gr-satnogs/lib/libfec/viterbi27_sse2.c

/* K=7 r=1/2 Viterbi decoder for SSE2
 * Feb 2004, Phil Karn, KA9Q
 */
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <xmmintrin.h>
#include "fec.h"

typedef union {
  unsigned char c[64];
  __m128i v[4];
} metric_t;
typedef union {
  unsigned long w[2];
  unsigned char c[8];
  unsigned short s[4];
  __m64 v[1];
} decision_t;
union branchtab27 {
  unsigned char c[32];
  __m128i v[2];
} Branchtab27_sse2[2];
static int Init = 0;

/* State info for instance of Viterbi decoder
 * Don't change this without also changing references in sse2bfly27.s!
 */
struct v27 {
  metric_t metrics1; /* path metric buffer 1 */
  metric_t metrics2; /* path metric buffer 2 */
  decision_t *dp;          /* Pointer to current decision */
  metric_t *old_metrics,
           *new_metrics; /* Pointers to path metrics, swapped on every bit */
  decision_t *decisions;   /* Beginning of decisions for block */
};

/* Initialize Viterbi decoder for start of new frame */
int init_viterbi27_sse2(void *p, int starting_state)
{
  struct v27 *vp = p;
  int i;

  if (p == NULL) {
    return -1;
  }
  for (i = 0; i < 64; i++) {
    vp->metrics1.c[i] = 63;
  }

  vp->old_metrics = &vp->metrics1;
  vp->new_metrics = &vp->metrics2;
  vp->dp = vp->decisions;
  vp->old_metrics->c[starting_state & 63] = 0; /* Bias known start state */
  return 0;
}

void set_viterbi27_polynomial_sse2(int polys[2])
{
  int state;

  for (state = 0; state < 32; state++) {
    Branchtab27_sse2[0].c[state] = (polys[0] < 0) ^ parity((2 * state) & abs(
                                     polys[0])) ? 255 : 0;
    Branchtab27_sse2[1].c[state] = (polys[1] < 0) ^ parity((2 * state) & abs(
                                     polys[1])) ? 255 : 0;
  }
  Init++;
}


/* Create a new instance of a Viterbi decoder */
void *create_viterbi27_sse2(int len)
{
  void *p;
  struct v27 *vp;

  if (!Init) {
    int polys[2] = { V27POLYA, V27POLYB };
    set_viterbi27_polynomial_sse2(polys);
  }
  /* Ordinary malloc() only returns 8-byte alignment, we need 16 */
  if (posix_memalign(&p, sizeof(__m128i), sizeof(struct v27))) {
    return NULL;
  }
  vp = (struct v27 *)p;

  if ((p = malloc((len + 6) * sizeof(decision_t))) == NULL) {
    free(vp);
    return NULL;
  }
  vp->decisions = (decision_t *)p;
  init_viterbi27_sse2(vp, 0);

  return vp;
}

/* Viterbi chainback */
int chainback_viterbi27_sse2(
  void *p,
  unsigned char *data, /* Decoded output data */
  unsigned int nbits, /* Number of data bits */
  unsigned int endstate)  /* Terminal encoder state */
{
  struct v27 *vp = p;
  decision_t *d;

  if (p == NULL) {
    return -1;
  }
  d = vp->decisions;
  /* Make room beyond the end of the encoder register so we can
   * accumulate a full byte of decoded data
   */
  endstate %= 64;
  endstate <<= 2;

  /* 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 += 6; /* Look past tail */
  while (nbits-- != 0) {
    int k;

    k = (d[nbits].c[(endstate >> 2) / 8] >> ((endstate >> 2) % 8)) & 1;
    data[nbits >> 3] = endstate = (endstate >> 1) | (k << 7);
  }
  return 0;
}

/* Delete instance of a Viterbi decoder */
void delete_viterbi27_sse2(void *p)
{
  struct v27 *vp = p;

  if (vp != NULL) {
    free(vp->decisions);
    free(vp);
  }
}


#if 0
/* This code is turned off because it's slower than my hand-crafted assembler in sse2bfly27.s. But it does work. */
void update_viterbi27_blk_sse2(void *p, unsigned char *syms, int nbits)
{
  struct v27 *vp = p;
  decision_t *d;

  if (p == NULL) {
    return;
  }
  d = (decision_t *)vp->dp;
  while (nbits--) {
    __m128i sym0v, sym1v;
    void *tmp;
    int i;

    /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
    sym0v = _mm_set1_epi8(syms[0]);
    sym1v = _mm_set1_epi8(syms[1]);
    syms += 2;

    for (i = 0; i < 2; i++) {
      __m128i decision0, decision1, metric, m_metric, m0, m1, m2, m3, survivor0,
              survivor1;

      /* Form branch metrics */
      metric = _mm_avg_epu8(_mm_xor_si128(Branchtab27_sse2[0].v[i], sym0v),
                            _mm_xor_si128(Branchtab27_sse2[1].v[i], sym1v));
      /* There's no packed bytes right shift in SSE2, so we use the word version and mask
       * (I'm *really* starting to like Altivec...)
       */
      metric = _mm_srli_epi16(metric, 3);
      metric = _mm_and_si128(metric, _mm_set1_epi8(31));
      m_metric = _mm_sub_epi8(_mm_set1_epi8(31), metric);

      /* Add branch metrics to path metrics */
      m0 = _mm_add_epi8(vp->old_metrics->v[i], metric);
      m3 = _mm_add_epi8(vp->old_metrics->v[2 + i], metric);
      m1 = _mm_add_epi8(vp->old_metrics->v[2 + i], m_metric);
      m2 = _mm_add_epi8(vp->old_metrics->v[i], m_metric);

      /* Compare and select, using modulo arithmetic */
      decision0 = _mm_cmpgt_epi8(_mm_sub_epi8(m0, m1), _mm_setzero_si128());
      decision1 = _mm_cmpgt_epi8(_mm_sub_epi8(m2, m3), _mm_setzero_si128());
      survivor0 = _mm_or_si128(_mm_and_si128(decision0, m1),
                               _mm_andnot_si128(decision0, m0));
      survivor1 = _mm_or_si128(_mm_and_si128(decision1, m3),
                               _mm_andnot_si128(decision1, m2));

      /* Pack each set of decisions into 16 bits */
      d->s[2 * i] = _mm_movemask_epi8(_mm_unpacklo_epi8(decision0, decision1));
      d->s[2 * i + 1] = _mm_movemask_epi8(_mm_unpackhi_epi8(decision0, decision1));

      /* Store surviving metrics */
      vp->new_metrics->v[2 * i] = _mm_unpacklo_epi8(survivor0, survivor1);
      vp->new_metrics->v[2 * i + 1] = _mm_unpackhi_epi8(survivor0, survivor1);
    }
    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;
}
#endif