decimate.c

// Simple sample rate decimators & half-band filters by powers of 2
// Not currently used in ka9q-radio, but here for reference or possible future use
// Copyright 2017-2023, Phil Karn, KA9Q

// Note: filters have unity middle tap, which usually results in overall gain of +6 dB

#include <string.h>
#include <assert.h>
#include "decimate.h"

// Pick up vectorized versions if available
#if defined(__SSSE3__)
#if 0 // messages apparently treated as warnings
#pragma message "Vectorized versions of decimation filters will require SSSE3 or better to run"
#endif

  #include <x86intrin.h>  // GCC-compatible compiler, targeting x86/x86-64

/* Folded half-band 15-tap filter
   Only four non-unity coefficents are needed
   Note ordering of coefficients: coeff[0] is at the tails, not the center

  |--   even[3]   even[2]    even[1]    even[0]  <-- in first
  v
drop

  --     odd[3]    odd[2]     odd[1]     odd[0]  <-- in second
  |
  |       +         +          +          +
  |
  |-->oldodd[3]  oldodd[2]  oldodd[1]  oldodd[0] --------|
          *          *           *          *            v
       coeff[3]   coeff[2]   coeff[1]   coeff[0]        drop
          v          v           v          v
        temp[3]    temp[2]    temp[1]    temp[0]
       sum(temp[3...0] + even[3]) to give output
 */

// Strong typing can be a major pain in the ass...
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"


#define shiftleft(arg,n) _mm_castsi128_ps(_mm_bslli_si128(_mm_castps_si128(arg),4*n))
#define shiftright(arg,n) _mm_castsi128_ps(_mm_bsrli_si128(_mm_castps_si128(arg),4*n))

void hb15_block(struct hb15_state *state,float *output,float *input,int cnt){
  __m128 coeffs= _mm_loadu_ps(state->coeffs);  // coeffs in xmm5;
  __m128 even_samples = _mm_loadu_ps(state->even_samples); // xmm3;
  __m128 odd_samples = _mm_loadu_ps(state->odd_samples); // xmm1;
  __m128 old_odd_samples = _mm_loadu_ps(state->old_odd_samples); // xmm2;
  __m128 temp = _mm_setzero_ps(); // Not really necessary but silences compiler warning
  __m128 mask3 = (__m128) _mm_set_epi32(-1,0,0,0); // For selecting high word of vector

  while(cnt--){
    temp = _mm_loadl_pi(temp,input); // X   X  IN[1] IN[0]
    input += 2;
    even_samples = _mm_move_ss(even_samples,temp);  // ES[3] ES[2] ES[1] in[0]
    temp = shiftright(temp,1);
    odd_samples = _mm_move_ss(odd_samples,temp);  // OS[3] OS[2] OS[1] in[1]

    temp = _mm_mul_ps(_mm_add_ps(odd_samples,old_odd_samples),coeffs); // all taps but the middle
    // Right shift old_odd_samples (no longer need old_odd_samples[0])
    // sets old_odd_samples[3] = 0 in preparation for ORing later with odd_samples[3]
    old_odd_samples = shiftright(old_odd_samples,1);  // 0 OOS[3] OOS[2] OOS[1]

    // old_odd_samples[3] = odd_samples[3]
    old_odd_samples = _mm_or_ps(old_odd_samples,_mm_and_ps(odd_samples,mask3));

    // left shift odd samples
    odd_samples = shiftleft(odd_samples,1);

    // Sum up terms
    temp = _mm_add_ps(temp,_mm_and_ps(even_samples,mask3)); // even_samples[3] (central tap, unity gain)
    temp = _mm_hadd_ps(temp,temp);
    temp = _mm_hadd_ps(temp,temp);    

    // Left shift one word (no longer need even_samples[3])
    even_samples = shiftleft(even_samples,1);

    // Stash result:     *output++ = result;
    _mm_store_ss(output++,temp);
  }
  _mm_storeu_ps(state->even_samples,even_samples);
  _mm_storeu_ps(state->odd_samples,odd_samples);
  _mm_storeu_ps(state->old_odd_samples,old_odd_samples);
}


// 3-tap halfband filter with fixed taps: 1, 2, 1
void hb3_block(float *state,float *output,float *input,int cnt){

  __m128 oldstate = _mm_load_ss(state);   // 0   0        0        S
  __m128 in = _mm_setzero_ps();            // 0   0        0        0

  while(cnt--){
    in = _mm_loadl_pi(in,input);   // 0   0       IN[1]  IN[0]
    input += 2;
    
    __m128 temp;
    temp = _mm_unpacklo_ps(in,oldstate); // 0   IN[1]            S           IN[0]
    temp = _mm_add_ps(temp,in);    // 0   IN[1]             S+IN[1]      2*IN[0]
    temp = _mm_hadd_ps(temp,temp); // IN[1]   S+IN[1]+2*IN[0]        IN[1]   S+IN[1]+2*IN[0]
    _mm_store_ss(output++,temp);   // Store result
    oldstate = shiftright(in,1);
  }
  _mm_store_ss(state,oldstate);
}
#pragma GCC diagnostic pop

#else

// Portable version - written to help the compiler vectorize at least partly
void hb15_block(struct hb15_state *state,float *output,float *input,int cnt){
  float even_samples[4];
  float odd_samples[4];
  float old_odd_samples[4];
  float coeffs[4];

  memcpy(coeffs, state->coeffs, sizeof(coeffs));
  memcpy(even_samples, state->even_samples, sizeof(even_samples));
  memcpy(odd_samples, state->odd_samples, sizeof(odd_samples));
  memcpy(old_odd_samples, state->old_odd_samples, sizeof(old_odd_samples));

  while(cnt--){
    even_samples[0] = *input++;
    odd_samples[0] = *input++;

    float result = even_samples[3];
    for(int i=2; i >= 0; i--)
      even_samples[i+1] = even_samples[i];

    for(int i=0; i < 4; i++)
      result += (odd_samples[i] + old_odd_samples[i]) * coeffs[i];
    
    *output++ = result;

    for(int i=0; i < 3; i++)
      old_odd_samples[i] = old_odd_samples[i+1];
    
    old_odd_samples[3] = odd_samples[3];
    
    for(int i=2; i >= 0; i--)
      odd_samples[i+1] = odd_samples[i];
  }
  memcpy(state->even_samples, even_samples, sizeof(even_samples));
  memcpy(state->odd_samples, odd_samples, sizeof(odd_samples));
  memcpy(state->old_odd_samples, old_odd_samples, sizeof(old_odd_samples));
}
// 3-tap halfband filter with fixed taps: 1, 2, 1
void hb3_block(float *state,float *output,float *input,int cnt){

  float oldstate = *state;

  while(cnt--){
    float in[2];
    in[0] = *input++;
    in[1] = *input++;

    *output++ = 2 * in[0] + in[1] + oldstate;
    oldstate = in[1];
  }
  *state = oldstate;
}


#endif