__rem_pio2m.c revision 25c28e83beb90e7c80452a7c818c5e6f73a07dc8
/*
* CDDL HEADER START
*
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* You may not use this file except in compliance with the License.
*
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* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
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*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
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*/
/*
* Copyright 2011 Nexenta Systems, Inc. All rights reserved.
*/
/*
* Copyright 2005 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/*
* int __rem_pio2m(x,y,e0,nx,prec,ipio2)
* double x[],y[]; int e0,nx,prec; const int ipio2[];
*
* __rem_pio2m return the last three digits of N with
* y = x - N*pi/2
* so that |y| < pi/4.
*
* 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[].
* Here PI could as well be a machine value pi.
*
* 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[0] is given in input parameter e0 (i.e., x[0]*2^e0
* match x's up to 24 bits.
*
* Example of breaking a double z into x[0]+x[1]+x[2]:
* 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
* precsion, one may have to do something like:
*
* long double t,w,r_head, r_tail;
* t = (long double)y[2] + (long double)y[1];
* w = (long double)y[0];
* r_head = t+w;
* r_tail = w - (r_head - t);
*
* e0 The exponent of x[0]
*
* nx dimension of x[]
*
* prec an interger indicating the precision:
* 0 24 bits (single)
* 1 53 bits (double)
* 2 64 bits (extended)
* 3 113 bits (quad)
*
* ipio2[]
* integer array, contains the (24*i)-th to (24*i+23)-th
* bit of 2/pi or 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 3,4,4,
* 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[0] * 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[0]. 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[0],..,fq[jk]
*
* ih integer. If >0 it indicats q[] is >= 0.5, hence
* it also indicates the *sign* of the result.
*
*/
#include "libm.h"
#if defined(__i386) && !defined(__amd64)
extern int __swapRP(int);
#endif
static const int init_jk[] = { 3, 4, 4, 6 }; /* initial value for jk */
static const double pio2[] = {
1.57079625129699707031e+00,
7.54978941586159635335e-08,
5.39030252995776476554e-15,
3.28200341580791294123e-22,
1.27065575308067607349e-29,
1.22933308981111328932e-36,
2.73370053816464559624e-44,
2.16741683877804819444e-51,
};
static const double
zero = 0.0,
one = 1.0,
half = 0.5,
eight = 8.0,
eighth = 0.125,
two24 = 16777216.0,
twon24 = 5.960464477539062500E-8;
int
__rem_pio2m(double *x, double *y, int e0, int nx, int prec, const int *ipio2)
{
int jz, jx, jv, jp, jk, carry, n, iq[20];
int i, j, k, m, q0, ih;
double z, fw, f[20], fq[20], q[20];
#if defined(__i386) && !defined(__amd64)
int rp;
rp = __swapRP(fp_extended);
#endif
/* initialize jk */
jp = jk = init_jk[prec];
/* 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[0] 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[0],q[1],...q[jk] */
for (i = 0; i <= jk; i++) {
for (j = 0, fw = zero; 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)((int)(twon24 * z));
iq[i] = (int)(z - two24 * fw);
z = q[j-1] + fw;
}
/* compute n */
z = scalbn(z, q0); /* actual value of z */
z -= eight * floor(z * eighth); /* trim off integer >= 8 */
n = (int)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 >= half) {
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 */
/* set k to no. of terms needed */
for (k = 1; iq[jk-k] == 0; k++)
;
/* add q[jz+1] to q[jz+k] */
for (i = jz + 1; i <= jz + k; i++) {
f[jx+i] = (double)ipio2[jv+i];
for (j = 0, fw = zero; j <= jx; j++)
fw += x[j] * f[jx+i-j];
q[i] = fw;
}
jz += k;
goto recompute;
}
}
/* cut out zero terms */
if (z == zero) {
jz -= 1;
q0 -= 24;
while (iq[jz] == 0) {
jz--;
q0 -= 24;
}
} else { /* break z into 24-bit if neccessary */
z = scalbn(z, -q0);
if (z >= two24) {
fw = (double)((int)(twon24 * z));
iq[jz] = (int)(z - two24 * fw);
jz += 1;
q0 += 24;
iq[jz] = (int)fw;
} else {
iq[jz] = (int)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 = zero, 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 = zero;
for (i = jz; i >= 0; i--)
fw += fq[i];
y[0] = (ih == 0)? fw : -fw;
break;
case 1:
case 2:
fw = zero;
for (i = jz; i >= 0; i--)
fw += fq[i];
y[0] = (ih == 0)? fw : -fw;
fw = fq[0] - fw;
for (i = 1; i <= jz; i++)
fw += fq[i];
y[1] = (ih == 0)? fw : -fw;
break;
default:
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 = zero, i = jz; i >= 2; i--)
fw += fq[i];
if (ih == 0) {
y[0] = fq[0];
y[1] = fq[1];
y[2] = fw;
} else {
y[0] = -fq[0];
y[1] = -fq[1];
y[2] = -fw;
}
}
#if defined(__i386) && !defined(__amd64)
(void) __swapRP(rp);
#endif
return (n & 7);
}