add comments to backend loading in test-backend-ops.cpp [no ci]

Author: danbev

tests/test-backend-ops.cpp

@@ -7150,10 +7150,12 @@ static void print_backend_features(ggml_backend_t backend) {
 
 static bool test_cpu_variant(const char * variant_name, const char * op_names_filter,
         const char * params_filter, printer * output_printer) {
-    std::string backend_ref_name = "CPU-ref";
-    std::string variant = std::string(variant_name).substr(4);
-    ggml_backend_load_variant("cpu", variant.c_str());
+    // Load the variant first so that extra buffer types created only use that
+    // backend as the features of the backend can determine if the extra buf
+    // types are enabled or not.
+    ggml_backend_load_variant("cpu", std::string(variant_name).substr(4).c_str());
 
+    // Load extra buffer types and allocate a buffer from each type.
     std::unordered_map<ggml_backend_buffer_type_t, ggml_backend_buffer_t> extra_buf_map;
     {
         auto * cpu_dev = ggml_backend_dev_by_type(GGML_BACKEND_DEVICE_TYPE_CPU);
@@ -7165,7 +7167,7 @@ static bool test_cpu_variant(const char * variant_name, const char * op_names_fi
             ggml_backend_buffer_type_t * extra_bufts = ggml_backend_dev_get_extra_bufts_fn(cpu_dev);
             while (extra_bufts && *extra_bufts) {
                 // TODO: What should the size be here? Do extra buffer types need a size even?
-                // We need to have a value larger than 0 to avoid the default buffer to be used.
+                // We need to have a value larger than 0 to avoid the dummy backend buffer to be used.
                 extra_buf_map[*extra_bufts] = ggml_backend_buft_alloc_buffer(*extra_bufts, 1);
                 ++extra_bufts;
             }
@@ -7178,7 +7180,8 @@ static bool test_cpu_variant(const char * variant_name, const char * op_names_fi
     }
     printf("\n");
 
-    ggml_backend_load_variant("cpu", "ref");
+    std::string backend_ref_name = "CPU-ref";
+    ggml_backend_load_variant("cpu", std::string(backend_ref_name).substr(4).c_str());
 
     ggml_backend_t backend_ref = ggml_backend_init_by_name(backend_ref_name.c_str(), nullptr);
     if (backend_ref == nullptr) {