[SYCL] Add "ext_oneapi_offload" backend enum value

This adds `offload` as a backend to the sycl runtime. This means it is
accepted as a value for `ONEAPI_DEVICE_SELECTOR` and friends, and
prevents an offload device reporting itself as such causing an exception
to be thrown.

Note that the UR offload adapter is still not built as part of the
build. The only way to use this is with a standalone build of
unified-runtime with libur_adapter_offload enabled and in the
`LD_LIBRARY_PATH`.

Author: RossBrunton

sycl/include/sycl/backend_types.hpp

@@ -25,6 +25,7 @@ enum class backend : char {
   // ext_intel_esimd_emulator  = 5,
   ext_oneapi_hip = 6,
   ext_oneapi_native_cpu = 7,
+  ext_oneapi_offload = 8,
 };
 
 template <backend Backend> class backend_traits;
@@ -56,6 +57,9 @@ inline std::ostream &operator<<(std::ostream &Out, backend be) {
   case backend::ext_oneapi_native_cpu:
     Out << "ext_oneapi_native_cpu";
     break;
+  case backend::ext_oneapi_offload:
+    Out << "ext_oneapi_offload";
+    break;
   case backend::all:
     Out << "all";
   }
@@ -77,6 +81,8 @@ inline std::string_view get_backend_name_no_vendor(backend Backend) {
     return "hip";
   case backend::ext_oneapi_native_cpu:
     return "native_cpu";
+  case backend::ext_oneapi_offload:
+    return "offload";
   case backend::all:
     return "all";
   }

sycl/source/backend.cpp

@@ -62,6 +62,8 @@ backend convertUrBackend(ur_backend_t UrBackend) {
     return backend::ext_oneapi_hip;
   case UR_BACKEND_NATIVE_CPU:
     return backend::ext_oneapi_native_cpu;
+  case UR_BACKEND_OFFLOAD:
+    return backend::ext_oneapi_offload;
   default:
     throw exception(make_error_code(errc::runtime),
                     "convertBackend: Unsupported backend");

sycl/source/detail/config.cpp

@@ -166,14 +166,17 @@ void dumpConfig() {
 // ONEAPI_DEVICE_SELECTOR
 // TODO: host device type will be removed once sycl_ext_oneapi_filter_selector
 // is removed.
-const std::array<std::pair<std::string, backend>, 7> &getSyclBeMap() {
-  static const std::array<std::pair<std::string, backend>, 7> SyclBeMap = {
+const std::array<std::pair<std::string, backend>, 8> &getSyclBeMap() {
+  static const std::array<std::pair<std::string, backend>, 8> SyclBeMap = {
       {{"host", backend::host},
        {"opencl", backend::opencl},
        {"level_zero", backend::ext_oneapi_level_zero},
        {"cuda", backend::ext_oneapi_cuda},
        {"hip", backend::ext_oneapi_hip},
        {"native_cpu", backend::ext_oneapi_native_cpu},
+       // Note: Offload is intentionally excluded from our documentation - it's
+       // only used for internal testing
+       {"offload", backend::ext_oneapi_offload},
        {"*", backend::all}}};
   return SyclBeMap;
 }

sycl/source/detail/config.hpp

@@ -251,7 +251,7 @@ getSyclDeviceTypeMap() {
 
 // Array is used by SYCL_DEVICE_FILTER and SYCL_DEVICE_ALLOWLIST and
 // ONEAPI_DEVICE_SELECTOR
-const std::array<std::pair<std::string, backend>, 7> &getSyclBeMap();
+const std::array<std::pair<std::string, backend>, 8> &getSyclBeMap();
 
 // ---------------------------------------
 // ONEAPI_DEVICE_SELECTOR support

sycl/source/detail/ur.cpp

@@ -217,6 +217,8 @@ static void initializeAdapters(std::vector<AdapterPtr> &Adapters,
       return backend::ext_oneapi_hip;
     case UR_BACKEND_NATIVE_CPU:
       return backend::ext_oneapi_native_cpu;
+    case UR_BACKEND_OFFLOAD:
+      return backend::ext_oneapi_offload;
     default:
       // Throw an exception, this should be unreachable.
       CHECK_UR_SUCCESS(UR_RESULT_ERROR_INVALID_ENUMERATION)

sycl/test-e2e/Basic/get_backend.cpp

@@ -22,6 +22,7 @@ bool check(backend be) {
   case backend::ext_oneapi_cuda:
   case backend::ext_oneapi_hip:
   case backend::ext_oneapi_native_cpu:
+  case backend::ext_oneapi_offload:
     return true;
   default:
     return false;

sycl/test-e2e/Regression/device_num.cpp

@@ -28,7 +28,8 @@ const std::map<backend, std::string> BackendStringMap = {
     {backend::ext_oneapi_level_zero, "ext_oneapi_level_zero"},
     {backend::ext_oneapi_cuda, "ext_oneapi_cuda"},
     {backend::ext_oneapi_hip, "ext_oneapi_hip"},
-    {backend::ext_oneapi_native_cpu, "ext_oneapi_native_cpu"}};
+    {backend::ext_oneapi_native_cpu, "ext_oneapi_native_cpu"},
+    {backend::ext_oneapi_offload, "ext_oneapi_offload"}};
 
 std::string getDeviceTypeName(const device &d) {
   auto DeviceType = d.get_info<info::device::device_type>();

sycl/unittests/helpers/UrMock.hpp

@@ -634,6 +634,8 @@ template <sycl::backend Backend = backend::opencl> class UrMock {
       return UR_BACKEND_HIP;
     case sycl::backend::ext_oneapi_native_cpu:
       return UR_BACKEND_NATIVE_CPU;
+    case sycl::backend::ext_oneapi_offload:
+      return UR_BACKEND_OFFLOAD;
     default:
       return UR_BACKEND_UNKNOWN;
     }