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poly_2xm1.c
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1 /*---------------------------------------------------------------------------+
2  | poly_2xm1.c |
3  | |
4  | Function to compute 2^x-1 by a polynomial approximation. |
5  | |
6  | Copyright (C) 1992,1993,1994,1997 |
7  | W. Metzenthen, 22 Parker St, Ormond, Vic 3163, Australia |
8  | E-mail [email protected] |
9  | |
10  | |
11  +---------------------------------------------------------------------------*/
12 
13 #include "exception.h"
14 #include "reg_constant.h"
15 #include "fpu_emu.h"
16 #include "fpu_system.h"
17 #include "control_w.h"
18 #include "poly.h"
19 
20 #define HIPOWER 11
21 static const unsigned long long lterms[HIPOWER] = {
22  0x0000000000000000LL, /* This term done separately as 12 bytes */
23  0xf5fdeffc162c7543LL,
24  0x1c6b08d704a0bfa6LL,
25  0x0276556df749cc21LL,
26  0x002bb0ffcf14f6b8LL,
27  0x0002861225ef751cLL,
28  0x00001ffcbfcd5422LL,
29  0x00000162c005d5f1LL,
30  0x0000000da96ccb1bLL,
31  0x0000000078d1b897LL,
32  0x000000000422b029LL
33 };
34 
35 static const Xsig hiterm = MK_XSIG(0xb17217f7, 0xd1cf79ab, 0xc8a39194);
36 
37 /* Four slices: 0.0 : 0.25 : 0.50 : 0.75 : 1.0,
38  These numbers are 2^(1/4), 2^(1/2), and 2^(3/4)
39  */
40 static const Xsig shiftterm0 = MK_XSIG(0, 0, 0);
41 static const Xsig shiftterm1 = MK_XSIG(0x9837f051, 0x8db8a96f, 0x46ad2318);
42 static const Xsig shiftterm2 = MK_XSIG(0xb504f333, 0xf9de6484, 0x597d89b3);
43 static const Xsig shiftterm3 = MK_XSIG(0xd744fcca, 0xd69d6af4, 0x39a68bb9);
44 
45 static const Xsig *shiftterm[] = { &shiftterm0, &shiftterm1,
46  &shiftterm2, &shiftterm3
47 };
48 
49 /*--- poly_2xm1() -----------------------------------------------------------+
50  | Requires st(0) which is TAG_Valid and < 1. |
51  +---------------------------------------------------------------------------*/
53 {
54  long int exponent, shift;
55  unsigned long long Xll;
56  Xsig accumulator, Denom, argSignif;
57  u_char tag;
58 
59  exponent = exponent16(arg);
60 
61 #ifdef PARANOID
62  if (exponent >= 0) { /* Don't want a |number| >= 1.0 */
63  /* Number negative, too large, or not Valid. */
64  EXCEPTION(EX_INTERNAL | 0x127);
65  return 1;
66  }
67 #endif /* PARANOID */
68 
69  argSignif.lsw = 0;
70  XSIG_LL(argSignif) = Xll = significand(arg);
71 
72  if (exponent == -1) {
73  shift = (argSignif.msw & 0x40000000) ? 3 : 2;
74  /* subtract 0.5 or 0.75 */
75  exponent -= 2;
76  XSIG_LL(argSignif) <<= 2;
77  Xll <<= 2;
78  } else if (exponent == -2) {
79  shift = 1;
80  /* subtract 0.25 */
81  exponent--;
82  XSIG_LL(argSignif) <<= 1;
83  Xll <<= 1;
84  } else
85  shift = 0;
86 
87  if (exponent < -2) {
88  /* Shift the argument right by the required places. */
89  if (FPU_shrx(&Xll, -2 - exponent) >= 0x80000000U)
90  Xll++; /* round up */
91  }
92 
93  accumulator.lsw = accumulator.midw = accumulator.msw = 0;
94  polynomial_Xsig(&accumulator, &Xll, lterms, HIPOWER - 1);
95  mul_Xsig_Xsig(&accumulator, &argSignif);
96  shr_Xsig(&accumulator, 3);
97 
98  mul_Xsig_Xsig(&argSignif, &hiterm); /* The leading term */
99  add_two_Xsig(&accumulator, &argSignif, &exponent);
100 
101  if (shift) {
102  /* The argument is large, use the identity:
103  f(x+a) = f(a) * (f(x) + 1) - 1;
104  */
105  shr_Xsig(&accumulator, -exponent);
106  accumulator.msw |= 0x80000000; /* add 1.0 */
107  mul_Xsig_Xsig(&accumulator, shiftterm[shift]);
108  accumulator.msw &= 0x3fffffff; /* subtract 1.0 */
109  exponent = 1;
110  }
111 
112  if (sign != SIGN_POS) {
113  /* The argument is negative, use the identity:
114  f(-x) = -f(x) / (1 + f(x))
115  */
116  Denom.lsw = accumulator.lsw;
117  XSIG_LL(Denom) = XSIG_LL(accumulator);
118  if (exponent < 0)
119  shr_Xsig(&Denom, -exponent);
120  else if (exponent > 0) {
121  /* exponent must be 1 here */
122  XSIG_LL(Denom) <<= 1;
123  if (Denom.lsw & 0x80000000)
124  XSIG_LL(Denom) |= 1;
125  (Denom.lsw) <<= 1;
126  }
127  Denom.msw |= 0x80000000; /* add 1.0 */
128  div_Xsig(&accumulator, &Denom, &accumulator);
129  }
130 
131  /* Convert to 64 bit signed-compatible */
132  exponent += round_Xsig(&accumulator);
133 
134  result = &st(0);
135  significand(result) = XSIG_LL(accumulator);
136  setexponent16(result, exponent);
137 
138  tag = FPU_round(result, 1, 0, FULL_PRECISION, sign);
139 
140  setsign(result, sign);
141  FPU_settag0(tag);
142 
143  return 0;
144 
145 }