std::fill does not turn into memset for POD types

Question

I am expecting a std::fill on an continuous container, say std::vector, will automatically compiled to a call of memset. However, when I tried the following code

#include <vector>
#include <algorithm>
#include <numeric>

using namespace std;

int main()
{
    vector<double> vec(300000);

    fill(vec.begin(),vec.end(),0.0);

    memset(&vec[0],0,vec.size()*sizeof(double));
}

gcc compiled the first std::fill to a simple loop. But I think it could be done by SSE or other advanced vectorized code. Please give me a hint. Thanks.

Answer 1

Addressing your specific example of double, it would have to be a platform specific optimization and most likely g++ decided not to do such a thing. The reason is of course any platforms using a representation of double for which 0.0 does not mean all zero bytes. Note that additionally, setting to any number OTHER than zero is a whole different game as it's not just setting every byte to zero: There is a specific pattern that needs to be followed. It gets worse with negative numbers.

Unless you have specific profiling information that the fill is taking significantly longer than memset I wouldn't be too worried about it. If it IS taking a lot longer you can either hand-tune to manually use memset or try to address the root cause, the need to set to zero repeatedly.

Answer 2

The standard doesn't force implementors to use memset(). But gcc for example does happen to use memset() for std::fill() on containers of char.