vDSP branching loop

[Luigi Castelli]

Hi there,

I have noticed there is no single vDSP function for the following:

for (n = 0; n < N; ++n)

    if (B[0] <= A[n])

        E[n] = C[0];

    else

        E[n] = D[0];

I have tried to use a combination of vDSP functions to implement the above but the results are not to my satisfaction.

How would you implement the above loop using vDSP ?

Thank you.

- Luigi

[Stephen Canon]

Hi Luigi —

Clang will happily autovectorize this loop for you if you write it like this (I made B, C, D scalar floats instead of pointers, since you only ever use the first element):

#include <stdlib.h>

void mySelect(const float *A, float B, float C, float D, float * restrict E, size_t N) {

    for (size_t n = 0; n < N; ++n)

        E[n] = A[n] >= B ? C : D;

}

– Steve

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[Luigi Castelli]

Ok, thanks. Good to know.

What about if I change the loop to this? (B is a vector now)

for (n = 0; n < N; ++n)

    if (B[n] <= A[n])

        E[n] = C[0];

    else

        E[n] = D[0];

- Luigi

[Stephen Canon]

Sure:

void select(const float *A, float *B, float C, float D, float * restrict E, size_t N) {

    for (size_t n = 0; n < N; ++n)

        E[n] = A[n] >= B[n] ? C : D;

}

Vectorized by clang at -O3.

[Luigi Castelli]

Perfect. Thanks Stephen!