mirror of
https://github.com/fluencelabs/musl
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52 lines
1.9 KiB
C
52 lines
1.9 KiB
C
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/* origin: FreeBSD /usr/src/lib/msun/src/k_exp.c */
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/*-
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* Copyright (c) 2011 David Schultz <das@FreeBSD.ORG>
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include "libm.h"
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/*
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* We use exp(x) = exp(x - kln2) * 2**k,
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* k is carefully chosen to minimize |exp(kln2) - 2**k|
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*/
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static const uint32_t k = 1799;
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static const double kln2 = 1246.97177782734161156;
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/* exp(x)/2 when x is huge */
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double __expo2(double x)
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{
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double scale;
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int n;
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/*
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* efficient scalbn(y, k-1):
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* 2**(k-1) cannot be represented
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* so we use that k-1 is even and scale in two steps
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*/
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n = (k - 1)/2;
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INSERT_WORDS(scale, (0x3ff + n) << 20, 0);
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return exp(x - kln2) * scale * scale;
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}
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