OpenBC FFT Solver: Fix a 2d mode bug. (#4671)

This bug only affected 3D WarpX/ImpactX when there was only one cell in
the z-direction for the solver.

The fix is that we have simplified the logic for handling domain
decomposition. If it's in the 2D mode in 3D, we simply do slab
decomposition.

Author: WeiqunZhang

Src/FFT/AMReX_FFT_R2C.H

@@ -491,11 +491,7 @@ R2C<T,D,C>::R2C (Box const& domain, Info const& info)
 #if (AMREX_SPACEDIM == 3)
     if (m_info.domain_strategy == DomainStrategy::automatic) {
         if (m_info.twod_mode) {
-            if (m_real_domain.length(2) < nprocs) {
-                m_info.domain_strategy = DomainStrategy::pencil;
-            } else {
-                m_info.domain_strategy = DomainStrategy::slab;
-            }
+            m_info.domain_strategy = DomainStrategy::slab;
         } else {
             int shortside = m_real_domain.shortside();
             if (shortside < m_info.pencil_threshold*nprocs) {
@@ -505,19 +501,20 @@ R2C<T,D,C>::R2C (Box const& domain, Info const& info)
             }
         }
     }
-    if (m_info.domain_strategy == DomainStrategy::slab && (m_real_domain.length(1) > 1)) {
-        if (m_info.twod_mode && m_real_domain.length(2) == 1) {
-            m_slab_decomp = false;
-        } else {
-            m_slab_decomp = true;
-        }
+
+    if (m_info.twod_mode) {
+        m_slab_decomp = true;
+    } else if (m_info.domain_strategy == DomainStrategy::slab && (m_real_domain.length(1) > 1)) {
+        m_slab_decomp = true;
     }
+
 #endif
 
     auto const ncomp = m_info.batch_size;
 
     auto bax = amrex::decompose(m_real_domain, nprocs,
-                                {AMREX_D_DECL(false,!m_slab_decomp,true)}, true);
+                                {AMREX_D_DECL(false,!m_slab_decomp,m_real_domain.length(2)>1)}, true);
+
     DistributionMapping dmx = detail::make_iota_distromap(bax.size());
     m_rx.define(bax, dmx, ncomp, 0, MFInfo().SetAlloc(false));
 
@@ -540,13 +537,8 @@ R2C<T,D,C>::R2C (Box const& domain, Info const& info)
         //
 
 #if (AMREX_SPACEDIM >= 2)
-#if (AMREX_SPACEDIM == 2)
-        bool batch_on_y = false;
-#else
-        bool batch_on_y = m_info.twod_mode && (m_real_domain.length(2) == 1);
-#endif
         DistributionMapping cdmy;
-        if ((m_real_domain.length(1) > 1) && !m_slab_decomp && !batch_on_y)
+        if ((m_real_domain.length(1) > 1) && !m_slab_decomp)
         {
             auto cbay = amrex::decompose(m_spectral_domain_y, nprocs,
                                          {AMREX_D_DECL(false,true,true)}, true);