Why not std :: copy vectorize?

Question

Consider this generic code:

#include <cstdlib>
#include <ctime>
#include <algorithm>    // std::copy    

int main() {
    const int n=1024;
    float a1[n],a2[n];
    std::srand(std::time(0)); 
    for(int i=0;i<n;i++)      a2[i]=std::rand()/(float)RAND_MAX;
    std::copy(a2,a2+n,a1);
}

when I compile this with g++/gcc 4.8.1and the flag -O3 -march=native -mtune=nativein Ubuntu, I get that the line corresponding to the copy cannot be put down because:

note: not vectorized: not enough data-refs in basic block.

If i use

for(int i=0;i<n;i++)      a1[i]=a2[i];

I also get the same compiler message. I am a little puzzled. Intuitively, I would think a copy between two non-overlapping arrays should be highly vectorial. Can someone explain why this is not the case (and, ultimately, admittedly, this is not a bottleneck in my code, I basically ask you to understand what this error message means.

Answer 1

, , "": . http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57579. , , ().