Add TODO comments for future USPP GPU optimizations

Author: chengleizheng

source/source_estate/elecstate_pw.cpp

@@ -369,6 +369,7 @@ void ElecStatePW<T, Device>::cal_becsum(const psi::Psi<T, Device>& psi)
                     }
                     else
                     {
+                        // TODO: gather auxk from becp on device to skip the H2D→fill→D2H round-trip
                         for (int ih = 0; ih < nh_atom; ih++)
                         {
                             const int ikb = this->ppcell->indv_ijkb0[iat] + ih;
@@ -453,6 +454,7 @@ void ElecStatePW<T, Device>::add_usrho(const psi::Psi<T, Device>& psi)
             std::vector<T> rhog_host(this->rhopw_smooth->npw);
             syncmem_complex_d2h_op()(rhog_host.data(), this->rhog[is], this->rhopw_smooth->npw);
             // CPU real2recip
+            // TODO: replace with cufft to keep rho/rhog on device
             this->rhopw_smooth->real2recip(rho_host.data(), rhog_host.data());
             // H2D rhog back to device
             syncmem_complex_h2d_op()(this->rhog[is], rhog_host.data(), this->rhopw_smooth->npw);
@@ -496,6 +498,7 @@ void ElecStatePW<T, Device>::addusdens_g(const Real* becsum, T** rhog)
     const std::complex<double> ci_tpi = ModuleBase::NEG_IMAG_UNIT * ModuleBase::TWO_PI;
 
     // ---------- all on CPU ----------
+    // TODO: port skk/tbecsum construction and radial_fft_q to device
     std::vector<double> qmod_host(npw);
     std::vector<std::complex<double>> qgm_host(npw);
     for (int ig = 0; ig < npw; ig++)