Remove clock for std::chrono (#2408)

`clock.hpp` has become redundant over `std::chrono` and was replaced
with `monotonic_clock`.

arbor/CMakeLists.txt

@@ -48,7 +48,6 @@ set(arbor_sources
     network_impl.cpp
     simulation.cpp
     partition_load_balance.cpp
-    profile/clock.cpp
     profile/memory_meter.cpp
     profile/meter_manager.cpp
     profile/power_meter.cpp

arbor/include/arbor/profile/meter_manager.hpp

@@ -33,7 +33,7 @@ class ARB_ARBOR_API meter_manager {
 private:
     bool started_ = false;
 
-    tick_type start_time_ = 0;
+    tick_type start_time_ = {};
     std::vector<double> times_;
 
     std::vector<std::unique_ptr<meter>> meters_;

arbor/include/arbor/profile/profiler.hpp

@@ -1,13 +1,10 @@
 #pragma once
 
-#include <cstdint>
 #include <ostream>
-#include <unordered_map>
 #include <vector>
 
 #include <arbor/export.hpp>
 #include <arbor/context.hpp>
-#include <arbor/profile/timer.hpp>
 
 namespace arb {
 

arbor/include/arbor/profile/timer.hpp

@@ -1,19 +1,20 @@
 #pragma once
 
-#include <arbor/profile/clock.hpp>
+#include <chrono>
+
+// NOTE This is only in the public API as meter_manager is and has a private
+//      member of tick_type.
 
 namespace arb {
 namespace profile {
 
-template <typename Clock = default_clock>
-struct timer {
-    static inline tick_type tic() {
-        return Clock::now();
-    }
+using clock_type = std::chrono::steady_clock;
+using tick_type = clock_type::time_point;
 
-    static inline double toc(tick_type t) {
-        return (Clock::now()-t)*Clock::seconds_per_tick();
-    }
+struct timer {
+    constexpr static double scale = 1e-9; // ns -> s
+    static inline tick_type tic() { return clock_type::now(); }
+    static inline double toc(tick_type t) { return scale*std::chrono::duration_cast<std::chrono::nanoseconds>(clock_type::now() - t).count(); }
 };
 
 } // namespace profile

arbor/profile/meter_manager.cpp

@@ -14,7 +14,6 @@
 namespace arb {
 namespace profile {
 
-using timer_type = timer<>;
 using util::strprintf;
 
 template <typename C>
@@ -69,14 +68,14 @@ void meter_manager::start(context ctx) {
     arb::threading::task_group(ctx->thread_pool.get()).wait();
     ctx->distributed->barrier();
 
-    start_time_ = timer_type::tic();
+    start_time_ = timer::tic();
 };
 
 void meter_manager::checkpoint(std::string name, context ctx) {
     arb_assert(started_);
 
     // Record the time taken on this domain since the last checkpoint
-    times_.push_back(timer<>::toc(start_time_));
+    times_.push_back(timer::toc(start_time_));
 
     // Update meters
     checkpoint_names_.push_back(std::move(name));
@@ -88,7 +87,7 @@ void meter_manager::checkpoint(std::string name, context ctx) {
     ctx->gpu->synchronize();
     arb::threading::task_group(ctx->thread_pool.get()).wait();
     ctx->distributed->barrier();
-    start_time_ = timer<>::tic();
+    start_time_ = timer::tic();
 }
 
 const std::vector<std::unique_ptr<meter>>& meter_manager::meters() const {

arbor/profile/profiler.cpp

@@ -4,16 +4,17 @@
 
 #include <arbor/context.hpp>
 #include <arbor/profile/profiler.hpp>
+#include <arbor/profile/timer.hpp>
 
 #include "execution_context.hpp"
 #include "threading/threading.hpp"
 #include "util/span.hpp"
 #include "util/rangeutil.hpp"
 
+
 namespace arb {
 namespace profile {
 
-using timer_type = timer<>;
 using util::make_span;
 
 #ifdef ARB_HAVE_PROFILING
@@ -150,12 +151,12 @@ void recorder::enter(region_id_type index) {
         accumulators_.resize(index+1);
     }
     index_ = index;
-    start_time_ = timer_type::tic();
+    start_time_ = timer::tic();
 }
 
 void recorder::leave() {
     // calculate the elapsed time before any other steps, to increase accuracy.
-    auto delta = timer_type::toc(start_time_);
+    auto delta = timer::toc(start_time_);
 
     if (index_==npos) {
         throw std::runtime_error("recorder::leave without matching recorder::enter");