A Timer for Modern C++

core::timer::Timer is a header-only timer for modern C++. The timer is designed to faciliate minimal-overhedad, ad-hoc timing of C++ code including micro-timing down to a single machine instruction. The timer is not designed for benchmarking (benchmark or nanobench are excellent tools for this purpose) nor for detailed performance analysis.

Continue on for a brief tour, or you can read the full documentation.

Brief Tour

The timer has two basic modes:

• Inovking run on isolated code for in vivo measurements.
• Using start and stop timer calls for in vitro measurements.

In Vivo Example

The run method is designed to repeatedly execute isolated code in order to measure the average cost per operation. Modern compilers, taking advantge of the C++ standard's "as if" rule, will often eliminate code that produces no visible side-effects making this type of measurement difficult.

The timer library provides the core::timer::doNotOptimizeAway function which forces the compiler to materialize the given expression either in memory or in a register. The compiler is still allowed to fully optimize the expression given that single constraint.

The following example demonstrates using the run method coupled with the doNotOptimizeAway function to measure the cost of performing an integer addition (i.e. output += 1). Without the doNotOptimizeAway call, a typical outcome is for the compiler to eliminate all of the computation since there are no visible side-effects.

#include <iomanip>
#include <iostream>
#include "core/timer/timer.h"

using namespace core;

int main(int argc, const char *argv[]) {

    unsigned int output{};
    auto ns = timer::Timer().run(1'000'000, [&]() {
	    output += 1;
	    timer::doNotOptimizeAway(output);
    }).elapsed_per_iteration();
    
    std::cout << std::setprecision(2) << ns << " nanoseconds per operation" << std::endl;
    // 0.31 nanoseconds per operation
	
    return 0;
}

In Vitro Example

The start and stop methods can be used to instrument code as it exists without isolating it to be executed by run. On a modern platform, each invocation of start and stop costs tens of clock cycles, thus, they are designed to be applied to more significant code segments.

The following example demonstrates using the start and stop methods to measure the cost of an exclusive or performed in a loop. Referencing the output variable at the end serves the same function as the doNotOptimizeAway function in the previous example; it forces the compiler to actually do the computation.

#include <iostream>
#include "core/timer/timer.h"

using namespace core;

int main(int argc, const char *argv[]) {
    
    timer::Timer timer;
    timer.start();
    unsigned int output{};
    for (auto i = 0; i < 10'000; ++i)
		output ^= i;
    timer.stop();

    auto ns = timer.elapsed().count();
    std::cout << ns << " nanoseconds" << std::endl;
    std::cout << output << std::endl;
    // 3150 nanoseconds
	
    return 0;
}

Build

git clone [email protected]:cpp-core/timer
mkdir timer/build && cd timer/build
CC=clang-15 CXX=clang++-15 cmake -DCMAKE_INSTALL_PREFIX=$HOME/opt ..
make check