0cbb654791
zero, one, two, half are replaced by const literals The policy was to use the f suffix for float consts (1.0f), but don't use suffix for long double consts (these consts can be exactly represented as double).
79 lines
2.2 KiB
C
79 lines
2.2 KiB
C
/* origin: FreeBSD /usr/src/lib/msun/src/e_log2f.c */
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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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* See comments in log2.c.
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*/
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#include "libm.h"
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#include "__log1pf.h"
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static const float
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two25 = 3.3554432000e+07, /* 0x4c000000 */
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ivln2hi = 1.4428710938e+00, /* 0x3fb8b000 */
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ivln2lo = -1.7605285393e-04; /* 0xb9389ad4 */
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float log2f(float x)
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{
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float f,hfsq,hi,lo,r,y;
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int32_t i,k,hx;
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GET_FLOAT_WORD(hx, x);
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k = 0;
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if (hx < 0x00800000) { /* x < 2**-126 */
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if ((hx&0x7fffffff) == 0)
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return -two25/0.0f; /* log(+-0)=-inf */
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if (hx < 0)
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return (x-x)/0.0f; /* log(-#) = NaN */
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/* subnormal number, scale up x */
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k -= 25;
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x *= two25;
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GET_FLOAT_WORD(hx, x);
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}
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if (hx >= 0x7f800000)
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return x+x;
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if (hx == 0x3f800000)
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return 0.0f; /* log(1) = +0 */
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k += (hx>>23) - 127;
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hx &= 0x007fffff;
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i = (hx+(0x4afb0d))&0x800000;
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SET_FLOAT_WORD(x, hx|(i^0x3f800000)); /* normalize x or x/2 */
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k += i>>23;
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y = (float)k;
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f = x - 1.0f;
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hfsq = 0.5f * f * f;
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r = __log1pf(f);
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/*
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* We no longer need to avoid falling into the multi-precision
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* calculations due to compiler bugs breaking Dekker's theorem.
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* Keep avoiding this as an optimization. See log2.c for more
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* details (some details are here only because the optimization
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* is not yet available in double precision).
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*
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* Another compiler bug turned up. With gcc on i386,
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* (ivln2lo + ivln2hi) would be evaluated in float precision
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* despite runtime evaluations using double precision. So we
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* must cast one of its terms to float_t. This makes the whole
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* expression have type float_t, so return is forced to waste
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* time clobbering its extra precision.
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*/
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// FIXME
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// if (sizeof(float_t) > sizeof(float))
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// return (r - hfsq + f) * ((float_t)ivln2lo + ivln2hi) + y;
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hi = f - hfsq;
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GET_FLOAT_WORD(hx,hi);
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SET_FLOAT_WORD(hi,hx&0xfffff000);
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lo = (f - hi) - hfsq + r;
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return (lo+hi)*ivln2lo + lo*ivln2hi + hi*ivln2hi + y;
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}
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