e_log10.c revision 219360
11556Srgrimes 250471Speter/* @(#)e_log10.c 1.3 95/01/18 */ 31556Srgrimes/* 41556Srgrimes * ==================================================== 53197Scsgr * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 651434Smarkm * 73197Scsgr * Developed at SunSoft, a Sun Microsystems, Inc. business. 834899Smarkm * Permission to use, copy, modify, and distribute this 934899Smarkm * software is freely granted, provided that this notice 10137164Sru * is preserved. 113197Scsgr * ==================================================== 121556Srgrimes */ 13 14#include <sys/cdefs.h> 15__FBSDID("$FreeBSD: head/lib/msun/src/e_log10.c 219360 2011-03-07 03:11:27Z das $"); 16 17/* 18 * Return the base 10 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 25static const double 26two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 27ivln10hi = 4.34294481878168880939e-01, /* 0x3fdbcb7b, 0x15200000 */ 28ivln10lo = 2.50829467116452752298e-11, /* 0x3dbb9438, 0xca9aadd5 */ 29log10_2hi = 3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */ 30log10_2lo = 3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */ 31 32static const double zero = 0.0; 33 34double 35__ieee754_log10(double x) 36{ 37 double f,hi,lo,y,z; 38 int32_t i,k,hx; 39 u_int32_t lx; 40 41 EXTRACT_WORDS(hx,lx,x); 42 43 k=0; 44 if (hx < 0x00100000) { /* x < 2**-1022 */ 45 if (((hx&0x7fffffff)|lx)==0) 46 return -two54/zero; /* log(+-0)=-inf */ 47 if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ 48 k -= 54; x *= two54; /* subnormal number, scale up x */ 49 GET_HIGH_WORD(hx,x); 50 } 51 if (hx >= 0x7ff00000) return x+x; 52 k += (hx>>20)-1023; 53 hx &= 0x000fffff; 54 i = (hx+0x95f64)&0x100000; 55 SET_HIGH_WORD(x,hx|(i^0x3ff00000)); /* normalize x or x/2 */ 56 k += (i>>20); 57 y = (double)k; 58 f = __kernel_log(x); 59 hi = x = x - 1; 60 SET_LOW_WORD(hi,0); 61 lo = x - hi; 62 z = y*log10_2lo + (x+f)*ivln10lo + (lo+f)*ivln10hi + hi*ivln10hi; 63 return z+y*log10_2hi; 64} 65