Bill Yi | 4e213d5 | 2015-06-23 13:53:11 -0700 | [diff] [blame] | 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 | } |