Add Const to SID Neighbor Table Element Type (#1808)

Allows NVCC to use `LDG` instructions and thus perform better
double-load elision. Credit goes to @iomaganaris. Further, fixes
`sid::as_const` for C arrays.

---------

Co-authored-by: Ioannis Magkanaris <ioannis.magkanaris@cscs.ch>

include/gridtools/fn/sid_neighbor_table.hpp

@@ -15,6 +15,7 @@
 #include "../common/array.hpp"
 #include "../common/const_ptr_deref.hpp"
 #include "../fn/unstructured.hpp"
+#include "../sid/as_const.hpp"
 #include "../sid/concept.hpp"
 
 namespace gridtools::fn::sid_neighbor_table {
@@ -61,14 +62,15 @@ namespace gridtools::fn::sid_neighbor_table {
             static_assert(!std::is_same_v<IndexDimension, NeighborDimension>,
                 "The index dimension and the neighbor dimension must be different.");
 
-            const auto origin = sid::get_origin(sid);
-            const auto strides = sid::get_strides(sid);
+            decltype(auto) const_sid = sid::as_const(std::forward<Sid>(sid));
+            const auto origin = sid::get_origin(const_sid);
+            const auto strides = sid::get_strides(const_sid);
 
             return sid_neighbor_table<IndexDimension,
                 NeighborDimension,
                 MaxNumNeighbors,
-                sid::ptr_holder_type<Sid>,
-                sid::strides_type<Sid>>{
+                decltype(origin),
+                decltype(strides)>{
                 origin, strides}; // Note: putting the return type into the function signature will crash nvcc 12.0
         }
     } // namespace sid_neighbor_table_impl_

include/gridtools/sid/as_const.hpp

@@ -51,14 +51,14 @@ namespace gridtools {
          *                 probably might we need the `host` and `device` variations as well
          */
         template <class Src,
-            class Ptr = sid::ptr_type<std::decay_t<Src>>,
+            class Ptr = sid::ptr_type<std::remove_cv_t<std::remove_reference_t<Src>>>,
             std::enable_if_t<std::is_pointer_v<Ptr> && !std::is_const_v<std::remove_pointer_t<Ptr>>, int> = 0>
         as_const_impl_::const_adapter<Src> as_const(Src &&src) {
             return {std::forward<Src>(src)};
         }
 
         template <class Src,
-            class Ptr = sid::ptr_type<std::decay_t<Src>>,
+            class Ptr = sid::ptr_type<std::remove_cv_t<std::remove_reference_t<Src>>>,
             std::enable_if_t<!std::is_pointer_v<Ptr> || std::is_const_v<std::remove_pointer_t<Ptr>>, int> = 0>
         decltype(auto) as_const(Src &&src) {
             return std::forward<Src>(src);

tests/unit_tests/fn/test_fn_sid_neighbor_table.cu

@@ -42,7 +42,10 @@ namespace gridtools::fn {
             using dim_hymap_t = hymap::keys<edge_dim_t, edge_to_cell_dim_t>;
             auto contents = sid::synthetic()
                                 .set<sid::property::origin>(sid::host_device::simple_ptr_holder(device_data.get()))
-                                .set<sid::property::strides>(dim_hymap_t::make_values(num_neighbors, 1));
+                                .set<sid::property::strides>(dim_hymap_t::make_values(num_neighbors, 1))
+                                // for whatever reason, setting strides_kind is required
+                                // by Clang-CUDA (tested Clang 17 + CUDA 12.4)
+                                .set<sid::property::strides_kind, sid::unknown_kind>();
 
             const auto table = as_neighbor_table<edge_dim_t, edge_to_cell_dim_t, num_neighbors>(contents);
             using table_t = std::decay_t<decltype(table)>;

tests/unit_tests/sid/test_sid_as_const.cpp

@@ -32,5 +32,15 @@ namespace gridtools {
             static_assert(std::is_same_v<sid::ptr_type<testee_t>, double const *>);
             EXPECT_EQ(sid::get_origin(src)(), sid::get_origin(testee)());
         }
+
+        TEST(as_const, c_array) {
+            int src[3][2] = {{0, 1}, {10, 11}, {20, 21}};
+            auto testee = sid::as_const(src);
+            using testee_t = decltype(testee);
+
+            static_assert(is_sid<testee_t>());
+            static_assert(std::is_same_v<sid::ptr_type<testee_t>, int const *>);
+            EXPECT_EQ(sid::get_origin(src)(), sid::get_origin(testee)());
+        }
     } // namespace
 } // namespace gridtools