1141296Sdas 2141296Sdas/* @(#)e_log10.c 1.3 95/01/18 */ 32116Sjkh/* 42116Sjkh * ==================================================== 52116Sjkh * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 62116Sjkh * 7141296Sdas * Developed at SunSoft, a Sun Microsystems, Inc. business. 82116Sjkh * Permission to use, copy, modify, and distribute this 9141296Sdas * software is freely granted, provided that this notice 102116Sjkh * is preserved. 112116Sjkh * ==================================================== 122116Sjkh */ 132116Sjkh 14176451Sdas#include <sys/cdefs.h> 15176451Sdas__FBSDID("$FreeBSD$"); 162116Sjkh 17216247Sdas/* 18216247Sdas * Return the base 2 logarithm of x. See k_log.c for details on the algorithm. 192116Sjkh */ 202116Sjkh 212116Sjkh#include "math.h" 222116Sjkh#include "math_private.h" 23216211Sdas#include "k_log.h" 242116Sjkh 252116Sjkhstatic const double 262116Sjkhtwo54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 27216247Sdasivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */ 28216247Sdasivln2lo = 1.67517131648865118353e-10; /* 0x3de705fc, 0x2eefa200 */ 292116Sjkh 302116Sjkhstatic const double zero = 0.0; 312116Sjkh 3297413Salfreddouble 33216211Sdas__ieee754_log2(double x) 342116Sjkh{ 35216211Sdas double f,hi,lo; 362116Sjkh int32_t i,k,hx; 372116Sjkh u_int32_t lx; 382116Sjkh 392116Sjkh EXTRACT_WORDS(hx,lx,x); 402116Sjkh 412116Sjkh k=0; 422116Sjkh if (hx < 0x00100000) { /* x < 2**-1022 */ 432116Sjkh if (((hx&0x7fffffff)|lx)==0) 442116Sjkh return -two54/zero; /* log(+-0)=-inf */ 452116Sjkh if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ 462116Sjkh k -= 54; x *= two54; /* subnormal number, scale up x */ 472116Sjkh GET_HIGH_WORD(hx,x); 482116Sjkh } 492116Sjkh if (hx >= 0x7ff00000) return x+x; 502116Sjkh k += (hx>>20)-1023; 51216211Sdas hx &= 0x000fffff; 52216211Sdas i = (hx+0x95f64)&0x100000; 53216211Sdas SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */ 54216211Sdas k += (i>>20); 55216211Sdas f = __kernel_log(x); 56216211Sdas hi = x = x - 1; 57216211Sdas SET_LOW_WORD(hi,0); 58216211Sdas lo = x - hi; 59216211Sdas return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k; 602116Sjkh} 61