/* 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;
}