srsRAN_4G/srslte/lib/fec/viterbi37_sse.c

/* Adapted Phil Karn's r=1/3 k=9 viterbi decoder to r=1/3 k=7
 * 
 * K=15 r=1/6 Viterbi decoder for x86 SSE2
 * Copyright Mar 2004, Phil Karn, KA9Q
 * May be used under the terms of the GNU Lesser General Public License (LGPL)
 */

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <memory.h>
#include <limits.h>
#include "parity.h"

//#define DEBUG

#ifdef LV_HAVE_SSE

#include <emmintrin.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];  
} Branchtab37_sse2[3];


/* State info for instance of Viterbi decoder */
struct v37 {
  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 */
};

void set_viterbi37_polynomial_sse(int polys[3]) {
  int state;

  for(state=0;state < 32;state++){
    Branchtab37_sse2[0].c[state] = (polys[0] < 0) ^ parity((2*state) & polys[0]) ? 255:0;
    Branchtab37_sse2[1].c[state] = (polys[1] < 0) ^ parity((2*state) & polys[1]) ? 255:0;
    Branchtab37_sse2[2].c[state] = (polys[2] < 0) ^ parity((2*state) & polys[2]) ? 255:0;
  }
}


/* Initialize Viterbi decoder for start of new frame */
int init_viterbi37_sse(void *p, int starting_state) {
  struct v37 *vp = p;
  uint32_t i;

  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;
  if (starting_state != -1) {
    vp->old_metrics->c[starting_state & 63] = 0; /* Bias known start state */
  }
  return 0;
}

/* Create a new instance of a Viterbi decoder */
void *create_viterbi37_sse(int polys[3], uint32_t len) {
  void *p;
  struct v37 *vp;

  set_viterbi37_polynomial_sse(polys);
  
  /* Ordinary malloc() only returns 8-byte alignment, we need 16 */
  if(posix_memalign(&p, sizeof(__m128i),sizeof(struct v37)))
    return NULL;

  vp = (struct v37 *)p;
  if(posix_memalign(&p, sizeof(__m128i),(len+6)*sizeof(decision_t))) {
    free(vp);
    return NULL;
  }
  vp->decisions = (decision_t *)p;
  return vp;
}


/* Viterbi chainback */
int chainback_viterbi37_sse(
      void *p,
      uint8_t *data, /* Decoded output data */
      uint32_t nbits, /* Number of data bits */
      uint32_t endstate) { /* Terminal encoder state */
  struct v37 *vp = p;
  
  if (p == NULL)
    return -1;

  decision_t *d = (decision_t *)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--) {
    int k;

    k = (d[nbits].c[(endstate>>2)/8] >> ((endstate>>2)%8)) & 1;
    endstate = (endstate >> 1) | (k << 7);
    data[nbits] = k;
    //printf("nbits=%d, endstate=%3d, k=%d, w[0]=%d, w[1]=%d, c=%d\n", nbits, endstate, k, d[nbits].s[1]&1, d[nbits].s[2]&1, d[nbits].c[(endstate>>2)/8]&1);
  }
  return 0;
}

/* Delete instance of a Viterbi decoder */
void delete_viterbi37_sse(void *p){
  struct v37 *vp = p;

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

void print_128i(char *s, __m128i val) {
  
  printf("%s: ", s);
  
  uint8_t *x = (uint8_t*) &val; 
  for (int i=0;i<16;i++) {
    printf("%3d, ", x[i]);
  }
  printf("\n");
}

void update_viterbi37_blk_sse(void *p,unsigned char *syms,int nbits, uint32_t *best_state) {
  struct v37 *vp = p;
  decision_t *d;

  if(p == NULL)
    return;
  
#ifdef DEBUG
  printf("[");
#endif
  
  d = (decision_t *) vp->dp;
  
  for (int s=0;s<nbits;s++) {
    memset(d+s,0,sizeof(decision_t));
  }
  
  while(nbits--) {
    __m128i sym0v,sym1v,sym2v;
    void *tmp;
    int i;

   // printf("nbits=%d, syms=%d,%d,%d\n", nbits, syms[0], syms[1], syms[2]);fflush(stdout);
    
    /* Splat the 0th symbol across sym0v, the 1st symbol across sym1v, etc */
    sym0v = _mm_set1_epi8(syms[0]);
    sym1v = _mm_set1_epi8(syms[1]);
    sym2v = _mm_set1_epi8(syms[2]);
    syms += 3;
    
    for(i=0;i<2;i++){
      __m128i decision0,decision1,metric,m_metric,m0,m1,m2,m3,survivor0,survivor1;

      /* Form branch metrics */
      m0 = _mm_avg_epu8(_mm_xor_si128(Branchtab37_sse2[0].v[i],sym0v),_mm_xor_si128(Branchtab37_sse2[1].v[i],sym1v));
      metric = _mm_avg_epu8(_mm_xor_si128(Branchtab37_sse2[2].v[i],sym2v),m0);

#ifdef DEBUG
      print_128i("metric_initial", metric);
#endif
      /* There's no packed bytes right shift in SSE2, so we use the word version and mask
       */
      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);
    
#ifdef DEBUG
      print_128i("metric        ", metric);
      print_128i("m_metric      ", m_metric);
#endif
      
      /* 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);
        
    }

    // See if we need to normalize
    if (vp->new_metrics->c[0] > 100) {
      int i;
      uint8_t adjust; 
      __m128i adjustv;
      union { __m128i v; signed short w[8]; } t;
      
      adjustv = vp->new_metrics->v[0];
      for(i=1;i<4;i++) {
        adjustv = _mm_min_epu8(adjustv,vp->new_metrics->v[i]);
      }
      
      adjustv = _mm_min_epu8(adjustv,_mm_srli_si128(adjustv,8));
      adjustv = _mm_min_epu8(adjustv,_mm_srli_si128(adjustv,4));
      adjustv = _mm_min_epu8(adjustv,_mm_srli_si128(adjustv,2));
      
      t.v = adjustv;
      adjust = t.w[0];
      adjustv = _mm_set1_epi8(adjust);
      
      /* We cannot use a saturated subtract, because we often have to adjust by more than SHRT_MAX
       * This is okay since it can't overflow anyway
       */
      for(i=0;i<4;i++)
        vp->new_metrics->v[i] = _mm_sub_epi8(vp->new_metrics->v[i],adjustv);
    }


    d++;
    /* Swap pointers to old and new metrics */
    tmp = vp->old_metrics;
    vp->old_metrics = vp->new_metrics;
    vp->new_metrics = tmp;
  }
  
  if (best_state) {
    uint32_t i, bst=0;
    uint8_t minmetric=UINT8_MAX;
    for (i=0;i<64;i++) {
      if (vp->old_metrics->c[i] <= minmetric) {
        bst = i;
        minmetric = vp->old_metrics->c[i];
      }
    }
    *best_state = bst;
  }

  #ifdef DEBUG
  printf("];\n===========================================\n");
#endif

  vp->dp = d;
}

#endif