FFTW: Initialize Threads

Co-authored-by: Severin Diederichs <severin.diederichs@desy.de>

Source/FieldSolver/SpectralSolver/WrapFFTW.cpp

@@ -1,4 +1,4 @@
-/* Copyright 2019-2020
+/* Copyright 2019-2021
  *
  * This file is part of WarpX.
  *
@@ -32,6 +32,16 @@ namespace AnyFFT
     {
         FFTplan fft_plan;
 
+#if defined(AMREX_USE_OMP) && defined(WarpX_FFTW_OMP)
+#   ifdef AMREX_USE_FLOAT
+        fftwf_init_threads();
+        fftwf_plan_with_nthreads(omp_get_max_threads());
+#   else
+        fftw_init_threads();
+        fftw_plan_with_nthreads(omp_get_max_threads());
+#   endif
+#endif
+
         // Initialize fft_plan.m_plan with the vendor fft plan.
         // Swap dimensions: AMReX FAB are Fortran-order but FFTW is C-order
         if (dir == direction::R2C){