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Diffstat (limited to 'viterbi39_av.c')
| -rw-r--r-- | viterbi39_av.c | 251 |
1 files changed, 251 insertions, 0 deletions
diff --git a/viterbi39_av.c b/viterbi39_av.c new file mode 100644 index 0000000..2deed51 --- /dev/null +++ b/viterbi39_av.c @@ -0,0 +1,251 @@ +/* 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 <stdio.h> +#include <stdlib.h> +#include <memory.h> +#include <limits.h> +#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; +} |