Re: [boost] [uuids] On boost::uuids::uuid operator == performance.

Subject: Re: [boost] [uuids] On boost::uuids::uuid operator == performance.
From: Peter Dimov (pdimov_at_[hidden])
Date: 2012-04-16 13:24:25

Interestingly, if I try the following program, the "stupid" my_memcmp 
produces the fastest result.

#include <boost/uuid/uuid.hpp>

#include <iostream>

#include <windows.h>
#include <mmsystem.h>

typedef unsigned uint32_t;

inline bool Eq16( unsigned char const * p, unsigned char const * q )
{
    return *reinterpret_cast<const uint32_t*>( p   ) == 
*reinterpret_cast<const uint32_t*>( q   )
        && *reinterpret_cast<const uint32_t*>( p+4 ) == 
*reinterpret_cast<const uint32_t*>( q+4 )
        && *reinterpret_cast<const uint32_t*>( p+8 ) == 
*reinterpret_cast<const uint32_t*>( q+8 )
        && *reinterpret_cast<const uint32_t*>( p+12) == 
*reinterpret_cast<const uint32_t*>( q+12);
}

inline bool my_memcmp( unsigned char const * p, unsigned char const * q, 
size_t n )
{
    for( size_t i = 0; i < n; ++i )
    {
        if( p[i] != q[i] ) return false;
    }

    return true;
}

int main()
{
    boost::uuids::uuid id1 = {}, id2 = {};

    int const N = 100000000;

    DWORD t1 = timeGetTime();

    int s = 0;

    for( int i = 0; i < N; ++i )
    {
        //s += ( id1 == id2 );
        //s += Eq16( id1.data, id2.data );
        //s += memcmp( id1.data, id2.data, 16 );
        s += my_memcmp( id1.data, id2.data, 16 );
        id2.data[ i % 16 ] += i & 0xFF;
    }

    DWORD t2 = timeGetTime();

    std::cout << s << ": " << t2 - t1 << std::endl;
}