The previous code is 30% slower than the baseline.

Improvements:
1. with the same config, the previous code will result in 10% more tokens in each frame compared with baseline. The reason is because GetCutoff function is over emitting tokens, while for baseline it is over emitting&non emiting tokens. (please check it use --verbose 6 and the number of all tokens is shown in line 895, but not in line 693, which only contains emitting) Hence we should use 10% less max_active config.
2. the baseline use kaldi version HashList, while the previous code use std::unordered_map. please still use HashList or its variants. The reason is because HashList is allocating memory in Pool/Block fashion, hence it is 10% faster than std::unordered_map. To show this problem, I temporally do a hack in line 247, plese check it

After these two improvements, the current code is with similar speed as the baseline. HOWEVER, please check the quality of 1-best & lattice, I have discuss about my worries here kaldi-asr#3061 (review)

src/bin/latgen-faster-mapped-combine.cc

@@ -0,0 +1,179 @@
+// bin/latgen-faster-mapped.cc
+
+// Copyright 2009-2012  Microsoft Corporation, Karel Vesely
+//                2013  Johns Hopkins University (author: Daniel Povey)
+//                2014  Guoguo Chen
+
+// See ../../COPYING for clarification regarding multiple authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//  http://www.apache.org/licenses/LICENSE-2.0
+//
+// THIS CODE IS PROVIDED *AS IS* BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED
+// WARRANTIES OR CONDITIONS OF TITLE, FITNESS FOR A PARTICULAR PURPOSE,
+// MERCHANTABLITY OR NON-INFRINGEMENT.
+// See the Apache 2 License for the specific language governing permissions and
+// limitations under the License.
+
+
+#include "base/kaldi-common.h"
+#include "util/common-utils.h"
+#include "tree/context-dep.h"
+#include "hmm/transition-model.h"
+#include "fstext/fstext-lib.h"
+#include "decoder/decoder-wrappers.h"
+#include "decoder/decodable-matrix.h"
+#include "base/timer.h"
+
+
+int main(int argc, char *argv[]) {
+  try {
+    using namespace kaldi;
+    typedef kaldi::int32 int32;
+    using fst::SymbolTable;
+    using fst::Fst;
+    using fst::StdArc;
+
+    const char *usage =
+        "Generate lattices, reading log-likelihoods as matrices\n"
+        " (model is needed only for the integer mappings in its transition-model)\n"
+        "Usage: latgen-faster-mapped [options] trans-model-in (fst-in|fsts-rspecifier) loglikes-rspecifier"
+        " lattice-wspecifier [ words-wspecifier [alignments-wspecifier] ]\n";
+    ParseOptions po(usage);
+    Timer timer;
+    bool allow_partial = false;
+    BaseFloat acoustic_scale = 0.1;
+    LatticeFasterDecoderCombineConfig config;
+
+    std::string word_syms_filename;
+    config.Register(&po);
+    po.Register("acoustic-scale", &acoustic_scale, "Scaling factor for acoustic likelihoods");
+
+    po.Register("word-symbol-table", &word_syms_filename, "Symbol table for words [for debug output]");
+    po.Register("allow-partial", &allow_partial, "If true, produce output even if end state was not reached.");
+
+    po.Read(argc, argv);
+
+    if (po.NumArgs() < 4 || po.NumArgs() > 6) {
+      po.PrintUsage();
+      exit(1);
+    }
+
+    std::string model_in_filename = po.GetArg(1),
+        fst_in_str = po.GetArg(2),
+        feature_rspecifier = po.GetArg(3),
+        lattice_wspecifier = po.GetArg(4),
+        words_wspecifier = po.GetOptArg(5),
+        alignment_wspecifier = po.GetOptArg(6);
+
+    TransitionModel trans_model;
+    ReadKaldiObject(model_in_filename, &trans_model);
+
+    bool determinize = config.determinize_lattice;
+    CompactLatticeWriter compact_lattice_writer;
+    LatticeWriter lattice_writer;
+    if (! (determinize ? compact_lattice_writer.Open(lattice_wspecifier)
+           : lattice_writer.Open(lattice_wspecifier)))
+      KALDI_ERR << "Could not open table for writing lattices: "
+                 << lattice_wspecifier;
+
+    Int32VectorWriter words_writer(words_wspecifier);
+
+    Int32VectorWriter alignment_writer(alignment_wspecifier);
+
+    fst::SymbolTable *word_syms = NULL;
+    if (word_syms_filename != "")
+      if (!(word_syms = fst::SymbolTable::ReadText(word_syms_filename)))
+        KALDI_ERR << "Could not read symbol table from file "
+                   << word_syms_filename;
+
+    double tot_like = 0.0;
+    kaldi::int64 frame_count = 0;
+    int num_success = 0, num_fail = 0;
+
+    if (ClassifyRspecifier(fst_in_str, NULL, NULL) == kNoRspecifier) {
+      SequentialBaseFloatMatrixReader loglike_reader(feature_rspecifier);
+      // Input FST is just one FST, not a table of FSTs.
+      Fst<StdArc> *decode_fst = fst::ReadFstKaldiGeneric(fst_in_str);
+      timer.Reset();
+
+      {
+        LatticeFasterDecoderCombine decoder(*decode_fst, config);
+
+        for (; !loglike_reader.Done(); loglike_reader.Next()) {
+          std::string utt = loglike_reader.Key();
+          Matrix<BaseFloat> loglikes (loglike_reader.Value());
+          loglike_reader.FreeCurrent();
+          if (loglikes.NumRows() == 0) {
+            KALDI_WARN << "Zero-length utterance: " << utt;
+            num_fail++;
+            continue;
+          }
+
+          DecodableMatrixScaledMapped decodable(trans_model, loglikes, acoustic_scale);
+
+          double like;
+          if (DecodeUtteranceLatticeFasterCombine(
+                  decoder, decodable, trans_model, word_syms, utt,
+                  acoustic_scale, determinize, allow_partial, &alignment_writer,
+                  &words_writer, &compact_lattice_writer, &lattice_writer,
+                  &like)) {
+            tot_like += like;
+            frame_count += loglikes.NumRows();
+            num_success++;
+          } else num_fail++;
+        }
+      }
+      delete decode_fst; // delete this only after decoder goes out of scope.
+    } else { // We have different FSTs for different utterances.
+      SequentialTableReader<fst::VectorFstHolder> fst_reader(fst_in_str);
+      RandomAccessBaseFloatMatrixReader loglike_reader(feature_rspecifier);
+      for (; !fst_reader.Done(); fst_reader.Next()) {
+        std::string utt = fst_reader.Key();
+        if (!loglike_reader.HasKey(utt)) {
+          KALDI_WARN << "Not decoding utterance " << utt
+                     << " because no loglikes available.";
+          num_fail++;
+          continue;
+        }
+        const Matrix<BaseFloat> &loglikes = loglike_reader.Value(utt);
+        if (loglikes.NumRows() == 0) {
+          KALDI_WARN << "Zero-length utterance: " << utt;
+          num_fail++;
+          continue;
+        }
+        LatticeFasterDecoderCombine decoder(fst_reader.Value(), config);
+        DecodableMatrixScaledMapped decodable(trans_model, loglikes, acoustic_scale);
+        double like;
+        if (DecodeUtteranceLatticeFasterCombine(
+                decoder, decodable, trans_model, word_syms, utt, acoustic_scale,
+                determinize, allow_partial, &alignment_writer, &words_writer,
+                &compact_lattice_writer, &lattice_writer, &like)) {
+          tot_like += like;
+          frame_count += loglikes.NumRows();
+          num_success++;
+        } else num_fail++;
+      }
+    }
+
+    double elapsed = timer.Elapsed();
+    KALDI_LOG << "Time taken "<< elapsed
+              << "s: real-time factor assuming 100 frames/sec is "
+              << (elapsed*100.0/frame_count);
+    KALDI_LOG << "Done " << num_success << " utterances, failed for "
+              << num_fail;
+    KALDI_LOG << "Overall log-likelihood per frame is " << (tot_like/frame_count) << " over "
+              << frame_count<<" frames.";
+
+    delete word_syms;
+    if (num_success != 0) return 0;
+    else return 1;
+  } catch(const std::exception &e) {
+    std::cerr << e.what();
+    return -1;
+  }
+}

src/decoder/lattice-faster-decoder-combine.cc

@@ -892,6 +892,7 @@ void LatticeFasterDecoderCombineTpl<FST, Token>::ProcessForFrame(
   }  // end of while loop
   frame_processed_[frame] = true;
   frame_processed_[frame + 1] = false;
+  KALDI_VLOG(6) << "toks after: " << cur_toks_.size();
 }
 
 

src/decoder/lattice-faster-decoder-combine.h

@@ -32,6 +32,7 @@
 #include "lat/kaldi-lattice.h"
 #include "decoder/grammar-fst.h"
 #include "decoder/lattice-faster-decoder.h"
+#include "memory.h"
 
 namespace kaldi {
 
@@ -242,7 +243,8 @@ class LatticeFasterDecoderCombineTpl {
   using Weight = typename Arc::Weight;
   using ForwardLinkT = decodercombine::ForwardLink<Token>;
 
-  using StateIdToTokenMap = typename std::unordered_map<StateId, Token*>;
+  //using StateIdToTokenMap = typename std::unordered_map<StateId, Token*>;
+  using StateIdToTokenMap = typename std::unordered_map<StateId, Token*, std::hash<StateId>, std::equal_to<StateId>, fkaldi::PoolAllocator<std::pair<const StateId, Token*>>>;
   using IterType = typename StateIdToTokenMap::const_iterator;
 
   // Instantiate this class once for each thing you have to decode.