1 /* K=9 r=1/3 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions
2 * 8-bit offset-binary soft decision samples
3 * Copyright Aug 2006, Phil Karn, KA9Q
4 * May be used under the terms of the GNU Lesser General Public License (LGPL)
5 */
6 #include <stdio.h>
7 #include <stdlib.h>
8 #include <memory.h>
9 #include <limits.h>
10 #include "fec.h"
11
12 typedef union { unsigned char c[2][16]; vector unsigned char v[2]; } decision_t;
13 typedef union { unsigned short s[256]; vector unsigned short v[32]; } metric_t;
14
15 static union branchtab39 { unsigned short s[128]; vector unsigned short v[16];} Branchtab39[3];
16 static int Init = 0;
17
18 /* State info for instance of Viterbi decoder */
19 struct v39 {
20 metric_t metrics1; /* path metric buffer 1 */
21 metric_t metrics2; /* path metric buffer 2 */
22 void *dp; /* Pointer to current decision */
23 metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */
24 void *decisions; /* Beginning of decisions for block */
25 };
26
27 /* Initialize Viterbi decoder for start of new frame */
init_viterbi39_av(void * p,int starting_state)28 int init_viterbi39_av(void *p,int starting_state){
29 struct v39 *vp = p;
30 int i;
31
32 for(i=0;i<32;i++)
33 vp->metrics1.v[i] = (vector unsigned short)(1000);
34
35 vp->old_metrics = &vp->metrics1;
36 vp->new_metrics = &vp->metrics2;
37 vp->dp = vp->decisions;
38 vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */
39 return 0;
40 }
41
set_viterbi39_polynomial_av(int polys[3])42 void set_viterbi39_polynomial_av(int polys[3]){
43 int state;
44
45 for(state=0;state < 128;state++){
46 Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0;
47 Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0;
48 Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0;
49 }
50 Init++;
51 }
52
53 /* Create a new instance of a Viterbi decoder */
create_viterbi39_av(int len)54 void *create_viterbi39_av(int len){
55 struct v39 *vp;
56
57 if(!Init){
58 int polys[3] = { V39POLYA, V39POLYB, V39POLYC };
59
60 set_viterbi39_polynomial_av(polys);
61 }
62 vp = (struct v39 *)malloc(sizeof(struct v39));
63 vp->decisions = malloc(sizeof(decision_t)*(len+8));
64 init_viterbi39_av(vp,0);
65 return vp;
66 }
67
68 /* Viterbi chainback */
chainback_viterbi39_av(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)69 int chainback_viterbi39_av(
70 void *p,
71 unsigned char *data, /* Decoded output data */
72 unsigned int nbits, /* Number of data bits */
73 unsigned int endstate){ /* Terminal encoder state */
74 struct v39 *vp = p;
75 decision_t *d = (decision_t *)vp->decisions;
76 int path_metric;
77
78 /* Make room beyond the end of the encoder register so we can
79 * accumulate a full byte of decoded data
80 */
81 endstate %= 256;
82
83 path_metric = vp->old_metrics->s[endstate];
84
85 /* The store into data[] only needs to be done every 8 bits.
86 * But this avoids a conditional branch, and the writes will
87 * combine in the cache anyway
88 */
89 d += 8; /* Look past tail */
90 while(nbits-- != 0){
91 int k;
92
93 k = (d[nbits].c[endstate >> 7][endstate & 15] & (0x80 >> ((endstate>>4)&7)) ) ? 1 : 0;
94 endstate = (k << 7) | (endstate >> 1);
95 data[nbits>>3] = endstate;
96 }
97 return path_metric;
98 }
99
100 /* Delete instance of a Viterbi decoder */
delete_viterbi39_av(void * p)101 void delete_viterbi39_av(void *p){
102 struct v39 *vp = p;
103
104 if(vp != NULL){
105 free(vp->decisions);
106 free(vp);
107 }
108 }
109
update_viterbi39_blk_av(void * p,unsigned char * syms,int nbits)110 int update_viterbi39_blk_av(void *p,unsigned char *syms,int nbits){
111 struct v39 *vp = p;
112 decision_t *d = (decision_t *)vp->dp;
113 int path_metric = 0;
114 vector unsigned char decisions = (vector unsigned char)(0);
115
116 while(nbits--){
117 vector unsigned short symv,sym0v,sym1v,sym2v;
118 vector unsigned char s;
119 void *tmp;
120 int i;
121
122 /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
123 s = (vector unsigned char)vec_perm(vec_ld(0,syms),vec_ld(5,syms),vec_lvsl(0,syms));
124
125 symv = (vector unsigned short)vec_mergeh((vector unsigned char)(0),s); /* Unsigned byte->word unpack */
126 sym0v = vec_splat(symv,0);
127 sym1v = vec_splat(symv,1);
128 sym2v = vec_splat(symv,2);
129 syms += 3;
130
131 for(i=0;i<16;i++){
132 vector bool short decision0,decision1;
133 vector unsigned short metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
134
135 /* Form branch metrics
136 * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
137 * the XOR operations constitute conditional negation.
138 * the metrics are in the range 0-765
139 */
140 m0 = vec_add(vec_xor(Branchtab39[0].v[i],sym0v),vec_xor(Branchtab39[1].v[i],sym1v));
141 m1 = vec_xor(Branchtab39[2].v[i],sym2v);
142 metric = vec_add(m0,m1);
143 m_metric = vec_sub((vector unsigned short)(765),metric);
144
145 /* Add branch metrics to path metrics */
146 m0 = vec_adds(vp->old_metrics->v[i],metric);
147 m3 = vec_adds(vp->old_metrics->v[16+i],metric);
148 m1 = vec_adds(vp->old_metrics->v[16+i],m_metric);
149 m2 = vec_adds(vp->old_metrics->v[i],m_metric);
150
151 /* Compare and select */
152 decision0 = vec_cmpgt(m0,m1);
153 decision1 = vec_cmpgt(m2,m3);
154 survivor0 = vec_min(m0,m1);
155 survivor1 = vec_min(m2,m3);
156
157 /* Store decisions and survivors.
158 * To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in
159 * a funny interleaved fashion that we undo in the chainback function.
160 */
161 decisions = vec_add(decisions,decisions); /* Shift each byte 1 bit to the left */
162
163 /* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting
164 * 0xff is equivalent to adding 1, which sets the lsb.
165 */
166 decisions = vec_sub(decisions,(vector unsigned char)vec_pack(vec_mergeh(decision0,decision1),vec_mergel(decision0,decision1)));
167
168 vp->new_metrics->v[2*i] = vec_mergeh(survivor0,survivor1);
169 vp->new_metrics->v[2*i+1] = vec_mergel(survivor0,survivor1);
170
171 if((i % 8) == 7){
172 /* We've accumulated a total of 128 decisions, stash and start again */
173 d->v[i>>3] = decisions; /* No need to clear, the new bits will replace the old */
174 }
175 }
176 #if 0
177 /* Experimentally determine metric spread
178 * The results are fixed for a given code and input symbol size
179 */
180 {
181 int i;
182 vector unsigned short min_metric;
183 vector unsigned short max_metric;
184 union { vector unsigned short v; unsigned short s[8];} t;
185 int minimum,maximum;
186 static int max_spread = 0;
187
188 min_metric = max_metric = vp->new_metrics->v[0];
189 for(i=1;i<32;i++){
190 min_metric = vec_min(min_metric,vp->new_metrics->v[i]);
191 max_metric = vec_max(max_metric,vp->new_metrics->v[i]);
192 }
193 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,8));
194 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,8));
195 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,4));
196 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,4));
197 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,2));
198 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,2));
199
200 t.v = min_metric;
201 minimum = t.s[0];
202 t.v = max_metric;
203 maximum = t.s[0];
204 if(maximum-minimum > max_spread){
205 max_spread = maximum-minimum;
206 printf("metric spread = %d\n",max_spread);
207 }
208 }
209 #endif
210
211 /* Renormalize if necessary. This deserves some explanation.
212 * The maximum possible spread, found by experiment, for 8 bit symbols is about 3825
213 * So by looking at one arbitrary metric we can tell if any of them have possibly saturated.
214 * However, this is very conservative. Large spreads occur only at very high Eb/No, where
215 * saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor.
216
217 * At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric
218 * by not not normalizing when we should are extremely low. So either way, the risk to performance is small.
219
220 * All this is borne out by experiment.
221 */
222 if(vp->new_metrics->s[0] >= USHRT_MAX-5000){
223 vector unsigned short scale;
224 union { vector unsigned short v; unsigned short s[8];} t;
225
226 /* Find smallest metric and splat */
227 scale = vp->new_metrics->v[0];
228 for(i=1;i<32;i++)
229 scale = vec_min(scale,vp->new_metrics->v[i]);
230
231 scale = vec_min(scale,vec_sld(scale,scale,8));
232 scale = vec_min(scale,vec_sld(scale,scale,4));
233 scale = vec_min(scale,vec_sld(scale,scale,2));
234
235 /* Subtract it from all metrics
236 * Work backwards to try to improve the cache hit ratio, assuming LRU
237 */
238 for(i=31;i>=0;i--)
239 vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i],scale);
240 t.v = scale;
241 path_metric += t.s[0];
242 }
243 d++;
244 /* Swap pointers to old and new metrics */
245 tmp = vp->old_metrics;
246 vp->old_metrics = vp->new_metrics;
247 vp->new_metrics = tmp;
248 }
249 vp->dp = d;
250 return path_metric;
251 }
252