source:
asp3_tinet_ecnl_arm/trunk/musl-1.1.18/src/math/tanh.c@
373
Last change on this file since 373 was 352, checked in by , 6 years ago | |
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1 | #include "libm.h" |
2 | |
3 | /* tanh(x) = (exp(x) - exp(-x))/(exp(x) + exp(-x)) |
4 | * = (exp(2*x) - 1)/(exp(2*x) - 1 + 2) |
5 | * = (1 - exp(-2*x))/(exp(-2*x) - 1 + 2) |
6 | */ |
7 | double tanh(double x) |
8 | { |
9 | union {double f; uint64_t i;} u = {.f = x}; |
10 | uint32_t w; |
11 | int sign; |
12 | double_t t; |
13 | |
14 | /* x = |x| */ |
15 | sign = u.i >> 63; |
16 | u.i &= (uint64_t)-1/2; |
17 | x = u.f; |
18 | w = u.i >> 32; |
19 | |
20 | if (w > 0x3fe193ea) { |
21 | /* |x| > log(3)/2 ~= 0.5493 or nan */ |
22 | if (w > 0x40340000) { |
23 | /* |x| > 20 or nan */ |
24 | /* note: this branch avoids raising overflow */ |
25 | t = 1 - 0/x; |
26 | } else { |
27 | t = expm1(2*x); |
28 | t = 1 - 2/(t+2); |
29 | } |
30 | } else if (w > 0x3fd058ae) { |
31 | /* |x| > log(5/3)/2 ~= 0.2554 */ |
32 | t = expm1(2*x); |
33 | t = t/(t+2); |
34 | } else if (w >= 0x00100000) { |
35 | /* |x| >= 0x1p-1022, up to 2ulp error in [0.1,0.2554] */ |
36 | t = expm1(-2*x); |
37 | t = -t/(t+2); |
38 | } else { |
39 | /* |x| is subnormal */ |
40 | /* note: the branch above would not raise underflow in [0x1p-1023,0x1p-1022) */ |
41 | FORCE_EVAL((float)x); |
42 | t = x; |
43 | } |
44 | return sign ? -t : t; |
45 | } |
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