src/libm/k_rem_pio2.c
 author Holmes Futrell Thu, 01 Jan 2009 23:49:28 +0000 changeset 2950 04c9f1e4c496 parent 2757 0581f49c9294 child 3162 dc1eb82ffdaa permissions -rw-r--r--
Added target testdraw2 for running the test/testdraw2.c test.
```
/* @(#)k_rem_pio2.c 5.1 93/09/24 */
/*
* ====================================================
*
* 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 char rcsid[] =
"\$NetBSD: k_rem_pio2.c,v 1.7 1995/05/10 20:46:25 jtc Exp \$";
#endif

/*
* __kernel_rem_pio2(x,y,e0,nx,prec,ipio2)
* double x[],y[]; int e0,nx,prec; int ipio2[];
*
* __kernel_rem_pio2 return the last three digits of N with
*		y = x - N*pi/2
* so that |y| < pi/2.
*
* The method is to compute the integer (mod 8) and fraction parts of
* (2/pi)*x without doing the full multiplication. In general we
* skip the part of the product that are known to be a huge integer (
* more accurately, = 0 mod 8 ). Thus the number of operations are
* independent of the exponent of the input.
*
* (2/pi) is represented by an array of 24-bit integers in ipio2[].
*
* Input parameters:
* 	x[]	The input value (must be positive) is broken into nx
*		pieces of 24-bit integers in double precision format.
*		x[i] will be the i-th 24 bit of x. The scaled exponent
*		of x is given in input parameter e0 (i.e., x*2^e0
*		match x's up to 24 bits.
*
*		Example of breaking a double positive z into x+x+x:
*			e0 = ilogb(z)-23
*			z  = scalbn(z,-e0)
*		for i = 0,1,2
*			x[i] = floor(z)
*			z    = (z-x[i])*2**24
*
*
*	y[]	ouput result in an array of double precision numbers.
*		The dimension of y[] is:
*			24-bit  precision	1
*			53-bit  precision	2
*			64-bit  precision	2
*			113-bit precision	3
*		The actual value is the sum of them. Thus for 113-bit
*		precison, one may have to do something like:
*
*		t = (long double)y + (long double)y;
*		w = (long double)y;
*		r_tail = w - (r_head - t);
*
*	e0	The exponent of x
*
*	nx	dimension of x[]
*
*  	prec	an integer indicating the precision:
*			0	24  bits (single)
*			1	53  bits (double)
*			2	64  bits (extended)
*
*	ipio2[]
*		integer array, contains the (24*i)-th to (24*i+23)-th
*		bit of 2/pi after binary point. The corresponding
*		floating value is
*
*			ipio2[i] * 2^(-24(i+1)).
*
* External function:
*	double scalbn(), floor();
*
*
* Here is the description of some local variables:
*
* 	jk	jk+1 is the initial number of terms of ipio2[] needed
*		in the computation. The recommended value is 2,3,4,
*		6 for single, double, extended,and quad.
*
* 	jz	local integer variable indicating the number of
*		terms of ipio2[] used.
*
*	jx	nx - 1
*
*	jv	index for pointing to the suitable ipio2[] for the
*		computation. In general, we want
*			( 2^e0*x * ipio2[jv-1]*2^(-24jv) )/8
*		is an integer. Thus
*			e0-3-24*jv >= 0 or (e0-3)/24 >= jv
*		Hence jv = max(0,(e0-3)/24).
*
*	jp	jp+1 is the number of terms in PIo2[] needed, jp = jk.
*
* 	q[]	double array with integral value, representing the
*		24-bits chunk of the product of x and 2/pi.
*
*	q0	the corresponding exponent of q. Note that the
*		exponent for q[i] would be q0-24*i.
*
*	PIo2[]	double precision array, obtained by cutting pi/2
*		into 24 bits chunks.
*
*	f[]	ipio2[] in floating point
*
*	iq[]	integer array by breaking up q[] in 24-bits chunk.
*
*	fq[]	final product of x*(2/pi) in fq,..,fq[jk]
*
*	ih	integer. If >0 it indicates q[] is >= 0.5, hence
*		it also indicates the *sign* of the result.
*
*/

/*
* Constants:
* The hexadecimal values are the intended ones for the following
* constants. The decimal values may be used, provided that the
* compiler will convert from decimal to binary accurately enough
* to produce the hexadecimal values shown.
*/

#include "math.h"
#include "math_private.h"

libm_hidden_proto(scalbn)
libm_hidden_proto(floor)
#ifdef __STDC__
static const int init_jk[] = { 2, 3, 4, 6 };       /* initial value for jk */
#else
static int init_jk[] = { 2, 3, 4, 6 };
#endif

#ifdef __STDC__
static const double PIo2[] = {
#else
static double PIo2[] = {
#endif
1.57079625129699707031e+00, /* 0x3FF921FB, 0x40000000 */
7.54978941586159635335e-08, /* 0x3E74442D, 0x00000000 */
5.39030252995776476554e-15, /* 0x3CF84698, 0x80000000 */
3.28200341580791294123e-22, /* 0x3B78CC51, 0x60000000 */
1.27065575308067607349e-29, /* 0x39F01B83, 0x80000000 */
1.22933308981111328932e-36, /* 0x387A2520, 0x40000000 */
2.73370053816464559624e-44, /* 0x36E38222, 0x80000000 */
2.16741683877804819444e-51, /* 0x3569F31D, 0x00000000 */
};

#ifdef __STDC__
static const double
#else
static double
#endif
zero = 0.0, one = 1.0, two24 = 1.67772160000000000000e+07,    /* 0x41700000, 0x00000000 */
twon24 = 5.96046447753906250000e-08;        /* 0x3E700000, 0x00000000 */

#ifdef __STDC__
int attribute_hidden
__kernel_rem_pio2(double *x, double *y, int e0, int nx, int prec,
const int32_t * ipio2)
#else
int attribute_hidden
__kernel_rem_pio2(x, y, e0, nx, prec, ipio2)
double x[], y[];
int e0, nx, prec;
int32_t ipio2[];
#endif
{
int32_t jz, jx, jv, jp, jk, carry, n, iq, i, j, k, m, q0, ih;
double z, fw, f, fq, q;

/* initialize jk */
jk = init_jk[prec];
jp = jk;

/* determine jx,jv,q0, note that 3>q0 */
jx = nx - 1;
jv = (e0 - 3) / 24;
if (jv < 0)
jv = 0;
q0 = e0 - 24 * (jv + 1);

/* set up f to f[jx+jk] where f[jx+jk] = ipio2[jv+jk] */
j = jv - jx;
m = jx + jk;
for (i = 0; i <= m; i++, j++)
f[i] = (j < 0) ? zero : (double) ipio2[j];

/* compute q,q,...q[jk] */
for (i = 0; i <= jk; i++) {
for (j = 0, fw = 0.0; j <= jx; j++)
fw += x[j] * f[jx + i - j];
q[i] = fw;
}

jz = jk;
recompute:
/* distill q[] into iq[] reversingly */
for (i = 0, j = jz, z = q[jz]; j > 0; i++, j--) {
fw = (double) ((int32_t) (twon24 * z));
iq[i] = (int32_t) (z - two24 * fw);
z = q[j - 1] + fw;
}

/* compute n */
z = scalbn(z, q0);          /* actual value of z */
z -= 8.0 * floor(z * 0.125);        /* trim off integer >= 8 */
n = (int32_t) z;
z -= (double) n;
ih = 0;
if (q0 > 0) {               /* need iq[jz-1] to determine n */
i = (iq[jz - 1] >> (24 - q0));
n += i;
iq[jz - 1] -= i << (24 - q0);
ih = iq[jz - 1] >> (23 - q0);
} else if (q0 == 0)
ih = iq[jz - 1] >> 23;
else if (z >= 0.5)
ih = 2;

if (ih > 0) {               /* q > 0.5 */
n += 1;
carry = 0;
for (i = 0; i < jz; i++) {      /* compute 1-q */
j = iq[i];
if (carry == 0) {
if (j != 0) {
carry = 1;
iq[i] = 0x1000000 - j;
}
} else
iq[i] = 0xffffff - j;
}
if (q0 > 0) {           /* rare case: chance is 1 in 12 */
switch (q0) {
case 1:
iq[jz - 1] &= 0x7fffff;
break;
case 2:
iq[jz - 1] &= 0x3fffff;
break;
}
}
if (ih == 2) {
z = one - z;
if (carry != 0)
z -= scalbn(one, q0);
}
}

/* check if recomputation is needed */
if (z == zero) {
j = 0;
for (i = jz - 1; i >= jk; i--)
j |= iq[i];
if (j == 0) {           /* need recomputation */
for (k = 1; iq[jk - k] == 0; k++);  /* k = no. of terms needed */

for (i = jz + 1; i <= jz + k; i++) {        /* add q[jz+1] to q[jz+k] */
f[jx + i] = (double) ipio2[jv + i];
for (j = 0, fw = 0.0; j <= jx; j++)
fw += x[j] * f[jx + i - j];
q[i] = fw;
}
jz += k;
goto recompute;
}
}

/* chop off zero terms */
if (z == 0.0) {
jz -= 1;
q0 -= 24;
while (iq[jz] == 0) {
jz--;
q0 -= 24;
}
} else {                    /* break z into 24-bit if necessary */
z = scalbn(z, -q0);
if (z >= two24) {
fw = (double) ((int32_t) (twon24 * z));
iq[jz] = (int32_t) (z - two24 * fw);
jz += 1;
q0 += 24;
iq[jz] = (int32_t) fw;
} else
iq[jz] = (int32_t) z;
}

/* convert integer "bit" chunk to floating-point value */
fw = scalbn(one, q0);
for (i = jz; i >= 0; i--) {
q[i] = fw * (double) iq[i];
fw *= twon24;
}

/* compute PIo2[0,...,jp]*q[jz,...,0] */
for (i = jz; i >= 0; i--) {
for (fw = 0.0, k = 0; k <= jp && k <= jz - i; k++)
fw += PIo2[k] * q[i + k];
fq[jz - i] = fw;
}

/* compress fq[] into y[] */
switch (prec) {
case 0:
fw = 0.0;
for (i = jz; i >= 0; i--)
fw += fq[i];
y = (ih == 0) ? fw : -fw;
break;
case 1:
case 2:
fw = 0.0;
for (i = jz; i >= 0; i--)
fw += fq[i];
y = (ih == 0) ? fw : -fw;
fw = fq - fw;
for (i = 1; i <= jz; i++)
fw += fq[i];
y = (ih == 0) ? fw : -fw;
break;
case 3:                    /* painful */
for (i = jz; i > 0; i--) {
fw = fq[i - 1] + fq[i];
fq[i] += fq[i - 1] - fw;
fq[i - 1] = fw;
}
for (i = jz; i > 1; i--) {
fw = fq[i - 1] + fq[i];
fq[i] += fq[i - 1] - fw;
fq[i - 1] = fw;
}
for (fw = 0.0, i = jz; i >= 2; i--)
fw += fq[i];
if (ih == 0) {
y = fq;
y = fq;
y = fw;
} else {
y = -fq;
y = -fq;
y = -fw;
}
}
return n & 7;
}
```