[jedbrown]

The floating point comment leaves out that one can use

    #pragma omp simd reduction(+:res)

as a more precise way to achieve vectorization in the reduction (compile with -fopenmp-simd to only use it for SIMD without linking an OpenMP library): https://godbolt.org/z/17oTz1

Unfortunately, the pragma is not supported with the new-style class iterators in a released compiler, though it works in clang-trunk: https://godbolt.org/z/hbP11W Note that Clang disables floating point contraction by default (so no vfmadd instructions), despite them being more accurate. One usually wants this globally (-ffp-contract=fast) except when trying to bitwise reproduce software compiled for pre-Haswell.