Add benchmark with null-clock for pure PERForate timing

src/perforate/scoped_trace.bench.cpp

@@ -29,6 +29,15 @@ static std::vector<size_t> to_measure() {
   return value;
 }
 
+struct null_clock {
+  using rep = size_t;
+  using period = std::ratio<1, 1>;
+  using duration = std::chrono::duration<rep, period>;
+  using time_point = std::chrono::time_point<null_clock>;
+
+  static time_point now() { return time_point(std::chrono::seconds{0}); }
+};
+
 TEST_CASE("perforate::scoped_trace", "[scoped_trace][benchmark]") {
   BENCHMARK("std::chrono::high_resolution_clock::now()") {
     return std::chrono::high_resolution_clock::now();
@@ -36,6 +45,13 @@ TEST_CASE("perforate::scoped_trace", "[scoped_trace][benchmark]") {
 
   BENCHMARK("scoped_trace overhead") { PERFORATE_SCOPED_TRACE(nullptr); };
 
+  BENCHMARK("scoped_trace null clock overhead") {
+    auto s = perforate::scoped_trace<class tsc_tag, 0, null_clock>(nullptr);
+    return s;
+  };
+
+  BENCHMARK("null_clock::now()") { return null_clock::now(); };
+
   // Measure the "PERFORATE_SCOPED_TRACE overhead to a loop with PERForate"
   // Assume it's around 100ns.
   {
@@ -57,7 +73,7 @@ TEST_CASE("perforate::scoped_trace", "[scoped_trace][benchmark]") {
     // Outside loop
     {
       PERFORATE_SCOPED_TRACE("measuring work outside loop")
-        .call_count(number_of_iterations);
+          .call_count(number_of_iterations);
 
       for (size_t i = 0; i < number_of_iterations; i++) {
         use(to_measure());