imatrix : use GGUF regardless of the output filename

The legacy format can only be produced with --output-format dat

common/arg.cpp

@@ -2629,10 +2629,10 @@ common_params_context common_params_parser_init(common_params & params, llama_ex
     ).set_examples({LLAMA_EXAMPLE_IMATRIX}));
     add_opt(common_arg(
         {"--output-format"}, "{gguf,dat}",
-        string_format("output format for imatrix file (default: gguf except when output filename ends with .dat)"),
+        string_format("output format for imatrix file (default: %s)", params.imat_dat ? "dat" : "gguf"),
         [](common_params & params, const std::string & value) {
-            /**/ if (value == "gguf") { params.imat_out_type = COMMON_IMATRIX_FORMAT_GGUF; }
-            else if (value == "dat")  { params.imat_out_type = COMMON_IMATRIX_FORMAT_DAT;  }
+            /**/ if (value == "gguf") { params.imat_dat = false; }
+            else if (value == "dat")  { params.imat_dat = true;  }
             else { throw std::invalid_argument("invalid output format"); }
         }
     ).set_examples({LLAMA_EXAMPLE_IMATRIX}));

common/common.h

@@ -233,12 +233,6 @@ enum common_reasoning_format {
     COMMON_REASONING_FORMAT_DEEPSEEK,        // Extract thinking tag contents and return as `message.reasoning_content`, including in streaming deltas.
 };
 
-enum common_imatrix_format_type {
-    COMMON_IMATRIX_FORMAT_AUTO,
-    COMMON_IMATRIX_FORMAT_GGUF,
-    COMMON_IMATRIX_FORMAT_DAT,  // legacy
-};
-
 struct common_params {
     int32_t n_predict             =    -1; // new tokens to predict
     int32_t n_ctx                 =  4096; // context size
@@ -437,7 +431,7 @@ struct common_params {
     int32_t n_out_freq  = 10; // output the imatrix every n_out_freq iterations
     int32_t n_save_freq =  0; // save the imatrix every n_save_freq iterations
     int32_t i_chunk     =  0; // start processing from this chunk
-    common_imatrix_format_type imat_out_type = COMMON_IMATRIX_FORMAT_AUTO; // format of the output imatrix
+    bool    imat_dat    = false; // whether the legacy imatrix.dat format should be output
 
     bool process_output  = false; // collect data for the output tensor
     bool compute_ppl     = true;  // whether to compute perplexity

tools/imatrix/README.md

@@ -20,7 +20,7 @@ The parameters in square brackets are optional and have the following meaning:
 * `-lv | --verbosity` specifies the verbosity level. If set to `0`, no output other than the perplexity of the processed chunks will be generated. If set to `1`, each time the results are saved a message is written to `stderr`. If `>=2`, a message is output each time data is collected for any tensor. Default verbosity level is `1`.
 * `-o | --output-file` specifies the name of the file where the computed data will be stored. If missing `imatrix.gguf` is used.
 * `-ofreq | --output-frequency` specifies how often the so far computed result is saved to disk. Default is 10 (i.e., every 10 chunks)
-* `--output-format` specifies the output format of the generated imatrix file. Either "gguf", or "dat" (the legacy format). Defaults to "gguf" unless the output filename ends with `.dat`.
+* `--output-format` specifies the output format of the generated imatrix file. Either "gguf", or "dat" (the legacy format). Defaults to "gguf".
 * `--save-frequency` specifies how often to save a copy of the imatrix in a separate file. Default is 0 (i.e., never)
 * `--process-output` specifies if data will be collected for the `output.weight` tensor. Typically, it is better not to utilize the importance matrix when quantizing `output.weight`, so this is set to `false` by default.
 * `--in-file` one or more existing imatrix files to load and combine. Useful for merging files from multiple runs/datasets.
@@ -46,14 +46,19 @@ Recent versions of `llama-imatrix` store data in GGUF format by default. For the
 
 ```bash
 # generate and save the imatrix using legacy format
-./llama-imatrix -m ggml-model-f16.gguf -f calibration-data.txt -o imatrix-legcy-format.dat -ngl 99
+./llama-imatrix -m ggml-model-f16.gguf -f calibration-data.txt --output-format dat -o imatrix-legcy-format.dat -ngl 99
 ```
 
 ```bash
-# covert legacy (binary) imatrix format to new (GGUF) format
+# convert legacy (binary) imatrix format to new (GGUF) format
 ./llama-imatrix --in-file imatrix-legacy-format.dat -o imatrix-new-format.gguf
 ```
 
+```bash
+# convert new (GGUF) imatrix format to legacy (binary) format
+./llama-imatrix --in-file imatrix-new-format.gguf --output-format dat -o imatrix-legacy-format.dat
+```
+
 ```bash
 # combine existing imatrices
 ./llama-imatrix --in-file imatrix-prev-0.gguf --in-file imatrix-prev-1.gguf -o imatrix-combined.gguf

tools/imatrix/imatrix.cpp

@@ -492,11 +492,9 @@ void IMatrixCollector::save_imatrix_legacy(int32_t ncall) const {
 
 void IMatrixCollector::save_imatrix(int32_t n_chunk) const {
     auto fname = m_params.out_file;
-    auto imat_type = m_params.imat_out_type;
+    bool use_legacy_format = m_params.imat_dat;
 
-    if ((imat_type == COMMON_IMATRIX_FORMAT_AUTO && string_ends_with(fname, ".dat")) ||
-        (imat_type == COMMON_IMATRIX_FORMAT_DAT)) {
-        LOG_WRN("\n%s: saving to legacy imatrix format\n", __func__);
+    if (use_legacy_format) {
         this->save_imatrix_legacy(n_chunk);
         return;
     }