C++11 vector<bool> performance issue (with code example)

Question

I notice that vector is much slower than bool array when running the following code.

int main() 
{
    int count = 0;
    int n = 1500000;
    // slower with c++ vector<bool>
    /*vector<bool> isPrime;
    isPrime.reserve(n);
    isPrime.assign(n, true);
    */
    // faster with bool array 
    bool* isPrime = new bool[n];

    for (int i = 0; i < n; ++i)
        isPrime[i] = true;


    for (int i = 2; i< n; ++i) {
        if (isPrime[i])
            count++;
        for (int j =2; i*j < n; ++j )
            isPrime[i*j] = false;
    }

    cout <<  count << endl;
    return 0;
}

Is there some way that I can do to make vector<bool> faster ? Btw, both std::vector::push_back and std::vector::emplace_back are even slower than std::vector::assign.

Answer 1

std::vector<bool> can have various performance issues (e.g. take a look at https://isocpp.org/blog/2012/11/on-vectorbool).

In general you can:

• use std::vector<std::uint8_t> instead of std::vector<bool> (give a try to std::valarray<bool> also).

  This requires more memory and is less cache-friendly but there isn't a overhead (in the form of bit manipulation) to access a single value, so there are situations in which it works better (after all it's just like your array of bool but without the nuisance of memory management)

• use std::bitset if you know at compile time how large your boolean array is going to be (or if you can at least establish a reasonable upper bound)
• if Boost is an option try boost::dynamic_bitset (the size can be specified at runtime)

---

But for speed optimizations you have to test...

With your specific example I can confirm a performance difference only when optimizations are turned off (of course this isn't the way to go).

Some tests with g++ v4.8.3 and clang++ v3.4.5 on an Intel Xeon system (-O3 optimization level) give a different picture:

                    time (ms)
                 G++      CLANG++
array of bool    3103     3010
vector<bool>     2835     2420    // not bad!
vector<char>     3136     3031    // same as array of bool
bitset           2742     2388    // marginally better

(time elapsed for 100 runs of the code in the answer)

std::vector<bool> doesn't look so bad (source code here).