	 * calculations to reduce branches and gain a little accuracy.

	 *

	 * When |z| is near 1, we subtract 1 and use log1p() and don't

	 * leave it to log() to subtract 1, since we gain at least 1 bit

	 * of accuracy in this way.

	 *

	 * When |z| is very near 1, subtracting 1 can cancel almost

	 * 3*MANT_DIG bits.  We arrange that subtracting 1 is exact in

	 * doubled precision, and then do the rest of the calculation

	 * in sloppy doubled precision.  Although large cancellations

	 * often lose lots of accuracy, here the final result is exact

	 * in doubled precision if the large calculation occurs (because

	 * then it is exact in tripled precision and the cancellation

	 * removes enough bits to fit in doubled precision).  Thus the

	 * result is accurate in sloppy doubled precision, and the only

	 * significant loss of accuracy is when it is summed and passed

	 * to log1p().

	 */

	sh = ax2h;

	sl = ay2h;

	_2sumF(sh, sl);

	if (sh < 0.5 || sh >= 3)

		RETURNI(CMPLXL(logl(ay2l + ax2l + sl + sh) / 2, v));

	sh -= 1;

	_2sum(sh, sl);

	_2sum(ax2l, ay2l);

	/* Briggs-Kahan algorithm (except we discard the final low term): */

	_2sum(sh, ax2l);

	_2sum(sl, ay2l);

	t = ax2l + sl;

	_2sumF(sh, t);

	RETURNI(CMPLXL(log1pl(ay2l + t + sh) / 2, v));

}