/* K=9 r=1/3 Viterbi decoder for PowerPC G4/G5 Altivec vector instructions * 8-bit offset-binary soft decision samples * Copyright Aug 2006, Phil Karn, KA9Q * May be used under the terms of the GNU Lesser General Public License (LGPL) */ #include #include #include #include #include "fec.h" typedef union { unsigned char c[2][16]; vector unsigned char v[2]; } decision_t; typedef union { unsigned short s[256]; vector unsigned short v[32]; } metric_t; static union branchtab39 { unsigned short s[128]; vector unsigned short v[16];} Branchtab39[3]; static int Init = 0; /* State info for instance of Viterbi decoder */ struct v39 { metric_t metrics1; /* path metric buffer 1 */ metric_t metrics2; /* path metric buffer 2 */ void *dp; /* Pointer to current decision */ metric_t *old_metrics,*new_metrics; /* Pointers to path metrics, swapped on every bit */ void *decisions; /* Beginning of decisions for block */ }; /* Initialize Viterbi decoder for start of new frame */ int init_viterbi39_av(void *p,int starting_state){ struct v39 *vp = p; int i; for(i=0;i<32;i++) vp->metrics1.v[i] = (vector unsigned short)(1000); vp->old_metrics = &vp->metrics1; vp->new_metrics = &vp->metrics2; vp->dp = vp->decisions; vp->old_metrics->s[starting_state & 255] = 0; /* Bias known start state */ return 0; } void set_viterbi39_polynomial_av(int polys[3]){ int state; for(state=0;state < 128;state++){ Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & abs(polys[0])) ? 255 : 0; Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & abs(polys[1])) ? 255 : 0; Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & abs(polys[2])) ? 255 : 0; } Init++; } /* Create a new instance of a Viterbi decoder */ void *create_viterbi39_av(int len){ struct v39 *vp; if(!Init){ int polys[3] = { V39POLYA, V39POLYB, V39POLYC }; set_viterbi39_polynomial_av(polys); } vp = (struct v39 *)malloc(sizeof(struct v39)); vp->decisions = malloc(sizeof(decision_t)*(len+8)); init_viterbi39_av(vp,0); return vp; } /* Viterbi chainback */ int chainback_viterbi39_av( void *p, unsigned char *data, /* Decoded output data */ unsigned int nbits, /* Number of data bits */ unsigned int endstate){ /* Terminal encoder state */ struct v39 *vp = p; decision_t *d = (decision_t *)vp->decisions; int path_metric; /* Make room beyond the end of the encoder register so we can * accumulate a full byte of decoded data */ endstate %= 256; path_metric = vp->old_metrics->s[endstate]; /* The store into data[] only needs to be done every 8 bits. * But this avoids a conditional branch, and the writes will * combine in the cache anyway */ d += 8; /* Look past tail */ while(nbits-- != 0){ int k; k = (d[nbits].c[endstate >> 7][endstate & 15] & (0x80 >> ((endstate>>4)&7)) ) ? 1 : 0; endstate = (k << 7) | (endstate >> 1); data[nbits>>3] = endstate; } return path_metric; } /* Delete instance of a Viterbi decoder */ void delete_viterbi39_av(void *p){ struct v39 *vp = p; if(vp != NULL){ free(vp->decisions); free(vp); } } int update_viterbi39_blk_av(void *p,unsigned char *syms,int nbits){ struct v39 *vp = p; decision_t *d = (decision_t *)vp->dp; int path_metric = 0; vector unsigned char decisions = (vector unsigned char)(0); while(nbits--){ vector unsigned short symv,sym0v,sym1v,sym2v; vector unsigned char s; void *tmp; int i; /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */ s = (vector unsigned char)vec_perm(vec_ld(0,syms),vec_ld(5,syms),vec_lvsl(0,syms)); symv = (vector unsigned short)vec_mergeh((vector unsigned char)(0),s); /* Unsigned byte->word unpack */ sym0v = vec_splat(symv,0); sym1v = vec_splat(symv,1); sym2v = vec_splat(symv,2); syms += 3; for(i=0;i<16;i++){ vector bool short decision0,decision1; vector unsigned short metric,m_metric,m0,m1,m2,m3,survivor0,survivor1; /* Form branch metrics * Because Branchtab takes on values 0 and 255, and the values of sym?v are offset binary in the range 0-255, * the XOR operations constitute conditional negation. * the metrics are in the range 0-765 */ m0 = vec_add(vec_xor(Branchtab39[0].v[i],sym0v),vec_xor(Branchtab39[1].v[i],sym1v)); m1 = vec_xor(Branchtab39[2].v[i],sym2v); metric = vec_add(m0,m1); m_metric = vec_sub((vector unsigned short)(765),metric); /* Add branch metrics to path metrics */ m0 = vec_adds(vp->old_metrics->v[i],metric); m3 = vec_adds(vp->old_metrics->v[16+i],metric); m1 = vec_adds(vp->old_metrics->v[16+i],m_metric); m2 = vec_adds(vp->old_metrics->v[i],m_metric); /* Compare and select */ decision0 = vec_cmpgt(m0,m1); decision1 = vec_cmpgt(m2,m3); survivor0 = vec_min(m0,m1); survivor1 = vec_min(m2,m3); /* Store decisions and survivors. * To save space without SSE2's handy PMOVMSKB instruction, we pack and store them in * a funny interleaved fashion that we undo in the chainback function. */ decisions = vec_add(decisions,decisions); /* Shift each byte 1 bit to the left */ /* Booleans are either 0xff or 0x00. Subtracting 0x00 leaves the lsb zero; subtracting * 0xff is equivalent to adding 1, which sets the lsb. */ decisions = vec_sub(decisions,(vector unsigned char)vec_pack(vec_mergeh(decision0,decision1),vec_mergel(decision0,decision1))); vp->new_metrics->v[2*i] = vec_mergeh(survivor0,survivor1); vp->new_metrics->v[2*i+1] = vec_mergel(survivor0,survivor1); if((i % 8) == 7){ /* We've accumulated a total of 128 decisions, stash and start again */ d->v[i>>3] = decisions; /* No need to clear, the new bits will replace the old */ } } #if 0 /* Experimentally determine metric spread * The results are fixed for a given code and input symbol size */ { int i; vector unsigned short min_metric; vector unsigned short max_metric; union { vector unsigned short v; unsigned short s[8];} t; int minimum,maximum; static int max_spread = 0; min_metric = max_metric = vp->new_metrics->v[0]; for(i=1;i<32;i++){ min_metric = vec_min(min_metric,vp->new_metrics->v[i]); max_metric = vec_max(max_metric,vp->new_metrics->v[i]); } min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,8)); max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,8)); min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,4)); max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,4)); min_metric = vec_min(min_metric,vec_sld(min_metric,min_metric,2)); max_metric = vec_max(max_metric,vec_sld(max_metric,max_metric,2)); t.v = min_metric; minimum = t.s[0]; t.v = max_metric; maximum = t.s[0]; if(maximum-minimum > max_spread){ max_spread = maximum-minimum; printf("metric spread = %d\n",max_spread); } } #endif /* Renormalize if necessary. This deserves some explanation. * The maximum possible spread, found by experiment, for 8 bit symbols is about 3825 * So by looking at one arbitrary metric we can tell if any of them have possibly saturated. * However, this is very conservative. Large spreads occur only at very high Eb/No, where * saturating a bad path metric doesn't do much to increase its chances of being erroneously chosen as a survivor. * At more interesting (low) Eb/No ratios, the spreads are much smaller so our chances of saturating a metric * by not not normalizing when we should are extremely low. So either way, the risk to performance is small. * All this is borne out by experiment. */ if(vp->new_metrics->s[0] >= USHRT_MAX-5000){ vector unsigned short scale; union { vector unsigned short v; unsigned short s[8];} t; /* Find smallest metric and splat */ scale = vp->new_metrics->v[0]; for(i=1;i<32;i++) scale = vec_min(scale,vp->new_metrics->v[i]); scale = vec_min(scale,vec_sld(scale,scale,8)); scale = vec_min(scale,vec_sld(scale,scale,4)); scale = vec_min(scale,vec_sld(scale,scale,2)); /* Subtract it from all metrics * Work backwards to try to improve the cache hit ratio, assuming LRU */ for(i=31;i>=0;i--) vp->new_metrics->v[i] = vec_subs(vp->new_metrics->v[i],scale); t.v = scale; path_metric += t.s[0]; } d++; /* Swap pointers to old and new metrics */ tmp = vp->old_metrics; vp->old_metrics = vp->new_metrics; vp->new_metrics = tmp; } vp->dp = d; return path_metric; }