gr-satnogs/lib/libfec/viterbi39_mmx.c

/* 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;
}