89c9aa491a
POSIX 2008 states that if the input for 'logb[f|l]' is a subnormal number it should be treated as if it were normalized. This means the implementation should calculate the log2 of the mantissa and add it to the subnormal exponent (-126 for float and -1022 for double and IBM long double). This patch takes care of that.
48 lines
1.2 KiB
C
48 lines
1.2 KiB
C
/* @(#)s_logb.c 5.1 93/09/24 */
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/*
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* double logb(x)
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* IEEE 754 logb. Included to pass IEEE test suite. Not recommend.
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* Use ilogb instead.
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*/
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#include <math.h>
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#include <math_private.h>
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double
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__logb (double x)
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{
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int32_t lx, ix, rix;
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EXTRACT_WORDS (ix, lx, x);
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ix &= 0x7fffffff; /* high |x| */
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if ((ix | lx) == 0)
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return -1.0 / fabs (x);
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if (ix >= 0x7ff00000)
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return x * x;
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if (__builtin_expect ((rix = ix >> 20) == 0, 0))
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{
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/* POSIX specifies that denormal number is treated as
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though it were normalized. */
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int m1 = (ix == 0) ? 0 : __builtin_clz (ix);
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int m2 = (lx == 0) ? 0 : __builtin_clz (lx);
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int ma = (m1 == 0) ? m2 + 32 : m1;
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return -1022.0 + (double)(11 - ma);
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}
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return (double) (rix - 1023);
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}
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weak_alias (__logb, logb)
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#ifdef NO_LONG_DOUBLE
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strong_alias (__logb, __logbl) weak_alias (__logb, logbl)
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#endif
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