C++ vectorize double loop - c++

Question

I would like to vectorize a double for loop with omp simd. My Problem is of the following form:
#include <vector>
using namespace std;
#define N 8000
int main() {
vector<int> a;
vector<int> b;
vector<int> c;
a.resize(N);
b.resize(N);
c.resize(N);
#pragma omp simd collapse(2)
for (unsigned int i = 0; i < c.size(); ++i) {
for (unsigned int j = 0; j < c.size(); ++j) {
c[i] += a[i] + b[j];
}
}
}
When I compile this with g++ -O2 -fopenmp-simd -fopt-info-vec-all the vectorization report states:
note: not vectorized: not suitable for gather load _14 = *_42;
How can the code be transformed for the compiler to auto-vectorize it?
(Compiler: g++ 5.4.0, CPU supports AVX2)
UPDATE
The main problem is, as mentioned below, a data dependency of c whereby only the inner loop seems to be vectorizable. Resolving the dependency, can be achieved by switching the loops as seen below. The compiler auto-vectorized this now for me.
for (unsigned int j = 0; j < c.size(); ++j) {
#pragma omp simd
for (unsigned int i = 0; i < c.size(); ++i) {
c[i] += a[i] + b[j];
}
}

Answer 1

the main problem of your code is loop iteration count cannot be computed before executing the loop. you need to replace c.size() with N.
second problem is if you want vectorize outer loop,statement of c[i] = a[i] + b[j] leads to Flow and Anti dependencies. for ovecome these issues I try to vectorize inner loop and my code successfully being vectorize.
you can read more about Anti and Flow Dependencies in below page:
https://en.wikipedia.org/wiki/Data_dependency
I achieve 6.3 speed up after vectorization.
finally my code looks like below:
for (unsigned int i = 0; i < N; ++i)
{
#pragma simd
for (unsigned int j = 0; j < N; ++j)
{
c[i] = a[i] + b[j];
}
}