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-
-/* @(#)k_rem_pio2.c 1.3 95/01/18 */
-/*
- * ====================================================
- * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
- *
- * Developed at SunSoft, a Sun Microsystems, Inc. business.
- * Permission to use, copy, modify, and distribute this
- * software is freely granted, provided that this notice
- * is preserved.
- * ====================================================
- */
-
-/*
- * __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[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 positive 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
- * precison, 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 integer 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 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[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 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"
-
-static const int init_jk[] = {2,3,4,6}; /* initial value for jk */
-
-static const double PIo2[] = {
- 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 */
-};
-
-static const double
-zero = 0.0,
-one = 1.0,
-two24 = 1.67772160000000000000e+07, /* 0x41700000, 0x00000000 */
-twon24 = 5.96046447753906250000e-08; /* 0x3E700000, 0x00000000 */
-
- int __kernel_rem_pio2(double *x, double *y, int e0, int nx, int prec, const int32_t *ipio2)
-{
- int32_t jz,jx,jv,jp,jk,carry,n,iq[20],i,j,k,m,q0,ih;
- double z,fw,f[20],fq[20],q[20];
-
- /* 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[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=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[0] = (ih==0)? fw: -fw;
- break;
- case 1:
- case 2:
- fw = 0.0;
- 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;
- 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[0] = fq[0]; y[1] = fq[1]; y[2] = fw;
- } else {
- y[0] = -fq[0]; y[1] = -fq[1]; y[2] = -fw;
- }
- }
- return n&7;
-}
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