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33 * @(#)log10.c 8.1 (Berkeley) 6/4/93
37 * RETURN THE BASE 10 LOGARITHM OF x
38 * DOUBLE PRECISION (VAX D format 56 bits, IEEE DOUBLE 53 BITS)
39 * CODED IN C BY K.C. NG, 1/20/85;
40 * REVISED BY K.C. NG on 1/23/85, 3/7/85, 4/16/85.
42 * Required kernel function:
47 * log10(x) = --------- or [1/log(10)]*log(x)
51 * [log(10)] rounded to 56 bits has error .0895 ulps,
52 * [1/log(10)] rounded to 53 bits has error .198 ulps;
53 * therefore, for better accuracy, in VAX D format, we divide
54 * log(x) by log(10), but in IEEE Double format, we multiply
55 * log(x) by [1/log(10)].
58 * log10(x) is NaN with signal if x < 0;
59 * log10(+INF) is +INF with no signal; log10(0) is -INF with signal;
60 * log10(NaN) is that NaN with no signal.
63 * log10(X) returns the exact log10(x) nearly rounded. In a test run
64 * with 1,536,000 random arguments on a VAX, the maximum observed
65 * error was 1.74 ulps (units in the last place).
68 * The hexadecimal values are the intended ones for the following constants.
69 * The decimal values may be used, provided that the compiler will convert
70 * from decimal to binary accurately enough to produce the hexadecimal values
76 vc(ln10hi, 2.3025850929940456790E0 ,5d8d,4113,a8ac,ddaa, 2, .935D8DDDAAA8AC)
78 ic(ivln10, 4.3429448190325181667E-1, -2, 1.BCB7B1526E50E)
81 #define ln10hi vccast(ln10hi)
88 #if defined(vax)||defined(tahoe)
89 return(log(x)/ln10hi);
90 #else /* defined(vax)||defined(tahoe) */
91 return(ivln10*log(x));
92 #endif /* defined(vax)||defined(tahoe) */