Loops with low trip count unknown at compile time do not benefit from vectorization
The main assumption of vectorization is that the same operation is applied to a vector of data instead of to a single piece of data. The size of the vector is fixed, and it is often the case that the size of the data set is not an exact multiple of the vector size. In this case, in addition to a vectorized loop, it is necessary to introduce a drain loop which processes a few last pieces of data which do not fit the whole vector.
When the loop trip count is very low, it might happen that the vector part of the loop doesn’t run at all, or it runs with only one or two iterations. If this is the case, the resulting vectorized loop will be the same speed or slower than its non-vectorized counterpart.
Please note that this recommendation applies only to those loops where the loop trip count is both low and unknown at compile time. Loops whose trip count is known at compile time do not have this issue.
There are several ways to fix this:
- Use techniques like loop interchange to increase the loop trip count of the innermost loop (read more here)
- Disable vectorization of the loop. The portable way to do it is though
#pragma omp simd if(simd:0), however, this is a recent extension of OpenMP and is not available in the older versions of the compiler. Many compilers have non-portable pragmas to disable vectorization (e.g. CLANG’s
#pragma clang loop vectorize(disable))
- If the loop trip count is unknown at compile time, but it can have only a few fixed values (e.g. 3, 4 and 5), create a separate version of the loop for each of the different trip counts.
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