1 /* K=9 r=1/3 Viterbi decoder for x86 MMX
2 * Aug 2006, Phil Karn, KA9Q
3 * May be used under the terms of the GNU Lesser General Public License (LGPL)
4 */
5 #include <mmintrin.h>
6 #include <stdio.h>
7 #include <stdlib.h>
8 #include <memory.h>
9 #include "fec.h"
10
11 typedef union { unsigned char c[256]; __m64 v[32];} decision_t;
12 typedef union { unsigned short s[256]; __m64 v[64];} metric_t;
13
14 static union branchtab39 { unsigned short s[128]; __m64 v[32];} Branchtab39[3];
15 static int Init = 0;
16
17 /* State info for instance of Viterbi decoder */
18 struct v39 {
19 metric_t metrics1; /* path metric buffer 1 */
20 metric_t metrics2; /* path metric buffer 2 */
21 void *dp; /* Pointer to current decision */
22 metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
23 void *decisions; /* Beginning of decisions for block */
24 };
25
26 /* Initialize Viterbi decoder for start of new frame */
init_viterbi39_mmx(void * p,int starting_state)27 int init_viterbi39_mmx(void *p,int starting_state){
28 struct v39 *vp = p;
29 int i;
30
31 if(p == NULL)
32 return -1;
33 for(i=0;i<256;i++)
34 vp->metrics1.s[i] = 1000;
35
36 vp->old_metrics = &vp->metrics1;
37 vp->new_metrics = &vp->metrics2;
38 vp->dp = vp->decisions;
39 vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */
40 return 0;
41 }
42
set_viterbi39_polynomial_mmx(int polys[3])43 void set_viterbi39_polynomial_mmx(int polys[3]){
44 int state;
45
46 for(state=0;state < 128;state++){
47 Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & polys[0]) ? 255:0;
48 Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & polys[1]) ? 255:0;
49 Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & polys[2]) ? 255:0;
50 }
51 Init++;
52 }
53
54 /* Create a new instance of a Viterbi decoder */
create_viterbi39_mmx(int len)55 void *create_viterbi39_mmx(int len){
56 struct v39 *vp;
57
58 if(!Init){
59 int polys[3] = { V39POLYA,V39POLYB,V39POLYC };
60 set_viterbi39_polynomial_mmx(polys);
61 }
62 if((vp = (struct v39 *)malloc(sizeof(struct v39))) == NULL)
63 return NULL;
64 if((vp->decisions = malloc((len+8)*sizeof(decision_t))) == NULL){
65 free(vp);
66 return NULL;
67 }
68 init_viterbi39_mmx(vp,0);
69 return vp;
70 }
71
72
73
74 /* Viterbi chainback */
chainback_viterbi39_mmx(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)75 int chainback_viterbi39_mmx(
76 void *p,
77 unsigned char *data, /* Decoded output data */
78 unsigned int nbits, /* Number of data bits */
79 unsigned int endstate){ /* Terminal encoder state */
80 struct v39 *vp = p;
81 decision_t *d;
82 int path_metric;
83
84 if(p == NULL)
85 return -1;
86
87 d = (decision_t *)vp->decisions;
88
89 endstate %= 256;
90
91 path_metric = vp->old_metrics->s[endstate];
92
93 /* The store into data[] only needs to be done every 8 bits.
94 * But this avoids a conditional branch, and the writes will
95 * combine in the cache anyway
96 */
97 d += 8; /* Look past tail */
98 while(nbits-- != 0){
99 int k;
100
101 k = d[nbits].c[endstate] & 1;
102 endstate = (k << 7) | (endstate >> 1);
103 data[nbits>>3] = endstate;
104 }
105 return path_metric;
106 }
107
108 /* Delete instance of a Viterbi decoder */
delete_viterbi39_mmx(void * p)109 void delete_viterbi39_mmx(void *p){
110 struct v39 *vp = p;
111
112 if(vp != NULL){
113 free(vp->decisions);
114 free(vp);
115 }
116 }
117
118
update_viterbi39_blk_mmx(void * p,unsigned char * syms,int nbits)119 int update_viterbi39_blk_mmx(void *p,unsigned char *syms,int nbits){
120 struct v39 *vp = p;
121 decision_t *d;
122 int path_metric = 0;
123
124 if(p == NULL)
125 return -1;
126
127 d = (decision_t *)vp->dp;
128
129 while(nbits--){
130 __m64 sym0v,sym1v,sym2v;
131 void *tmp;
132 int i;
133
134 /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
135 sym0v = _mm_set1_pi16(syms[0]);
136 sym1v = _mm_set1_pi16(syms[1]);
137 sym2v = _mm_set1_pi16(syms[2]);
138 syms += 3;
139
140 for(i=0;i<32;i++){
141 __m64 decision0,decision1,metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
142
143 /* Form branch metrics
144 * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
145 * the XOR operations constitute conditional negation.
146 * metric and m_metric (-metric) are in the range 0-1530
147 */
148 m0 = _mm_add_pi16(_mm_xor_si64(Branchtab39[0].v[i],sym0v),_mm_xor_si64(Branchtab39[1].v[i],sym1v));
149 metric = _mm_add_pi16(_mm_xor_si64(Branchtab39[2].v[i],sym2v),m0);
150 m_metric = _mm_sub_pi16(_mm_set1_pi16(765),metric);
151
152 /* Add branch metrics to path metrics */
153 m0 = _mm_add_pi16(vp->old_metrics->v[i],metric);
154 m3 = _mm_add_pi16(vp->old_metrics->v[32+i],metric);
155 m1 = _mm_add_pi16(vp->old_metrics->v[32+i],m_metric);
156 m2 = _mm_add_pi16(vp->old_metrics->v[i],m_metric);
157
158 /* Compare and select
159 * There's no packed min instruction in MMX, so we use modulo arithmetic
160 * to form the decisions and then do the select the hard way
161 */
162 decision0 = _mm_cmpgt_pi16(_mm_sub_pi16(m0,m1),_mm_setzero_si64());
163 decision1 = _mm_cmpgt_pi16(_mm_sub_pi16(m2,m3),_mm_setzero_si64());
164 survivor0 = _mm_or_si64(_mm_and_si64(decision0,m1),_mm_andnot_si64(decision0,m0));
165 survivor1 = _mm_or_si64(_mm_and_si64(decision1,m3),_mm_andnot_si64(decision1,m2));
166
167 /* Merge decisions and store as bytes */
168 d->v[i] = _mm_unpacklo_pi8(_mm_packs_pi16(decision0,_mm_setzero_si64()),_mm_packs_pi16(decision1,_mm_setzero_si64()));
169
170 /* Store surviving metrics */
171 vp->new_metrics->v[2*i] = _mm_unpacklo_pi16(survivor0,survivor1);
172 vp->new_metrics->v[2*i+1] = _mm_unpackhi_pi16(survivor0,survivor1);
173 }
174 if(vp->new_metrics->s[0] < vp->old_metrics->s[0])
175 path_metric += 65536; /* Hack: wraparound probably occured */
176 d++;
177 /* Swap pointers to old and new metrics */
178 tmp = vp->old_metrics;
179 vp->old_metrics = vp->new_metrics;
180 vp->new_metrics = tmp;
181 }
182 vp->dp = d;
183 _mm_empty();
184 return path_metric;
185 }
186