Improve fft based speed sync performance by avoiding std::vector.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/wmspeed.cc

@@ -358,13 +358,16 @@ SpeedSync::compare_bits_fft (FFTProcessor& fft_processor, BitValue *bit_values,
       window[i] *= 2.0 / window_weight;
     }
 
+  const size_t n_bands = Params::max_band - Params::min_band + 1;
+  float fft_out_db[n_bands * in_data_sub.n_channels()];
+
   for (auto si = sync_bits.begin(); si != sync_bits.end(); si++)
     {
       int pos = ((offset / steps_per_frame) + si->frame) * sub_frame_size * relative_speed_inv;
 
       if (pos >= 0 && size_t (pos + sub_frame_size) < in_data_sub.n_frames())
         {
-          vector<float> fft_out_db;
+          int out_db_index = 0;
 
           for (int ch = 0; ch < in_data_sub.n_channels(); ch++)
             {
@@ -378,7 +381,7 @@ SpeedSync::compare_bits_fft (FFTProcessor& fft_processor, BitValue *bit_values,
                 {
                   const float min_db = -96;
 
-                  fft_out_db.push_back (db_from_complex (out[i * 2], out[i * 2 + 1], min_db));
+                  fft_out_db[out_db_index++] = db_from_complex (out[i * 2], out[i * 2 + 1], min_db);
                 }
             }
           auto& bv = bit_values[si->bit];
@@ -515,7 +518,7 @@ score_smooth_find_best (const vector<SpeedSync::Score>& in_scores, double step,
   double best_speed = 0;
   double best_quality = 0;
 
-  static constexpr bool debug = true;
+  static constexpr bool debug = false;
   if (debug)
     {
       for (auto s : scores)