1 /* 2 * ==================================================== 3 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. 4 * 5 * Developed at SunPro, a Sun Microsystems, Inc. business. 6 * Permission to use, copy, modify, and distribute this 7 * software is freely granted, provided that this notice 8 * is preserved. 9 * ==================================================== 10 */ 11 12 #include <sys/cdefs.h> 13 __FBSDID("$FreeBSD$"); 14 15 /* 16 * Return the base 2 logarithm of x. See k_log.c for details on the algorithm. 17 */ 18 19 #include "math.h" 20 #include "math_private.h" 21 #include "k_logf.h" 22 23 static const float 24 two25 = 3.3554432000e+07, /* 0x4c000000 */ 25 ivln2hi = 1.4428710938e+00, /* 0x3fb8b000 */ 26 ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */ 27 28 static const float zero = 0.0; 29 30 float 31 __ieee754_log2f(float x) 32 { 33 float f,hi,lo; 34 int32_t i,k,hx; 35 36 GET_FLOAT_WORD(hx,x); 37 38 k=0; 39 if (hx < 0x00800000) { /* x < 2**-126 */ 40 if ((hx&0x7fffffff)==0) 41 return -two25/zero; /* log(+-0)=-inf */ 42 if (hx<0) return (x-x)/zero; /* log(-#) = NaN */ 43 k -= 25; x *= two25; /* subnormal number, scale up x */ 44 GET_FLOAT_WORD(hx,x); 45 } 46 if (hx >= 0x7f800000) return x+x; 47 k += (hx>>23)-127; 48 hx &= 0x007fffff; 49 i = (hx+(0x4afb0d))&0x800000; 50 SET_FLOAT_WORD(x,hx|(i^0x3f800000)); /* normalize x or x/2 */ 51 k += (i>>23); 52 f = __kernel_logf(x); 53 x = x - (float)1.0; 54 GET_FLOAT_WORD(hx,x); 55 SET_FLOAT_WORD(hi,hx&0xfffff000); 56 lo = x - hi; 57 return (x+f)*ivln2lo + (lo+f)*ivln2hi + hi*ivln2hi + k; 58 } 59