The new HEaP analytic placer for NextPNR uses Eigen for underlying
placement algorithms. Enabling -DUSE_OPENMP passes -fopenmp onto the
compiler, which Eigen picks up automatically with no extra work.
This should result in placer speedups for large designs, e.g. on ECP5
85k chips.
Signed-off-by: Austin Seipp <aseipp@pobox.com>
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