Trial-division code runs 2x faster as 32-bit on Windows than 64-bit on Linux

Question

I have a piece of code that runs 2x faster on windows than on linux. Here are the times I measured:

g++ -Ofast -march=native -m64
    29.1123
g++ -Ofast -march=native
    29.0497
clang++ -Ofast -march=native
    28.9192
visual studio 2013 Debug 32b
    13.8802
visual studio 2013 Release 32b
    12.5569

It really seems to be too huge a difference.

Here is the code:

#include <iostream>
#include <map>
#include <chrono>
static std::size_t Count = 1000;

static std::size_t MaxNum = 50000000;

bool IsPrime(std::size_t num)
{
    for (std::size_t i = 2; i < num; i++)
    {
        if (num % i == 0)
            return false;
    }
    return true;
}

int main()
{
    auto start = std::chrono::steady_clock::now();
    std::map<std::size_t, bool> value;
    for (std::size_t i = 0; i < Count; i++)
    {
        value[i] = IsPrime(i);
        value[MaxNum - i] = IsPrime(MaxNum - i);
    }
    std::chrono::duration<double> serialTime = std::chrono::steady_clock::now() - start;
    std::cout << "Serial time = " << serialTime.count() << std::endl;

    system("pause");
    return 0;
}

All of this was measured on the same machine with windows 8 vs linux 3.19.5(gcc 4.9.2, clang 3.5.0). Both linux and windows are 64bit.

What could be the reason for this? Some scheduler issues?

Answer 1

You don't say whether the windows/linux operating systems are 32 or 64 bit.

On a 64-bit linux machine, if you change the size_t to an int you'll find that execution times drop on linux to a similar value to those that you have for windows.

size_t is an int32 on win32, an int64 on win64.

EDIT: just seen your windows disassembly.

Your windows OS is the 32-bit variety (or at least you've compiled for 32-bit).