cxx-benchmarks/benchmark-atomic-costs.cpp

#include "measure_time.h"

#include <atomic>
#include <cstdint>
#include <thread>
#include <ext/concurrence.h>

#define COUNT (1u << 24)

// use "incl" for 32-bit and "incq" for 64-bit in asm below:
// use 32-bit for now: _Atomic_word should be 32-bit too;
// 64-bit is basically as fast as 32-bit if it fits in your cache
typedef std::uint32_t counting;

volatile counting __attribute__ ((aligned (64))) volatile_counter;
counting __attribute__ ((aligned (64))) simple_counter;
std::atomic<counting> __attribute__ ((aligned (64))) atomic_counter;

_Atomic_word __attribute__ ((aligned (64))) gxx_counter;

void benchmark_volatile() {
	// should be the same as asm below
	volatile_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		++volatile_counter;
	}
}

void benchmark_asm() {
	simple_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		asm volatile(
			"incl %0;"
			: "+m"(simple_counter)
		);
	}
}

void benchmark_local_atomic() {
	// compiler doesn't realize no one else can read this counter and
	// uses real atomic (i.e. "lock")
	std::atomic<counting> local_atomic_counter;
	local_atomic_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		++local_atomic_counter;
	}
}

void benchmark_atomic() {
	// should be the same as "lock_asm"
	atomic_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		++atomic_counter;
	}
}

void benchmark_lock_asm() {
	simple_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		asm volatile(
			"lock incl %0;"
			: "+m"(simple_counter)
		);
	}
}

void benchmark_gxx_atomic() {
	// if linked with pthread this will use "lock" same as atomic, otherwise
	// it uses a simple non-volatile non-atomic add instruction
	// https://gcc.gnu.org/onlinedocs/libstdc++/manual/ext_concurrency.html
	// claims __atomic_add_dispatch uses a volatile counter, but this isn't
	// true in gcc 4.9
	gxx_counter = 0;
	for (size_t i = 0; i < COUNT; ++i) {
		__gnu_cxx::__atomic_add_dispatch(&gxx_counter, 1);
	}
}


int main() {
//	std::thread fake([] { return; });
//	fake.join();

	std::cout << "volatile counter    : ";
	measure_time_with_dry_run(benchmark_volatile);

	std::cout << "asm counter         : ";
	measure_time_with_dry_run(benchmark_asm);

	std::cout << "local atomic counter: ";
	measure_time_with_dry_run(benchmark_local_atomic);

	std::cout << "atomic counter      : ";
	measure_time_with_dry_run(benchmark_atomic);

	std::cout << "lock asm counter    : ";
	measure_time_with_dry_run(benchmark_lock_asm);

	std::cout << "gxx atomic counter  : ";
	measure_time_with_dry_run(benchmark_gxx_atomic);
}