mechanical_sympathy.write_combining.cpp

// This is a C++ port from Java of Martin Thomson's code to demonstrate
// write combining buffers.
// See: http://mechanical-sympathy.blogspot.sg/2011/07/write-combining.html

#include <iostream>
#include <limits>
#include <chrono>

const int ITERATIONS = std::numeric_limits<int>::max();
const int ITEMS = 1 << 24;
const int MASK = ITEMS - 1;

char arrayA[ITEMS];
char arrayB[ITEMS];
char arrayC[ITEMS];
char arrayD[ITEMS];
char arrayE[ITEMS];
char arrayF[ITEMS];

std::chrono::duration<long, std::nano> runCaseOne()
{
    auto start = std::chrono::high_resolution_clock::now();

    int i = ITERATIONS;
    while (--i != 0)
    {
        int slot = i & MASK;
        char b = (char)i;
        arrayA[slot] = b;
        arrayB[slot] = b;
        arrayC[slot] = b;
        arrayD[slot] = b;
        arrayE[slot] = b;
        arrayF[slot] = b;
    }

    std::chrono::duration<long, std::nano> duration = std::chrono::high_resolution_clock::now() - start;
    return duration;
}

std::chrono::duration<long, std::nano> runCaseTwo()
{
    auto start = std::chrono::high_resolution_clock::now();

    int i = ITERATIONS;
    while (--i != 0)
    {
        int slot = i & MASK;
        char b = (char)i;
        arrayA[slot] = b;
        arrayB[slot] = b;
        arrayC[slot] = b;
    }

    i = ITERATIONS;
    while (--i != 0)
    {
        int slot = i & MASK;
        char b = (char)i;
        arrayD[slot] = b;
        arrayE[slot] = b;
        arrayF[slot] = b;
    }

    std::chrono::duration<long, std::nano> duration = std::chrono::high_resolution_clock::now() - start;
    return duration;
}

int main (int argc, char** argv)
{
    for (int i = 1; i <= 3 ; i++) {
        std::cout << i << " SingleLoop duration (ns) = " << runCaseOne().count() << std::endl;
        std::cout << i << " SplitLoop duration (ns) = " << runCaseTwo().count() << std::endl;
    }

    int result = arrayA[1] + arrayB[2] + arrayC[3] +
                 arrayD[4] + arrayE[5] + arrayF[6];

    std::cout << "result = " << result << std::endl;
}