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 */ 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 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 */ 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 */ 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 */ 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 } 109 110 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
