pragma omp for simd does not generate vector instructions in GCC

Short: Does the pragma omp for simd OpenMP directive generate code that uses SIMD registers?

Longer:As stated in the OpenMP documentation"The worksharing-loop SIMD construct specifies that the iterations of one or more associated loops will be distributed across threads that already exist [..] using SIMD instructions". From this statement, I would expect the following code (simd.c) to use XMM, YMM or ZMM registers when compiling running gcc simd.c -o simd -fopenmp but it does not.

#include <stdio.h>#define N 100int main() {    int x[N];    int y[N];    int z[N];    int i;    int sum;    for(i=0; i < N; i++) {        x[i] = i;        y[i] = i;    }    #pragma omp parallel    {        #pragma omp for simd        for(i=0; i < N; i++) {            z[i] = x[i] + y[i];        }        #pragma omp for simd reduction(+:sum)        for(i=0; i < N; i++) {            sum += x[i];        }    }    printf("%d %d\n",z[N/2], sum);    return 0;}

When checking the assembler generated running gcc simd.c -S -fopenmp no SIMD register is used.

I can use SIMD registers without OpenMP using the option -O3 because according to GCC documentationit includes the -ftree-vectorize flag.

• XMM registers: gcc simd.c -o simd -O3
• YMM registers: gcc simd.c -o simd -O3 -march=skylake-avx512
• ZMM registers: gcc simd.c -o simd -O3 -march=skylake-avx512 -mprefer-vector-width=512

However, using the flags -march=skylake-avx512 -mprefer-vector-width=512 combined with -fopenmp does not generates SIMD instructions.

Therefore, I can easily vectorize my code with -O3 without the pragma omp for simd but not for the other way around.

At this point, my purpose is not to generate SIMD instructions but to understand how do OpenMP SIMD directives work in GCC and how to generate SIMD instructions only with OpenMP (without -O3).