FFT-based Poisson solver follow-up (ECP-WarpX#4945)

* changed  2

* add assert to check fft compilation

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update Source/ablastr/fields/PoissonSolver.H

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

* Update Source/ablastr/fields/PoissonSolver.H

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

Source/FieldSolver/ElectrostaticSolver.cpp

@@ -382,8 +382,8 @@ WarpX::computePhi (const amrex::Vector<std::unique_ptr<amrex::MultiFab> >& rho,
     const std::optional<amrex::Vector<amrex::FArrayBoxFactory const *> > eb_farray_box_factory;
 #endif
 
-    bool const is_solver_multigrid =
-        WarpX::poisson_solver_id != PoissonSolverAlgo::IntegratedGreenFunction;
+    bool const is_solver_igf_on_lev0 =
+        WarpX::poisson_solver_id == PoissonSolverAlgo::IntegratedGreenFunction;
 
     ablastr::fields::computePhi(
         sorted_rho,
@@ -397,7 +397,7 @@ WarpX::computePhi (const amrex::Vector<std::unique_ptr<amrex::MultiFab> >& rho,
         this->dmap,
         this->grids,
         this->m_poisson_boundary_handler,
-        is_solver_multigrid,
+        is_solver_igf_on_lev0,
         WarpX::do_single_precision_comms,
         this->ref_ratio,
         post_phi_calculation,

Source/ablastr/fields/PoissonSolver.H

@@ -82,7 +82,7 @@ namespace ablastr::fields {
  * \param[in] dmap the distribution mapping per level (e.g., from AmrMesh)
  * \param[in] grids the grids per level (e.g., from AmrMesh)
  * \param[in] boundary_handler a handler for boundary conditions, for example @see ElectrostaticSolver::PoissonBoundaryHandler
- * \param[in] is_solver_multigrid boolean to select the Poisson solver: 1 for Multigrid, 0 for FFT
+ * \param[in] is_solver_igf_on_lev0 boolean to select the Poisson solver: 1 for FFT on level 0 & Multigrid on other levels, 0 for Multigrid on all levels
  * \param[in] do_single_precision_comms perform communications in single precision
  * \param[in] rel_ref_ratio mesh refinement ratio between levels (default: 1)
  * \param[in] post_phi_calculation perform a calculation per level directly after phi was calculated; required for embedded boundaries (default: none)
@@ -106,7 +106,7 @@ computePhi (amrex::Vector<amrex::MultiFab*> const & rho,
             amrex::Vector<amrex::DistributionMapping> const& dmap,
             amrex::Vector<amrex::BoxArray> const& grids,
             T_BoundaryHandler const boundary_handler,
-            [[maybe_unused]] bool is_solver_multigrid,
+            bool is_solver_igf_on_lev0,
             bool const do_single_precision_comms = false,
             std::optional<amrex::Vector<amrex::IntVect> > rel_ref_ratio = std::nullopt,
             [[maybe_unused]] T_PostPhiCalculationFunctor post_phi_calculation = std::nullopt,
@@ -160,9 +160,19 @@ computePhi (amrex::Vector<amrex::MultiFab*> const & rho,
                 {{ beta[0], beta[1], beta[2] }};
 #endif
 
+#if !defined(WARPX_USE_FFT)
+        ABLASTR_ALWAYS_ASSERT_WITH_MESSAGE( !is_solver_igf_on_lev0,
+        "Must compile with -DWarpX_FFT=ON to use the FFT solver!");
+#endif
+
+#if !defined(WARPX_DIM_3D)
+        ABLASTR_ALWAYS_ASSERT_WITH_MESSAGE( !is_solver_igf_on_lev0,
+        "The FFT Poisson solver is currently only implemented for 3D!");
+#endif
+
 #if (defined(WARPX_USE_FFT)  && defined(WARPX_DIM_3D))
         // Use the Integrated Green Function solver (FFT) on the coarsest level if it was selected
-        if(!is_solver_multigrid && lev==0){
+        if(is_solver_igf_on_lev0 && lev==0){
             amrex::Array<amrex::Real,AMREX_SPACEDIM> const dx_igf
                     {AMREX_D_DECL(geom[lev].CellSize(0)/std::sqrt(1._rt-beta_solver[0]*beta_solver[0]),
                                 geom[lev].CellSize(1)/std::sqrt(1._rt-beta_solver[1]*beta_solver[1]),