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comparison libtwamr/inv_sqrt.c @ 253:54f6bc41ed10

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libtwamr: integrate a* modules
author Mychaela Falconia <falcon@freecalypso.org>
date Fri, 05 Apr 2024 06:08:15 +0000
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children 07f936338de1
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252:57b4053559ff 253:54f6bc41ed10
1 /*
2 ********************************************************************************
3 *
4 * GSM AMR-NB speech codec R98 Version 7.6.0 December 12, 2001
5 * R99 Version 3.3.0
6 * REL-4 Version 4.1.0
7 *
8 ********************************************************************************
9 *
10 * File : inv_sqrt.c
11 * Purpose : Computes 1/sqrt(L_x), where L_x is positive.
12 * : If L_x is negative or zero,
13 * : the result is 1 (3fff ffff).
14 * Description :
15 * The function 1/sqrt(L_x) is approximated by a table and linear
16 * interpolation. The inverse square root is computed using the
17 * following steps:
18 * 1- Normalization of L_x.
19 * 2- If (30-exponent) is even then shift right once.
20 * 3- exponent = (30-exponent)/2 +1
21 * 4- i = bit25-b31 of L_x; 16<=i<=63 because of normalization.
22 * 5- a = bit10-b24
23 * 6- i -=16
24 * 7- L_y = table[i]<<16 - (table[i] - table[i+1]) * a * 2
25 * 8- L_y >>= exponent
26 *
27 ********************************************************************************
28 */
29 /*
30 ********************************************************************************
31 * MODULE INCLUDE FILE AND VERSION ID
32 ********************************************************************************
33 */
34 #include "namespace.h"
35 #include "inv_sqrt.h"
36 const char inv_sqrt_id[] = "@(#)$Id $" inv_sqrt_h;
37
38 /*
39 ********************************************************************************
40 * INCLUDE FILES
41 ********************************************************************************
42 */
43 #include "typedef.h"
44 #include "basic_op.h"
45 #include "no_count.h"
46
47 /*
48 ********************************************************************************
49 * LOCAL VARIABLES AND TABLES
50 ********************************************************************************
51 */
52 #include "inv_sqrt.tab" /* Table for inv_sqrt() */
53
54 /*
55 ********************************************************************************
56 * PUBLIC PROGRAM CODE
57 ********************************************************************************
58 */
59
60 Word32 Inv_sqrt ( /* (o) : output value */
61 Word32 L_x /* (i) : input value */
62 )
63 {
64 Word16 exp, i, a, tmp;
65 Word32 L_y;
66
67 test ();
68 if (L_x <= (Word32) 0)
69 return ((Word32) 0x3fffffffL);
70
71 exp = norm_l (L_x);
72 L_x = L_shl (L_x, exp); /* L_x is normalize */
73
74 exp = sub (30, exp);
75 test (); logic16 ();
76 if ((exp & 1) == 0) /* If exponent even -> shift right */
77 {
78 L_x = L_shr (L_x, 1);
79 }
80 exp = shr (exp, 1);
81 exp = add (exp, 1);
82
83 L_x = L_shr (L_x, 9);
84 i = extract_h (L_x); /* Extract b25-b31 */
85 L_x = L_shr (L_x, 1);
86 a = extract_l (L_x); /* Extract b10-b24 */
87 a = a & (Word16) 0x7fff; logic16 ();
88
89 i = sub (i, 16);
90
91 L_y = L_deposit_h (table[i]); /* table[i] << 16 */
92 tmp = sub (table[i], table[i + 1]); /* table[i] - table[i+1]) */
93 L_y = L_msu (L_y, tmp, a); /* L_y -= tmp*a*2 */
94
95 L_y = L_shr (L_y, exp); /* denormalization */
96
97 return (L_y);
98 }