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/* $Id: mmath.c,v 1.2 1997/05/28 03:25:43 brianp Exp $ */
/*
* Mesa 3-D graphics library
* Version: 2.3
* Copyright (C) 1995-1997 Brian Paul
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Library General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Library General Public License for more details.
*
* You should have received a copy of the GNU Library General Public
* License along with this library; if not, write to the Free
* Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
/*
* $Log: mmath.c,v $
* Revision 1.2 1997/05/28 03:25:43 brianp
* added precompiled header (PCH) support
*
* Revision 1.1 1997/05/01 01:41:54 brianp
* Initial revision
*
*/
#ifdef PCH
#include "all.h"
#else
#include <GL/gl.h>
#include "mmath.h"
#endif
/*
* A High Speed, Low Precision Square Root
* by Paul Lalonde and Robert Dawson
* from "Graphics Gems", Academic Press, 1990
*/
/*
* SPARC implementation of a fast square root by table
* lookup.
* SPARC floating point format is as follows:
*
* BIT 31 30 23 22 0
* sign exponent mantissa
*/
static short sqrttab[0x100]; /* declare table of square roots */
static void init_sqrt(void)
{
#ifdef FAST_MATH
unsigned short i;
float f;
unsigned int *fi = (unsigned int *)&f;
/* to access the bits of a float in */
/* C quickly we must misuse pointers */
for(i=0; i<= 0x7f; i++) {
*fi = 0;
/*
* Build a float with the bit pattern i as mantissa
* and an exponent of 0, stored as 127
*/
*fi = (i << 16) | (127 << 23);
f = sqrt(f);
/*
* Take the square root then strip the first 7 bits of
* the mantissa into the table
*/
sqrttab[i] = (*fi & 0x7fffff) >> 16;
/*
* Repeat the process, this time with an exponent of
* 1, stored as 128
*/
*fi = 0;
*fi = (i << 16) | (128 << 23);
f = sqrt(f);
sqrttab[i+0x80] = (*fi & 0x7fffff) >> 16;
}
#endif /*FAST_MATH*/
}
float gl_sqrt( float x )
{
#ifdef FAST_MATH
unsigned int *num = (unsigned int *)&x;
/* to access the bits of a float in C
* we must misuse pointers */
short e; /* the exponent */
if (x == 0.0F) return 0.0F; /* check for square root of 0 */
e = (*num >> 23) - 127; /* get the exponent - on a SPARC the */
/* exponent is stored with 127 added */
*num &= 0x7fffff; /* leave only the mantissa */
if (e & 0x01) *num |= 0x800000;
/* the exponent is odd so we have to */
/* look it up in the second half of */
/* the lookup table, so we set the */
/* high bit */
e >>= 1; /* divide the exponent by two */
/* note that in C the shift */
/* operators are sign preserving */
/* for signed operands */
/* Do the table lookup, based on the quaternary mantissa,
* then reconstruct the result back into a float
*/
*num = ((sqrttab[*num >> 16]) << 16) | ((e + 127) << 23);
return x;
#else
return sqrt(x);
#endif
}
/*
* Initialize tables, etc for fast math functions.
*/
void gl_init_math(void)
{
static GLboolean initialized = GL_FALSE;
if (!initialized) {
init_sqrt();
initialized = GL_TRUE;
}
}
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