CODEWITHSUNDEEP
c++vectorization
user189035
user189035

Reputation: 5789

Why would std::copy not vectorize?

Consider this general 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 4.8.1 and the -O3 -march=native -mtune=native flag on Ubuntu, I get that the line corresponding to the copy can't be vectored 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'm a bit puzzled. Intuitively I would think a copy between two non-overlapping array must be eminently vector-able. Can anyone explain why this is not the case (and eventually provide a fix, though admittedly this is not a bottleneck in my code, I'm mostly asking for the sake of understanding what that error message mean).

Upvotes: 4

Views: 856

Answers (1)

MP24
MP24

Reputation: 3200

It seems that your code is fine and it is just an "information" message: See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57579. There, similar code will be vectorized by another method, and therefore the second vectorizing code spits out the informational message that it cannot vectorize (again).

Upvotes: 3

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