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/* K=9 r=1/3 Viterbi decoder for x86 MMX
* Aug 2006, Phil Karn, KA9Q
* May be used under the terms of the GNU Lesser General Public License (LGPL)
*/
#include <mmintrin.h>
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include "fec.h"
typedef union { unsigned char c[256]; __m64 v[32];} decision_t;
typedef union { unsigned short s[256]; __m64 v[64];} metric_t;
static union branchtab39 { unsigned short s[128]; __m64 v[32];} 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_mmx(void *p,int starting_state){
struct v39 *vp = p;
int i;
if(p == NULL)
return -1;
for(i=0;i<256;i++)
vp->metrics1.s[i] = 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_mmx(int polys[3]){
int state;
for(state=0;state < 128;state++){
Branchtab39[0].s[state] = (polys[0] < 0) ^ parity((2*state) & polys[0]) ? 255:0;
Branchtab39[1].s[state] = (polys[1] < 0) ^ parity((2*state) & polys[1]) ? 255:0;
Branchtab39[2].s[state] = (polys[2] < 0) ^ parity((2*state) & polys[2]) ? 255:0;
}
Init++;
}
/* Create a new instance of a Viterbi decoder */
void *create_viterbi39_mmx(int len){
struct v39 *vp;
if(!Init){
int polys[3] = { V39POLYA,V39POLYB,V39POLYC };
set_viterbi39_polynomial_mmx(polys);
}
if((vp = (struct v39 *)malloc(sizeof(struct v39))) == NULL)
return NULL;
if((vp->decisions = malloc((len+8)*sizeof(decision_t))) == NULL){
free(vp);
return NULL;
}
init_viterbi39_mmx(vp,0);
return vp;
}
/* Viterbi chainback */
int chainback_viterbi39_mmx(
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;
int path_metric;
if(p == NULL)
return -1;
d = (decision_t *)vp->decisions;
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] & 1;
endstate = (k << 7) | (endstate >> 1);
data[nbits>>3] = endstate;
}
return path_metric;
}
/* Delete instance of a Viterbi decoder */
void delete_viterbi39_mmx(void *p){
struct v39 *vp = p;
if(vp != NULL){
free(vp->decisions);
free(vp);
}
}
int update_viterbi39_blk_mmx(void *p,unsigned char *syms,int nbits){
struct v39 *vp = p;
decision_t *d;
int path_metric = 0;
if(p == NULL)
return -1;
d = (decision_t *)vp->dp;
while(nbits--){
__m64 sym0v,sym1v,sym2v;
void *tmp;
int i;
/* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
sym0v = _mm_set1_pi16(syms[0]);
sym1v = _mm_set1_pi16(syms[1]);
sym2v = _mm_set1_pi16(syms[2]);
syms += 3;
for(i=0;i<32;i++){
__m64 decision0,decision1,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.
* metric and m_metric (-metric) are in the range 0-1530
*/
m0 = _mm_add_pi16(_mm_xor_si64(Branchtab39[0].v[i],sym0v),_mm_xor_si64(Branchtab39[1].v[i],sym1v));
metric = _mm_add_pi16(_mm_xor_si64(Branchtab39[2].v[i],sym2v),m0);
m_metric = _mm_sub_pi16(_mm_set1_pi16(765),metric);
/* Add branch metrics to path metrics */
m0 = _mm_add_pi16(vp->old_metrics->v[i],metric);
m3 = _mm_add_pi16(vp->old_metrics->v[32+i],metric);
m1 = _mm_add_pi16(vp->old_metrics->v[32+i],m_metric);
m2 = _mm_add_pi16(vp->old_metrics->v[i],m_metric);
/* Compare and select
* There's no packed min instruction in MMX, so we use modulo arithmetic
* to form the decisions and then do the select the hard way
*/
decision0 = _mm_cmpgt_pi16(_mm_sub_pi16(m0,m1),_mm_setzero_si64());
decision1 = _mm_cmpgt_pi16(_mm_sub_pi16(m2,m3),_mm_setzero_si64());
survivor0 = _mm_or_si64(_mm_and_si64(decision0,m1),_mm_andnot_si64(decision0,m0));
survivor1 = _mm_or_si64(_mm_and_si64(decision1,m3),_mm_andnot_si64(decision1,m2));
/* Merge decisions and store as bytes */
d->v[i] = _mm_unpacklo_pi8(_mm_packs_pi16(decision0,_mm_setzero_si64()),_mm_packs_pi16(decision1,_mm_setzero_si64()));
/* Store surviving metrics */
vp->new_metrics->v[2*i] = _mm_unpacklo_pi16(survivor0,survivor1);
vp->new_metrics->v[2*i+1] = _mm_unpackhi_pi16(survivor0,survivor1);
}
if(vp->new_metrics->s[0] < vp->old_metrics->s[0])
path_metric += 65536; /* Hack: wraparound probably occured */
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;
_mm_empty();
return path_metric;
}
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