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/*---------------------------------------------------------------------------+
| poly_2xm1.c |
| |
| Function to compute 2^x-1 by a polynomial approximation. |
| |
| Copyright (C) 1992,1993 |
| W. Metzenthen, 22 Parker St, Ormond, Vic 3163, |
| Australia. E-mail billm@vaxc.cc.monash.edu.au |
| |
| |
+---------------------------------------------------------------------------*/
#include "exception.h"
#include "reg_constant.h"
#include "fpu_emu.h"
#define HIPOWER 13
static unsigned short const lterms[HIPOWER][4] =
{
{ 0x79b5, 0xd1cf, 0x17f7, 0xb172 },
{ 0x1b56, 0x058b, 0x7bff, 0x3d7f },
{ 0x8bb0, 0x8250, 0x846b, 0x0e35 },
{ 0xbc65, 0xf747, 0x556d, 0x0276 },
{ 0x17cb, 0x9e39, 0x61ff, 0x0057 },
{ 0xe018, 0x9776, 0x1848, 0x000a },
{ 0x66f2, 0xff30, 0xffe5, 0x0000 },
{ 0x682f, 0xffb6, 0x162b, 0x0000 },
{ 0xb7ca, 0x2956, 0x01b5, 0x0000 },
{ 0xcd3e, 0x4817, 0x001e, 0x0000 },
{ 0xb7e2, 0xecbe, 0x0001, 0x0000 },
{ 0x0ed5, 0x1a27, 0x0000, 0x0000 },
{ 0x101d, 0x0222, 0x0000, 0x0000 },
};
/*--- poly_2xm1() -----------------------------------------------------------+
| Requires a positive argument which is TW_Valid and < 1. |
+---------------------------------------------------------------------------*/
int poly_2xm1(FPU_REG const *arg, FPU_REG *result)
{
short exponent;
long long Xll;
FPU_REG accum;
exponent = arg->exp - EXP_BIAS;
#ifdef PARANOID
if ( (arg->sign != SIGN_POS) /* Can't hack a number < 0.0 */
|| (exponent >= 0) /* or a |number| >= 1.0 */
|| (arg->tag != TW_Valid) )
{
/* Number negative, too large, or not Valid. */
EXCEPTION(EX_INTERNAL|0x127);
return 1;
}
#endif PARANOID
*(unsigned *)&Xll = arg->sigl;
*(((unsigned *)&Xll)+1) = arg->sigh;
if ( exponent < -1 )
{
/* Shift the argument right by the required places. */
if ( shrx(&Xll, -1-exponent) >= 0x80000000U )
Xll++; /* round up */
}
*(short *)&(accum.sign) = 0; /* Will be a valid positive nr with expon = 0 */
accum.exp = 0;
/* Do the basic fixed point polynomial evaluation */
polynomial((unsigned *)&accum.sigl, (unsigned *)&Xll, lterms, HIPOWER-1);
/* Convert to 64 bit signed-compatible */
accum.exp += EXP_BIAS - 1;
reg_move(&accum, result);
normalize(result);
return 0;
}