Merge pull request #33 from lanl/mauneyc/update/execquery

Draft: Add portable query for exec space

doc/sphinx/src/using.rst

@@ -83,6 +83,14 @@ with `to` being the target location, from being the source location, and size_by
 the size of the transfer in bytes. This has implemenatations for kokkos and none 
 portability strategies.
 
+It may be useful to query the execution space, for example to know where memory needs to be copied.
+To this end, a compile-time constant boolean can be queried:
+
+.. cpp:var:: PortsOfCall::EXECUTION_IS_HOST
+
+which is `true` if the host execution space can trivially access device memory space. For example,
+for `PORTABILITY_STRATEGY_CUDA`, `PortsOfCall::EXECUTION_IS_HOST == false`.
+
 portable_errors.hpp
 ^^^^^^^^^^^^^^^^^^^^
 

ports-of-call/portability.hpp

@@ -91,6 +91,18 @@ typedef float Real;
 typedef double Real;
 #endif
 
+namespace PortsOfCall{
+// compile-time constant to check if execution of memory space
+// will be done on the host or is offloaded
+#if defined(PORTABILITY_STRATEGY_KOKKOS)
+inline constexpr bool EXECUTION_IS_HOST{Kokkos::SpaceAccessibility<Kokkos::DefaultExecutionSpace::memory_space,Kokkos::HostSpace>::accessible};
+#elif defined(PORTABILITY_STRATEGY_CUDA)
+inline constexpr bool EXECUTION_IS_HOST{false};
+#else
+inline constexpr bool EXECUTION_IS_HOST{true};
+#endif
+} // PortsOfCall
+
 template <typename T>
 void portableCopyToDevice(T * const to, T const * const from, size_t const size_bytes) {
   auto const length = size_bytes / sizeof(T);

test/test_portsofcall.cpp

@@ -18,6 +18,22 @@
 #define CATCH_CONFIG_RUNNER
 #include "catch2/catch.hpp"
 
+TEST_CASE("EXECUTION_IS_HOST is set correctly", 
+    "[PortsOfCall]") {
+  // testing this is maybe nontrivial?
+  auto isHost = PortsOfCall::EXECUTION_IS_HOST;
+
+#if defined(PORTABILITY_STRATEGY_KOKKOS)
+  auto checkHost = std::is_same<Kokkos::DefaultExecutionSpace, Kokkos::HostSpace::execution_space>::value;
+  REQUIRE( isHost == checkHost );
+#elif defined(PORTABILITY_STRATEGY_CUDA)
+  REQUIRE( isHost == false );
+#else
+  REQUIRE( isHost == true );
+#endif
+
+}
+
 // this test is lifted directly from `spiner`;
 // and there appears to be a significant amount of
 // ports-of-call testing done there.