OpenMP and Vectorization Problem

Hi,

I am using a simple ikj triple loop to compute a matrix multiplication. The intel compiler icpc (ICC) 14.0.2 20140120 is used.

Suppose that in the 2 following cases the number of threads is 1 (one) (No parallel for is used yet!)

1- If I use a #pragma omp parallel, the compiled code is seemed to be vectorized. That is what -vec-report6 tells me. But the running time is equal to the non-vectorized case: 

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference B has aligned access

MATMUL.cc(71): (col. 4) remark: vectorization support: unroll factor set to 4

MATMUL.cc(71): (col. 4) remark: LOOP WAS VECTORIZED

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference B has unaligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: unaligned access used inside loop body

MATMUL.cc(71): (col. 4) remark: REMAINDER LOOP WAS VECTORIZED

2- On the other hand, if I simply remove the #pragma omp parallel, This message is printed out by the -vec-report6:  

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference B has aligned access

MATMUL.cc(71): (col. 4) remark: vectorization support: unroll factor set to 4

MATMUL.cc(71): (col. 4) remark: LOOP WAS VECTORIZED

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference C has aligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: reference B has unaligned access

MATMUL.cc(73): (col. 12) remark: vectorization support: unaligned access used inside loop body

MATMUL.cc(71): (col. 4) remark: REMAINDER LOOP WAS VECTORIZED

MATMUL.cc(71): (col. 4) remark: loop skipped: multiversioned

Although it says "loop skipped: multiversioned", which I am not sure what it exactly means, the running time is roughly 6X better, which implies the proper vectorization. Using the #pragma omp simd does not change the results.

void MatMul_Par(float* A, float* B, float* C) {
 //#pragma omp parallel shared(A,B,C)
   {
     for (int i=0;i<N;i++) {
           for(int k=0;k<N;k++) {
                float temp = A[i*N+k];
                //#pragma omp simd
                for(int j=0;j<N;j++) {
                    C[i*N+j] += temp * B[k*N+j];
                 }
           }
    }
   } //parallel
}

PS: The problem does not exist when using Intel Cilk Plus, etc. It seems to be related to the parallel pragma in OpenMP.