Cactus known-architectures files lib/make/known-architectures/linux and lib/make/known-architectures/darwin differ in whether they set defaults for XXX_OPENMP_FLAGS. Namely the Linux one contains:

ib/make/known-architectures/linux:       : ${CXX_OPENMP_FLAGS='-fopenmp'}
lib/make/known-architectures/linux:     : ${CXX_OPENMP_FLAGS='-openmp'}
lib/make/known-architectures/linux:     : ${CXX_OPENMP_FLAGS='-mp'}
lib/make/known-architectures/linux:     : ${CXX_OPENMP_FLAGS='-openmp'}
lib/make/known-architectures/linux:     : ${CXX_OPENMP_FLAGS='-qsmp=omp'}

while the darwin (macOS) one does not set any defaults. Other architectures (aix) set a them as well while yet others (all others) do not.

Ideally the defaults should be the same for all architectures (and nowadays probably -fopenmp). The value of OPENMP itself (ie if OpenMP is used) is not set by any architecture file.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2588/defaults-for-xxx_openmp_flags-differ