1 /* K=15 r=1/6 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions
2 * 8-bit offset-binary soft decision samples
3 * Copyright Mar 2004, 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[128][16]; vector unsigned char v[128]; } decision_t;
13 typedef union { unsigned short s[16384]; vector unsigned short v[2048]; } metric_t;
14
15 static union branchtab615 { unsigned short s[8192]; vector unsigned short v[1024];} Branchtab615[6];
16 static int Init = 0;
17
18 /* State info for instance of Viterbi decoder */
19 struct v615 {
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_viterbi615_av(void * p,int starting_state)28 int init_viterbi615_av(void *p,int starting_state){
29 struct v615 *vp = p;
30 int i;
31
32 if(p == NULL)
33 return -1;
34
35 for(i=0;i<2048;i++)
36 vp->metrics1.v[i] = (vector unsigned short)(5000);
37
38 vp->old_metrics = &vp->metrics1;
39 vp->new_metrics = &vp->metrics2;
40 vp->dp = vp->decisions;
41 vp->old_metrics->s[starting_state & 16383] = 0; /* Bias known start state */
42 return 0;
43 }
44
45 /* Create a new instance of a Viterbi decoder */
create_viterbi615_av(int len)46 void *create_viterbi615_av(int len){
47 struct v615 *vp;
48
49 if(!Init){
50 int polys[6] = { V615POLYA,V615POLYB,V615POLYC,V615POLYD,V615POLYE,V615POLYF };
51 set_viterbi615_polynomial_av(polys);
52 }
53 vp = (struct v615 *)malloc(sizeof(struct v615));
54 vp->decisions = malloc(sizeof(decision_t)*(len+14));
55 init_viterbi615_av(vp,0);
56 return vp;
57 }
58
set_viterbi615_polynomial_av(int polys[6])59 void set_viterbi615_polynomial_av(int polys[6]){
60 int state;
61 int i;
62
63 for(state=0;state < 8192;state++){
64 for(i=0;i<6;i++)
65 Branchtab615[i].s[state] = (polys[i] < 0) ^ parity((2*state) & abs(polys[i])) ? 255 : 0;
66 }
67 Init++;
68 }
69
70
71 /* Viterbi chainback */
chainback_viterbi615_av(void * p,unsigned char * data,unsigned int nbits,unsigned int endstate)72 int chainback_viterbi615_av(
73 void *p,
74 unsigned char *data, /* Decoded output data */
75 unsigned int nbits, /* Number of data bits */
76 unsigned int endstate){ /* Terminal encoder state */
77 struct v615 *vp = p;
78 decision_t *d = (decision_t *)vp->decisions;
79 int path_metric;
80
81 endstate %= 16384;
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 += 14; /* 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 << 13) | (endstate >> 1);
95 data[nbits>>3] = endstate >> 6;
96 }
97 return path_metric;
98 }
99
100 /* Delete instance of a Viterbi decoder */
delete_viterbi615_av(void * p)101 void delete_viterbi615_av(void *p){
102 struct v615 *vp = p;
103
104 if(vp != NULL){
105 free(vp->decisions);
106 free(vp);
107 }
108 }
109
update_viterbi615_blk_av(void * p,unsigned char * syms,int nbits)110 int update_viterbi615_blk_av(void *p,unsigned char *syms,int nbits){
111 struct v615 *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,sym3v,sym4v,sym5v;
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 sym3v = vec_splat(symv,3);
130 sym4v = vec_splat(symv,4);
131 sym5v = vec_splat(symv,5);
132 syms += 6;
133
134 for(i=0;i<1024;i++){
135 vector bool short decision0,decision1;
136 vector unsigned short metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;
137
138 /* Form branch metrics
139 * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255,
140 * the XOR operations constitute conditional negation.
141 * metric and m_metric (-metric) are in the range 0-1530
142 */
143 m0 = vec_add(vec_xor(Branchtab615[0].v[i],sym0v),vec_xor(Branchtab615[1].v[i],sym1v));
144 m1 = vec_add(vec_xor(Branchtab615[2].v[i],sym2v),vec_xor(Branchtab615[3].v[i],sym3v));
145 m2 = vec_add(vec_xor(Branchtab615[4].v[i],sym4v),vec_xor(Branchtab615[5].v[i],sym5v));
146 metric = vec_add(m0,m1);
147 metric = vec_add(metric,m2);
148 m_metric = vec_sub((vector unsigned short)(1530),metric);
149
150 /* Add branch metrics to path metrics */
151 m0 = vec_adds(vp->old_metrics->v[i],metric);
152 m3 = vec_adds(vp->old_metrics->v[1024+i],metric);
153 m1 = vec_adds(vp->old_metrics->v[1024+i],m_metric);
154 m2 = vec_adds(vp->old_metrics->v[i],m_metric);
155
156 /* Compare and select */
157 decision0 = vec_cmpgt(m0,m1);
158 decision1 = vec_cmpgt(m2,m3);
159 survivor0 = vec_min(m0,m1);
160 survivor1 = vec_min(m2,m3);
161
162 /* Store decisions and survivors.
163 * To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in
164 * a funny interleaved fashion that we undo in the chainback function.
165 */
166 decisions = vec_add(decisions,decisions); /* Shift each byte 1 bit to the left */
167
168 /* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting
169 * 0xff is equivalent to adding 1, which sets the lsb.
170 */
171 decisions = vec_sub(decisions,(vector unsigned char)vec_pack(vec_mergeh(decision0,decision1),vec_mergel(decision0,decision1)));
172
173 vp->new_metrics->v[2*i] = vec_mergeh(survivor0,survivor1);
174 vp->new_metrics->v[2*i+1] = vec_mergel(survivor0,survivor1);
175
176 if((i % 8) == 7){
177 /* We've accumulated a total of 128 decisions, stash and start again */
178 d->v[i>>3] = decisions; /* No need to clear, the new bits will replace the old */
179 }
180 }
181 #if 0
182 /* Experimentally determine metric spread
183 * The results are fixed for a given code and input symbol size
184 */
185 {
186 int i;
187 vector unsigned short min_metric;
188 vector unsigned short max_metric;
189 union { vector unsigned short v; unsigned short s[8];} t;
190 int minimum,maximum;
191 static int max_spread = 0;
192
193 min_metric = max_metric = vp->new_metrics->v[0];
194 for(i=1;i<2048;i++){
195 min_metric = vec_min(min_metric,vp->new_metrics->v[i]);
196 max_metric = vec_max(max_metric,vp->new_metrics->v[i]);
197 }
198 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,8));
199 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,8));
200 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,4));
201 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,4));
202 min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,2));
203 max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,2));
204
205 t.v = min_metric;
206 minimum = t.s[0];
207 t.v = max_metric;
208 maximum = t.s[0];
209 if(maximum-minimum > max_spread){
210 max_spread = maximum-minimum;
211 printf("metric spread = %d\n",max_spread);
212 }
213 }
214 #endif
215
216 /* Renormalize if necessary. This deserves some explanation.
217
218 * The maximum possible spread, found by experiment, for 4-bit symbols is 405; for 8 bit symbols, it's 12750.
219 * So by looking at one arbitrary metric we can tell if any of them have possibly saturated.
220 * However, this is very conservative. Large spreads occur only at very high Eb/No, where
221 * saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor.
222
223 * At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric
224 * by not not normalizing when we should are extremely low. So either way, the risk to performance is small.
225
226 * All this is borne out by experiment.
227 */
228 if(vp->new_metrics->s[0] >= USHRT_MAX-12750){
229 vector unsigned short scale;
230 union { vector unsigned short v; unsigned short s[8];} t;
231
232 /* Find smallest metric and splat */
233 scale = vp->new_metrics->v[0];
234 for(i=1;i<2048;i++)
235 scale = vec_min(scale,vp->new_metrics->v[i]);
236
237 scale = vec_min(scale,vec_sld(scale,scale,8));
238 scale = vec_min(scale,vec_sld(scale,scale,4));
239 scale = vec_min(scale,vec_sld(scale,scale,2));
240
241 /* Subtract it from all metrics
242 * Work backwards to try to improve the cache hit ratio, assuming LRU
243 */
244 for(i=2047;i>=0;i--)
245 vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i],scale);
246 t.v = scale;
247 path_metric += t.s[0];
248 }
249 d++;
250 /* Swap pointers to old and new metrics */
251 tmp = vp->old_metrics;
252 vp->old_metrics = vp->new_metrics;
253 vp->new_metrics = tmp;
254 }
255 vp->dp = d;
256 return path_metric;
257 }
258