k_cos.cpp (2797B)
1 /* @(#)k_cos.c 1.3 95/01/18 */ 2 /* 3 * ==================================================== 4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 5 * 6 * Developed at SunSoft, a Sun Microsystems, Inc. business. 7 * Permission to use, copy, modify, and distribute this 8 * software is freely granted, provided that this notice 9 * is preserved. 10 * ==================================================== 11 */ 12 13 //#include <sys/cdefs.h> 14 //__FBSDID("$FreeBSD$"); 15 16 /* 17 * __kernel_cos( x, y ) 18 * kernel cos function on [-pi/4, pi/4], pi/4 ~ 0.785398164 19 * Input x is assumed to be bounded by ~pi/4 in magnitude. 20 * Input y is the tail of x. 21 * 22 * Algorithm 23 * 1. Since cos(-x) = cos(x), we need only to consider positive x. 24 * 2. if x < 2^-27 (hx<0x3e400000 0), return 1 with inexact if x!=0. 25 * 3. cos(x) is approximated by a polynomial of degree 14 on 26 * [0,pi/4] 27 * 4 14 28 * cos(x) ~ 1 - x*x/2 + C1*x + ... + C6*x 29 * where the remez error is 30 * 31 * | 2 4 6 8 10 12 14 | -58 32 * |cos(x)-(1-.5*x +C1*x +C2*x +C3*x +C4*x +C5*x +C6*x )| <= 2 33 * | | 34 * 35 * 4 6 8 10 12 14 36 * 4. let r = C1*x +C2*x +C3*x +C4*x +C5*x +C6*x , then 37 * cos(x) ~ 1 - x*x/2 + r 38 * since cos(x+y) ~ cos(x) - sin(x)*y 39 * ~ cos(x) - x*y, 40 * a correction term is necessary in cos(x) and hence 41 * cos(x+y) = 1 - (x*x/2 - (r - x*y)) 42 * For better accuracy, rearrange to 43 * cos(x+y) ~ w + (tmp + (r-x*y)) 44 * where w = 1 - x*x/2 and tmp is a tiny correction term 45 * (1 - x*x/2 == w + tmp exactly in infinite precision). 46 * The exactness of w + tmp in infinite precision depends on w 47 * and tmp having the same precision as x. If they have extra 48 * precision due to compiler bugs, then the extra precision is 49 * only good provided it is retained in all terms of the final 50 * expression for cos(). Retention happens in all cases tested 51 * under FreeBSD, so don't pessimize things by forcibly clipping 52 * any extra precision in w. 53 */ 54 55 #include "math_private.h" 56 57 static const double 58 one = 1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */ 59 C1 = 4.16666666666666019037e-02, /* 0x3FA55555, 0x5555554C */ 60 C2 = -1.38888888888741095749e-03, /* 0xBF56C16C, 0x16C15177 */ 61 C3 = 2.48015872894767294178e-05, /* 0x3EFA01A0, 0x19CB1590 */ 62 C4 = -2.75573143513906633035e-07, /* 0xBE927E4F, 0x809C52AD */ 63 C5 = 2.08757232129817482790e-09, /* 0x3E21EE9E, 0xBDB4B1C4 */ 64 C6 = -1.13596475577881948265e-11; /* 0xBDA8FAE9, 0xBE8838D4 */ 65 66 double 67 __kernel_cos(double x, double y) 68 { 69 double hz,z,r,w; 70 71 z = x*x; 72 w = z*z; 73 r = z*(C1+z*(C2+z*C3)) + w*w*(C4+z*(C5+z*C6)); 74 hz = 0.5*z; 75 w = one-hz; 76 return w + (((one-w)-hz) + (z*r-x*y)); 77 }