Fix get_config_count for CPU-only benchmarks. (#218)

nvbench/benchmark_base.cxx

@@ -20,6 +20,9 @@
 #include <nvbench/criterion_manager.cuh>
 #include <nvbench/detail/transform_reduce.cuh>
 
+#include <algorithm>
+#include <cstdint>
+
 namespace nvbench
 {
 
@@ -86,7 +89,8 @@ std::size_t benchmark_base::get_config_count() const
       return axis_ptr->get_size();
     });
 
-  return per_device_count * m_devices.size();
+  // Devices will be empty for cpu-only benchmarks.
+  return per_device_count * std::max(std::size_t(1), m_devices.size());
 }
 
 benchmark_base &benchmark_base::set_stopping_criterion(std::string criterion)

testing/benchmark.cu

@@ -18,6 +18,7 @@
 
 #include <nvbench/benchmark.cuh>
 #include <nvbench/callable.cuh>
+#include <nvbench/device_manager.cuh>
 #include <nvbench/named_values.cuh>
 #include <nvbench/state.cuh>
 #include <nvbench/type_list.cuh>
@@ -27,6 +28,7 @@
 #include <fmt/format.h>
 
 #include <algorithm>
+#include <cstdint>
 #include <utility>
 #include <variant>
 #include <vector>
@@ -279,6 +281,7 @@ void test_clone()
 void test_get_config_count()
 {
   lots_of_types_bench bench;
+  bench.set_devices(nvbench::device_manager::get().get_devices());
   bench.set_type_axes_names({"Integer", "Float", "Other"});
   bench.get_axes().get_type_axis(0).set_active_inputs({"I16", "I32"}); // 2,  2
   bench.get_axes().get_type_axis(1).set_active_inputs({"F32", "F64"}); // 2,  4
@@ -288,9 +291,13 @@ void test_get_config_count()
   bench.add_string_axis("baz", {"str", "ing"});                        // 2, 72
   bench.add_string_axis("baz", {"single"});                            // 1, 72
 
-  auto const num_devices = bench.get_devices().size();
+  auto const num_devices = std::max(std::size_t(1), bench.get_devices().size());
 
   ASSERT_MSG(bench.get_config_count() == 72 * num_devices, "Got {}", bench.get_config_count());
+
+  // Check that zero devices (e.g. CPU-only) is the same as a single device:
+  bench.set_devices(std::vector<int>{});
+  ASSERT_MSG(bench.get_config_count() == 72, "Got {}", bench.get_config_count());
 }
 
 int main()