45 lines
1.4 KiB
C
45 lines
1.4 KiB
C
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/*
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* ====================================================
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* Copyright (C) 1998 by Cygnus Solutions. All rights reserved.
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*
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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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Fast version of pow using Intel float instructions.
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double _f_pow (double x, double y);
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Function calculates x to power of y.
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The function optimizes the case where x is >0.0 and y is finite.
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In such a case, there is no error checking or setting of errno.
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All other cases defer to normal pow() function which will
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set errno as normal.
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*/
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#include <math.h>
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#include <ieeefp.h>
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#include "f_math.h"
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double _f_pow (double x, double y)
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{
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/* following sequence handles the majority of cases for pow() */
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if (x > 0.0 && check_finite(y))
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{
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double result;
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/* calculate x ** y as 2 ** (y log2(x)). On Intel, can only
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raise 2 to an integer or a small fraction, thus, we have
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to perform two steps 2**integer portion * 2**fraction. */
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asm ("fldl 8(%%ebp); fyl2x; fld %%st; frndint; fsub %%st,%%st(1);" \
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"fxch; fchs; f2xm1; fld1; faddp; fxch; fld1; fscale; fstp %%st(1);"\
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"fmulp" : "=t" (result) : "0" (y));
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return result;
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}
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else /* all other strange cases, defer to normal pow() */
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return pow (x,y);
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}
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