1 2 /* @(#)e_log10.c 1.3 95/01/18 */ 3 /* 4 * ==================================================== 5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 6 * 7 * Developed at SunSoft, a Sun Microsystems, Inc. business. 8 * Permission to use, copy, modify, and distribute this 9 * software is freely granted, provided that this notice 10 * is preserved. 11 * ==================================================== 12 */ 13 14 #include <sys/cdefs.h> 15 __FBSDID("$FreeBSD$"); 16 17 /* 18 * Return the base 2 logarithm of x. See k_log.c for details on the algorithm. 19 */ 20 21 #include "math.h" 22 #include "math_private.h" 23 #include "k_log.h" 24 25 static const double 26 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 27 ivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */ 28 ivln2lo = 1.67517131648865118353e-10; /* 0x3de705fc, 0x2eefa200 */ 29 30 static const double zero = 0.0; 31 32 double 33 __ieee754_log2(double x) 34 { 35 double f,hi,lo; 36 int32_t i,k,hx; 37 u_int32_t lx; 38 39 EXTRACT_WORDS(hx,lx,x); 40 41 k=0; 42 if (hx < 0x00100000) { /* x < 2**-1022 */ 43 if (((hx&0x7fffffff)|lx)==0) 44 return -two54/zero; /* log(+-0)=-inf */ 45 if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ 46 k -= 54; x *= two54; /* subnormal number, scale up x */ 47 GET_HIGH_WORD(hx,x); 48 } 49 if (hx >= 0x7ff00000) return x+x; 50 k += (hx>>20)-1023; 51 hx &= 0x000fffff; 52 i = (hx+0x95f64)&0x100000; 53 SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */ 54 k += (i>>20); 55 f = __kernel_log(x); 56 hi = x = x - 1; 57 SET_LOW_WORD(hi,0); 58 lo = x - hi; 59 return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k; 60 } 61