2 /* @(#)e_log.c 5.1 93/09/24 */
4 * ====================================================
5 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
7 * Developed at SunPro, a Sun Microsystems, Inc. business.
8 * Permission to use, copy, modify, and distribute this
9 * software is freely granted, provided that this notice
11 * ====================================================
15 * Return the logrithm of x
18 * 1. Argument Reduction: find k and f such that
20 * where sqrt(2)/2 < 1+f < sqrt(2) .
22 * 2. Approximation of log(1+f).
23 * Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s)
24 * = 2s + 2/3 s**3 + 2/5 s**5 + .....,
26 * We use a special Reme algorithm on [0,0.1716] to generate
27 * a polynomial of degree 14 to approximate R The maximum error
28 * of this polynomial approximation is bounded by 2**-58.45. In
31 * R(z) ~ Lg1*s +Lg2*s +Lg3*s +Lg4*s +Lg5*s +Lg6*s +Lg7*s
32 * (the values of Lg1 to Lg7 are listed in the program)
35 * | Lg1*s +...+Lg7*s - R(z) | <= 2
37 * Note that 2s = f - s*f = f - hfsq + s*hfsq, where hfsq = f*f/2.
38 * In order to guarantee error in log below 1ulp, we compute log
40 * log(1+f) = f - s*(f - R) (if f is not too large)
41 * log(1+f) = f - (hfsq - s*(hfsq+R)). (better accuracy)
43 * 3. Finally, log(x) = k*ln2 + log(1+f).
44 * = k*ln2_hi+(f-(hfsq-(s*(hfsq+R)+k*ln2_lo)))
45 * Here ln2 is split into two floating point number:
47 * where n*ln2_hi is always exact for |n| < 2000.
50 * log(x) is NaN with signal if x < 0 (including -INF) ;
51 * log(+INF) is +INF; log(0) is -INF with signal;
52 * log(NaN) is that NaN with no signal.
55 * according to an error analysis, the error is always less than
56 * 1 ulp (unit in the last place).
59 * The hexadecimal values are the intended ones for the following
60 * constants. The decimal values may be used, provided that the
61 * compiler will convert from decimal to binary accurately enough
62 * to produce the hexadecimal values shown.
67 #ifndef _DOUBLE_IS_32BITS
74 ln2_hi
= 6.93147180369123816490e-01, /* 3fe62e42 fee00000 */
75 ln2_lo
= 1.90821492927058770002e-10, /* 3dea39ef 35793c76 */
76 two54
= 1.80143985094819840000e+16, /* 43500000 00000000 */
77 Lg1
= 6.666666666666735130e-01, /* 3FE55555 55555593 */
78 Lg2
= 3.999999999940941908e-01, /* 3FD99999 9997FA04 */
79 Lg3
= 2.857142874366239149e-01, /* 3FD24924 94229359 */
80 Lg4
= 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */
81 Lg5
= 1.818357216161805012e-01, /* 3FC74664 96CB03DE */
82 Lg6
= 1.531383769920937332e-01, /* 3FC39A09 D078C69F */
83 Lg7
= 1.479819860511658591e-01; /* 3FC2F112 DF3E5244 */
86 static const double zero
= 0.0;
88 static double zero
= 0.0;
92 double __ieee754_log(double x
)
94 double __ieee754_log(x
)
98 double hfsq
,f
,s
,z
,R
,w
,t1
,t2
,dk
;
102 EXTRACT_WORDS(hx
,lx
,x
);
105 if (hx
< 0x00100000) { /* x < 2**-1022 */
106 if (((hx
&0x7fffffff)|lx
)==0)
107 return -two54
/zero
; /* log(+-0)=-inf */
108 if (hx
<0) return (x
-x
)/zero
; /* log(-#) = NaN */
109 k
-= 54; x
*= two54
; /* subnormal number, scale up x */
112 if (hx
>= 0x7ff00000) return x
+x
;
115 i
= (hx
+0x95f64)&0x100000;
116 SET_HIGH_WORD(x
,hx
|(i
^0x3ff00000)); /* normalize x or x/2 */
119 if((0x000fffff&(2+hx
))<3) { /* |f| < 2**-20 */
125 return dk
*ln2_hi
+dk
*ln2_lo
;
128 R
= f
*f
*(0.5-0.33333333333333333*f
);
129 if(k
==0) return f
-R
; else {dk
=(double)k
;
130 return dk
*ln2_hi
-((R
-dk
*ln2_lo
)-f
);}
138 t1
= w
*(Lg2
+w
*(Lg4
+w
*Lg6
));
139 t2
= z
*(Lg1
+w
*(Lg3
+w
*(Lg5
+w
*Lg7
)));
144 if(k
==0) return f
-(hfsq
-s
*(hfsq
+R
)); else
145 return dk
*ln2_hi
-((hfsq
-(s
*(hfsq
+R
)+dk
*ln2_lo
))-f
);
147 if(k
==0) return f
-s
*(f
-R
); else
148 return dk
*ln2_hi
-((s
*(f
-R
)-dk
*ln2_lo
)-f
);
152 #endif /* defined(_DOUBLE_IS_32BITS) */