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c++cgccvectorization

What does vectorization mean?

Is it a good idea to vectorize the code? What are good practices in terms of when to do it? What happens underneath?

Solution

  • Vectorization means that the compiler detects that your independent instructions can be executed as one SIMD instruction. Usual example is that if you do something like

    for (i = 0; i < N; i++) {
    a[i] = a[i] + b[i];
}

    It will be vectorized as (using vector notation)

    for (i = 0; i < (N - N % VF); i += VF) {
    a[i : i + VF] = a[i : i + VF] + b[i : i + VF];
}

    Basically the compiler picks one operation that can be done on VF elements of the array at the same time and does this N/VF times instead of doing the single operation N times.

    It increases performance, but puts more requirement on the architecture.