1 /* @(#)e_log10.c 5.1 93/09/24 */ 2 /* 3 * ==================================================== 4 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 5 * 6 * Developed at SunPro, 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 #ifndef lint 14 static char rcsid[] = "$Id: e_log10.c,v 1.4 1994/03/03 17:04:19 jtc Exp $"; 15 #endif 16 17 /* __ieee754_log10(x) 18 * Return the base 10 logarithm of x 19 * 20 * Method : 21 * Let log10_2hi = leading 40 bits of log10(2) and 22 * log10_2lo = log10(2) - log10_2hi, 23 * ivln10 = 1/log(10) rounded. 24 * Then 25 * n = ilogb(x), 26 * if(n<0) n = n+1; 27 * x = scalbn(x,-n); 28 * log10(x) := n*log10_2hi + (n*log10_2lo + ivln10*log(x)) 29 * 30 * Note 1: 31 * To guarantee log10(10**n)=n, where 10**n is normal, the rounding 32 * mode must set to Round-to-Nearest. 33 * Note 2: 34 * [1/log(10)] rounded to 53 bits has error .198 ulps; 35 * log10 is monotonic at all binary break points. 36 * 37 * Special cases: 38 * log10(x) is NaN with signal if x < 0; 39 * log10(+INF) is +INF with no signal; log10(0) is -INF with signal; 40 * log10(NaN) is that NaN with no signal; 41 * log10(10**N) = N for N=0,1,...,22. 42 * 43 * Constants: 44 * The hexadecimal values are the intended ones for the following constants. 45 * The decimal values may be used, provided that the compiler will convert 46 * from decimal to binary accurately enough to produce the hexadecimal values 47 * shown. 48 */ 49 50 #include <math.h> 51 #include <machine/endian.h> 52 53 #if BYTE_ORDER == LITTLE_ENDIAN 54 #define n0 1 55 #else 56 #define n0 0 57 #endif 58 59 #ifdef __STDC__ 60 static const double 61 #else 62 static double 63 #endif 64 two54 = 1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */ 65 ivln10 = 4.34294481903251816668e-01, /* 0x3FDBCB7B, 0x1526E50E */ 66 log10_2hi = 3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */ 67 log10_2lo = 3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */ 68 69 static double zero = 0.0; 70 71 #ifdef __STDC__ 72 double __ieee754_log10(double x) 73 #else 74 double __ieee754_log10(x) 75 double x; 76 #endif 77 { 78 double y,z; 79 int i,k,hx; 80 unsigned lx; 81 82 hx = *(n0+(unsigned*)&x); /* high word of x */ 83 lx = *(1-n0+(unsigned*)&x); /* low word of x */ 84 85 k=0; 86 if (hx < 0x00100000) { /* x < 2**-1022 */ 87 if (((hx&0x7fffffff)|lx)==0) 88 return -two54/zero; /* log(+-0)=-inf */ 89 if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ 90 k -= 54; x *= two54; /* subnormal number, scale up x */ 91 hx = *(n0+(int*)&x); /* high word of x */ 92 } 93 if (hx >= 0x7ff00000) return x+x; 94 k += (hx>>20)-1023; 95 i = ((unsigned)k&0x80000000)>>31; 96 hx = (hx&0x000fffff)|((0x3ff-i)<<20); 97 y = (double)(k+i); 98 *(n0+(int*)&x) = hx; 99 z = y*log10_2lo + ivln10*__ieee754_log(x); 100 return z+y*log10_2hi; 101 } 102